add (local) explain command to provide insights on the resolution chain

src/cli/explain.zig

@@ -0,0 +1,482 @@
+const std = @import("std");
+const Config = @import("../Config.zig");
+const GeoIp = @import("../location/GeoIp.zig");
+const GeoLite2 = @import("../location/GeoLite2.zig");
+
+/// `wttr explain <address>`: reports how an address resolves, stage by stage.
+///
+/// Deliberately a command rather than an HTTP endpoint. The report reveals the
+/// pin table, which records where particular networks are, and `--online` spends
+/// the provider's quota and appends to a cache that is never evicted. Gating all
+/// of that behind shell access on the server makes it safe by construction, with
+/// no credential to configure, no endpoint to rate limit, and no way for a
+/// spoofed header to reach it.
+pub fn runExplain(
+    allocator: std.mem.Allocator,
+    io: std.Io,
+    cfg: Config,
+    args: []const []const u8,
+) !u8 {
+    var err_buf: [512]u8 = undefined;
+    var err_out = std.Io.File.stderr().writer(io, &err_buf);
+    const ew = &err_out.interface;
+
+    var address: ?[]const u8 = null;
+    var online = false;
+
+    for (args) |arg| {
+        if (std.mem.eql(u8, arg, "--online")) {
+            online = true;
+        } else if (std.mem.startsWith(u8, arg, "-")) {
+            try ew.print("unknown option {s}\n", .{arg});
+            try ew.flush();
+            return 2;
+        } else if (address == null) {
+            address = arg;
+        } else {
+            try ew.writeAll("usage: wttr explain <address> [--online]\n");
+            try ew.flush();
+            return 2;
+        }
+    }
+
+    const ip = address orelse {
+        try ew.writeAll("usage: wttr explain <address> [--online]\n");
+        try ew.flush();
+        return 2;
+    };
+
+    var geoip = GeoIp.init(allocator, io, cfg.geolite_path, cfg) catch |e| {
+        // Not downloaded here on purpose: a diagnostic should not pull tens of
+        // megabytes as a side effect. If the server runs, the file is present.
+        try ew.print(
+            \\could not open the GeoLite2 database at {s}: {t}
+            \\the server downloads this on first start; run it once, or set WTTR_GEOLITE_PATH
+            \\
+        , .{ cfg.geolite_path, e });
+        try ew.flush();
+        return 1;
+    };
+    defer geoip.deinit();
+
+    const explanation = try geoip.explain(ip, .{ .online = online });
+    defer explanation.deinit();
+
+    var out_buf: [4096]u8 = undefined;
+    var out = std.Io.File.stdout().writer(io, &out_buf);
+    const w = &out.interface;
+    try report(w, io, cfg, ip, online, explanation);
+    try w.flush();
+
+    // Non-zero when nothing resolved, so this is usable in a script without
+    // parsing the report.
+    return if (explanation.result == null) 1 else 0;
+}
+
+fn report(
+    w: *std.Io.Writer,
+    io: std.Io,
+    cfg: Config,
+    ip: []const u8,
+    online: bool,
+    e: GeoIp.Explanation,
+) !void {
+    try w.print("address     {s}\n", .{ip});
+    if (!e.parsed) {
+        // Stated plainly because it is the one case where no stage runs: nothing
+        // is handed to a provider unless it parses as an address first.
+        try w.writeAll("            not an IP address; no source was consulted\n");
+        return;
+    }
+
+    try w.writeAll("\npins        ");
+    try w.print("{s}", .{cfg.pins_file});
+    try printAge(w, io, cfg.pins_file);
+    try w.print("\n            {d} pin(s) loaded\n", .{e.pins_loaded});
+    if (e.pin) |p| {
+        try w.print("            MATCH {s} -> {s}\n", .{ p.cidr, p.location.name });
+        try w.print("                  {d:.4},{d:.4} country {s}\n", .{
+            p.location.coords.latitude,
+            p.location.coords.longitude,
+            isoText(p.location.iso_country),
+        });
+    } else {
+        try w.writeAll("            no pin matches this address\n");
+    }
+
+    try w.writeAll("\ngeolite2    ");
+    try w.print("{s}", .{cfg.geolite_path});
+    try printAge(w, io, cfg.geolite_path);
+    try w.writeAll("\n");
+    switch (e.database) {
+        .absent => try w.writeAll("            no entry for this address\n"),
+        .unusable => try w.writeAll("            entry found, but it has no usable coordinates\n"),
+        .accepted, .rejected => |hit| {
+            try w.print("            {s}\n", .{hit.location.name});
+            try w.print("            {d:.4},{d:.4} country {s}\n", .{
+                hit.location.coords.latitude,
+                hit.location.coords.longitude,
+                isoText(hit.location.iso_country),
+            });
+            if (hit.accuracy_radius) |radius| {
+                // The number that explains a confidently wrong answer. A small
+                // radius on a wrong city is exactly the case pins exist for,
+                // because no confidence test and no provider can correct it.
+                try w.print("            accuracy radius {d} km ({s} threshold {d} km)\n", .{
+                    radius,
+                    if (e.database == .rejected) "REJECTED, over" else "accepted, within",
+                    GeoIp.max_accuracy_radius_km,
+                });
+            } else {
+                try w.writeAll("            no accuracy radius recorded (accepted)\n");
+            }
+        },
+    }
+
+    // The provider's answer is shown whether or not it was used. When the
+    // database answers confidently and wrongly, "what would the other source
+    // have said" is the question, and only a side-by-side answers it.
+    try w.print("\nfallback    {s}", .{e.fallback_provider});
+    if (!e.fallbackWasUsed()) try w.writeAll(" (not used, an earlier stage answered)");
+    try w.writeAll("\n");
+    switch (e.fallback) {
+        .not_cached => try w.writeAll("            not cached; re-run with --online to ask the provider\n"),
+        .failed => try w.writeAll("            queried, no usable answer\n"),
+        .cached, .fetched => |loc| {
+            try w.print("            {s}\n", .{loc.name});
+            try w.print("            {d:.4},{d:.4} country {s}\n", .{
+                loc.coords.latitude,
+                loc.coords.longitude,
+                isoText(loc.iso_country),
+            });
+            if (e.fallback == .fetched)
+                try w.writeAll("            fetched just now, and cached permanently\n");
+        },
+    }
+    // Last, as a footnote: the path matters when diagnosing a wrong cache
+    // directory, but it is not what the reader came for.
+    try w.print("            cache {s}", .{e.fallback_cache_path});
+    try printAge(w, io, e.fallback_cache_path);
+    try w.writeAll("\n");
+
+    try w.writeAll("\nresult      ");
+    if (e.result) |r| {
+        try w.print("{s}\n", .{r.location.name});
+        try w.print("            {d:.4},{d:.4}\n", .{
+            r.location.coords.latitude,
+            r.location.coords.longitude,
+        });
+        try w.print("            from {s}\n", .{r.source.label()});
+    } else {
+        try w.writeAll("no source resolved this address\n");
+        if (!online and e.fallback == .not_cached)
+            try w.writeAll("            (the provider was not queried; --online would try it)\n");
+    }
+
+    try w.print("            units {s}", .{
+        if (e.country) |iso| if (std.mem.eql(u8, &iso, "US")) "imperial" else "metric" else "metric",
+    });
+    if (e.country) |iso| {
+        try w.print(" (country {s})\n", .{iso[0..]});
+    } else {
+        try w.writeAll(" (no country known for this address)\n");
+    }
+
+    // Said every time rather than only on a mismatch, because there is nothing
+    // here that can detect one: a running server holds its own mapped database
+    // and its own pins, and this process cannot see either.
+    try w.writeAll(
+        \\
+        \\note        this re-runs the chain against the files above. A running
+        \\            server may differ if it has not reloaded them (SIGHUP is
+        \\            sent by `wttr pin`, and the database is remapped on refresh).
+        \\
+    );
+}
+
+/// Renders the value of an optional country code for display.
+fn isoText(iso: ?[2]u8) []const u8 {
+    if (iso) |*code| return code[0..];
+    return "unknown";
+}
+
+/// Appends "(updated N ago)" for a path, or notes that it is missing.
+///
+/// The age is what lets an operator judge whether the file this command read is
+/// the one a long-running server actually has mapped.
+///
+/// Stats directly rather than going through `GeoLite2.ageInSeconds`, which logs
+/// any failure as a missing *GeoLite2 database*; that would misreport an absent
+/// pins file, and an absent pins file is the normal state.
+fn printAge(w: *std.Io.Writer, io: std.Io, path: []const u8) !void {
+    const stat = std.Io.Dir.cwd().statFile(io, path, .{}) catch {
+        try w.writeAll("  (absent)");
+        return;
+    };
+    const age = GeoLite2.ageOf(std.Io.Timestamp.now(io, .real), stat.mtime);
+    if (age < std.time.s_per_hour) {
+        try w.print("  (updated {d}m ago)", .{@divFloor(age, std.time.s_per_min)});
+    } else if (age < std.time.s_per_day) {
+        try w.print("  (updated {d}h ago)", .{@divFloor(age, std.time.s_per_hour)});
+    } else {
+        try w.print("  (updated {d}d ago)", .{@divFloor(age, std.time.s_per_day)});
+    }
+}
+
+test "explain: rejects an argument that is not an address" {
+    const allocator = std.testing.allocator;
+    var buf: [4096]u8 = undefined;
+    var w = std.Io.Writer.fixed(&buf);
+
+    const cfg = try Config.loadForTest(allocator);
+    defer cfg.deinit(allocator);
+
+    // Reported rather than passed on. The provider URLs interpolate the argument,
+    // so "not an address" has to stop before any stage runs.
+    const e: GeoIp.Explanation = .{
+        .allocator = allocator,
+        .parsed = false,
+        .pins_loaded = 0,
+        .pin = null,
+        .database = .absent,
+        .fallback = .not_cached,
+        .fallback_provider = "ipwho.is",
+        .fallback_cache_path = "/tmp/none",
+        .result = null,
+        .country = null,
+    };
+
+    try report(&w, std.testing.io, cfg, "not-an-address", false, e);
+
+    const output = buf[0..w.end];
+    try std.testing.expect(std.mem.indexOf(u8, output, "not an IP address") != null);
+    // None of the stage headings should appear: there is nothing to say about them.
+    try std.testing.expect(std.mem.indexOf(u8, output, "geolite2") == null);
+}
+
+test "explain: names the accuracy radius and shows the provider it displaced" {
+    const allocator = std.testing.allocator;
+    var buf: [4096]u8 = undefined;
+    var w = std.Io.Writer.fixed(&buf);
+
+    const cfg = try Config.loadForTest(allocator);
+    defer cfg.deinit(allocator);
+
+    // The case this command exists for: a small radius on the wrong city, which
+    // passes the threshold and so prevents the provider from being asked. Both
+    // answers are shown, because seeing them side by side is what reveals that
+    // the database is the thing that is wrong.
+    const database_answer: @import("../location/resolver.zig").Location = .{
+        .allocator = allocator,
+        .name = "Portland, Oregon, United States",
+        .coords = .{ .latitude = 45.5136, .longitude = -122.5946 },
+        .iso_country = .{ 'U', 'S' },
+    };
+
+    const e: GeoIp.Explanation = .{
+        .allocator = allocator,
+        .parsed = true,
+        .pins_loaded = 0,
+        .pin = null,
+        .database = .{ .accepted = .{
+            .location = database_answer,
+            .accuracy_radius = 20,
+        } },
+        .fallback = .{ .cached = .{
+            .allocator = allocator,
+            .name = "Portland, Washington, United States",
+            .coords = .{ .latitude = 47.2255, .longitude = -122.4112 },
+            .iso_country = .{ 'U', 'S' },
+        } },
+        .fallback_provider = "ipwho.is",
+        .fallback_cache_path = "/tmp/none",
+        .result = .{ .location = database_answer, .source = .database },
+        .country = .{ 'U', 'S' },
+    };
+
+    try report(&w, std.testing.io, cfg, "70.102.70.100", false, e);
+
+    const output = buf[0..w.end];
+    try std.testing.expect(std.mem.indexOf(u8, output, "accuracy radius 20 km") != null);
+    try std.testing.expect(std.mem.indexOf(u8, output, "accepted, within threshold 200 km") != null);
+    try std.testing.expect(std.mem.indexOf(u8, output, "from geolite2") != null);
+    try std.testing.expect(std.mem.indexOf(u8, output, "units imperial (country US)") != null);
+
+    // The provider's answer is present and marked as not the one in force.
+    try std.testing.expect(std.mem.indexOf(u8, output, "ipwho.is (not used, an earlier stage answered)") != null);
+    try std.testing.expect(std.mem.indexOf(u8, output, "Portland, Washington, United States") != null);
+    try std.testing.expect(std.mem.indexOf(u8, output, "47.2255,-122.4112 country US") != null);
+}
+
+test "explain: a rejected entry is labelled as over the threshold" {
+    const allocator = std.testing.allocator;
+    var buf: [4096]u8 = undefined;
+    var w = std.Io.Writer.fixed(&buf);
+
+    const cfg = try Config.loadForTest(allocator);
+    defer cfg.deinit(allocator);
+
+    const e: GeoIp.Explanation = .{
+        .allocator = allocator,
+        .parsed = true,
+        .pins_loaded = 0,
+        .pin = null,
+        .database = .{ .rejected = .{
+            .location = .{
+                .allocator = allocator,
+                .name = "Somewhere Vague",
+                .coords = .{ .latitude = 1, .longitude = 2 },
+                .iso_country = null,
+            },
+            .accuracy_radius = 500,
+        } },
+        .fallback = .not_cached,
+        .fallback_provider = "ipwho.is",
+        .fallback_cache_path = "/tmp/none",
+        .result = null,
+        .country = null,
+    };
+
+    try report(&w, std.testing.io, cfg, "203.0.113.7", false, e);
+
+    const output = buf[0..w.end];
+    try std.testing.expect(std.mem.indexOf(u8, output, "REJECTED, over threshold 200 km") != null);
+    // The operator has to be told the provider is reachable but was not asked,
+    // otherwise "no source resolved this" looks like a dead end.
+    try std.testing.expect(std.mem.indexOf(u8, output, "--online") != null);
+    try std.testing.expect(std.mem.indexOf(u8, output, "no source resolved this address") != null);
+    try std.testing.expect(std.mem.indexOf(u8, output, "no country known") != null);
+}
+
+test "explain: a pin is reported as the match, with the database it overrides" {
+    const allocator = std.testing.allocator;
+    var buf: [4096]u8 = undefined;
+    var w = std.Io.Writer.fixed(&buf);
+
+    const cfg = try Config.loadForTest(allocator);
+    defer cfg.deinit(allocator);
+
+    const pin_location: @import("../location/resolver.zig").Location = .{
+        .allocator = allocator,
+        .name = "Seattle, Washington, United States",
+        .coords = .{ .latitude = 47.6, .longitude = -122.3 },
+        .iso_country = .{ 'U', 'S' },
+    };
+
+    const e: GeoIp.Explanation = .{
+        .allocator = allocator,
+        .parsed = true,
+        .pins_loaded = 1,
+        .pin = .{ .cidr = "70.102.70.100/32", .location = pin_location },
+        // Still reported, because an operator checking a pin needs to see what it
+        // is overriding.
+        .database = .{ .accepted = .{
+            .location = .{
+                .allocator = allocator,
+                .name = "Portland, Oregon, United States",
+                .coords = .{ .latitude = 45.5136, .longitude = -122.5946 },
+                .iso_country = .{ 'U', 'S' },
+            },
+            .accuracy_radius = 20,
+        } },
+        .fallback = .not_cached,
+        .fallback_provider = "ipwho.is",
+        .fallback_cache_path = "/tmp/none",
+        .result = .{ .location = pin_location, .source = .pin },
+        .country = .{ 'U', 'S' },
+    };
+
+    try report(&w, std.testing.io, cfg, "70.102.70.100", false, e);
+
+    const output = buf[0..w.end];
+    try std.testing.expect(std.mem.indexOf(u8, output, "MATCH 70.102.70.100/32") != null);
+    try std.testing.expect(std.mem.indexOf(u8, output, "1 pin(s) loaded") != null);
+    try std.testing.expect(std.mem.indexOf(u8, output, "Portland, Oregon") != null);
+    try std.testing.expect(std.mem.indexOf(u8, output, "from pin") != null);
+}
+
+test "explain: a cached provider answer says so, and a fetched one says it was stored" {
+    const allocator = std.testing.allocator;
+    const cfg = try Config.loadForTest(allocator);
+    defer cfg.deinit(allocator);
+
+    const loc: @import("../location/resolver.zig").Location = .{
+        .allocator = allocator,
+        .name = "Tacoma, Washington, United States",
+        .coords = .{ .latitude = 47.2255, .longitude = -122.4111 },
+        .iso_country = .{ 'U', 'S' },
+    };
+
+    const base: GeoIp.Explanation = .{
+        .allocator = allocator,
+        .parsed = true,
+        .pins_loaded = 0,
+        .pin = null,
+        .database = .absent,
+        .fallback = .not_cached,
+        .fallback_provider = "ipwho.is",
+        .fallback_cache_path = "/tmp/none",
+        .result = null,
+        .country = .{ 'U', 'S' },
+    };
+
+    {
+        var buf: [4096]u8 = undefined;
+        var w = std.Io.Writer.fixed(&buf);
+        var cached = base;
+        cached.fallback = .{ .cached = loc };
+        cached.result = .{ .location = loc, .source = .fallback_cache };
+        try report(&w, std.testing.io, cfg, "198.51.100.7", false, cached);
+        const output = buf[0..w.end];
+        // Used, so it must NOT be labelled as displaced.
+        try std.testing.expect(std.mem.indexOf(u8, output, "not used") == null);
+        try std.testing.expect(std.mem.indexOf(u8, output, "from fallback (cached)") != null);
+        // Nothing was fetched, so nothing may claim to have been stored.
+        try std.testing.expect(std.mem.indexOf(u8, output, "cached permanently") == null);
+    }
+
+    {
+        // The report has to admit the side effect: this cache is append-only and
+        // never evicted, so a fetch is a permanent change.
+        var buf: [4096]u8 = undefined;
+        var w = std.Io.Writer.fixed(&buf);
+        var fetched = base;
+        fetched.fallback = .{ .fetched = loc };
+        fetched.result = .{ .location = loc, .source = .fallback_online };
+        try report(&w, std.testing.io, cfg, "198.51.100.7", true, fetched);
+        const output = buf[0..w.end];
+        try std.testing.expect(std.mem.indexOf(u8, output, "cached permanently") != null);
+        try std.testing.expect(std.mem.indexOf(u8, output, "from fallback (fetched)") != null);
+    }
+}
+
+test "explain: reports the pins file even when there are none" {
+    const allocator = std.testing.allocator;
+    var buf: [4096]u8 = undefined;
+    var w = std.Io.Writer.fixed(&buf);
+
+    const cfg = try Config.loadForTest(allocator);
+    defer cfg.deinit(allocator);
+
+    const e: GeoIp.Explanation = .{
+        .allocator = allocator,
+        .parsed = true,
+        .pins_loaded = 0,
+        .pin = null,
+        .database = .absent,
+        .fallback = .not_cached,
+        .fallback_provider = "ipwho.is",
+        .fallback_cache_path = "/tmp/none",
+        .result = null,
+        .country = null,
+    };
+
+    try report(&w, std.testing.io, cfg, "203.0.113.7", false, e);
+
+    const output = buf[0..w.end];
+    // Naming the path matters more than the count: the usual mistake is running
+    // this against a different cache directory than the server uses.
+    try std.testing.expect(std.mem.indexOf(u8, output, cfg.pins_file) != null);
+    try std.testing.expect(std.mem.indexOf(u8, output, "0 pin(s) loaded") != null);
+    try std.testing.expect(std.mem.indexOf(u8, output, "no pin matches") != null);
+}

src/cli/pin.zig

@@ -33,21 +33,29 @@ pub fn printUsage(io: std.Io, exe: []const u8) !u8 {
         \\  pin <cidr> <location>   Override the location for an IP or range
         \\  unpin <cidr>            Remove an override
         \\  pins                    List overrides
+        \\  explain <address>       Show how an address resolves, stage by stage
         \\  help                    Show this message
         \\
         \\Pins take precedence over the GeoLite2 database, which is the point:
         \\they exist for addresses the database resolves confidently and wrongly.
+        \\`explain` is how you tell that case apart from the rest; it names the
+        \\source that answered and the accuracy radius the database claimed.
         \\
         \\Examples:
         \\  {s} pin 12.94.132.0/24 San Francisco
         \\  {s} pin 12.94.132.170 Portland, Oregon
         \\  {s} pin 2001:db8::/32 London
         \\  {s} unpin 12.94.132.0/24
+        \\  {s} explain 12.94.132.170
         \\
         \\A bare address is treated as a single host (/32 or /128). When ranges
         \\overlap, the longest prefix wins, so a host pin beats a range pin.
         \\
-    , .{ base, base, base, base, base });
+        \\`explain` does not query the online provider unless given --online,
+        \\because that spends the provider's quota and writes to a cache that is
+        \\never evicted.
+        \\
+    , .{ base, base, base, base, base, base });
     try w.flush();
     return 0;
 }
@@ -109,7 +117,7 @@ pub fn runPin(
     var pins = try Pins.load(allocator, io, cfg.pins_file);
     defer pins.deinit();
 
-    try pins.put(cidr, location.name, location.coords);
+    try pins.put(cidr, location.name, location.coords, location.iso_country);
     try pins.save(io, cfg.pins_file);
 
     var out_buf: [512]u8 = undefined;
@@ -121,6 +129,13 @@ pub fn runPin(
         location.coords.latitude,
         location.coords.longitude,
     });
+    // Surfaced because it decides units: without a country the pinned client
+    // falls back to whatever the database says about its address.
+    if (location.iso_country) |iso| {
+        try w.print("country {s} (units follow this)\n", .{iso[0..]});
+    } else {
+        try w.writeAll("country unknown (units will follow the GeoLite2 entry for the address)\n");
+    }
     try w.print("wrote {s}\n", .{cfg.pins_file});
     try reportSignal(w, notifyServers(io));
     try w.flush();
@@ -188,8 +203,14 @@ pub fn runList(allocator: std.mem.Allocator, io: std.Io, cfg: Config) !u8 {
             .{ .family = e.family, .network = e.network, .prefix_len = e.prefix_len },
             &cidr_buf,
         );
-        try w.print("{s: <20} {s} ({d:.4}, {d:.4})\n", .{
-            text, e.name, e.coords.latitude, e.coords.longitude,
+        // The country is shown because it is what decides units, and a pin
+        // written before it was recorded shows "--" rather than looking complete.
+        try w.print("{s: <20} {s: <4} {s} ({d:.4}, {d:.4})\n", .{
+            text,
+            if (e.iso_country) |*iso| iso[0..] else "--",
+            e.name,
+            e.coords.latitude,
+            e.coords.longitude,
         });
     }
     try w.flush();

src/http/handler.zig

@@ -147,7 +147,11 @@ fn handleWeatherInternal(
 
     var render_options = params.render_options;
     // Determine if imperial units should be used
-    // Priority: explicit ?u or ?m > lang=us > US IP > default metric
+    // Priority: explicit ?u or ?m > lang=us > US client address > default metric
+    //
+    // The address is resolved through the full pin -> database -> fallback-cache
+    // chain, so an operator's pin decides units as well as location. It used to
+    // read the database directly, which rendered a pinned US client in metric.
     if (params.use_imperial == null) {
         // User did not ask for anything explicitly
 
@@ -528,3 +532,95 @@ test "handler: format line 3" {
     try std.testing.expect(std.mem.indexOf(u8, pr.body, "California, United States:") != null);
     try std.testing.expect(std.mem.indexOf(u8, pr.body, "°C") != null);
 }
+
+test "handler: a pin's country selects units, not the database" {
+    const allocator = std.testing.allocator;
+    const MockHarness = @import("Server.zig").MockHarness;
+    const Pins = @import("../location/Pins.zig");
+    const Location = @import("../location/resolver.zig").Location;
+
+    var harness = try MockHarness.init(allocator);
+    defer harness.deinit();
+
+    // GeoLite2 places this AT&T address in Texas, so the database says US. The
+    // operator says London. Units have to follow the operator.
+    try harness.geoip.pins.put(
+        try Pins.parseCidr("12.94.132.0/24"),
+        "London, United Kingdom",
+        .{ .latitude = 51.5074, .longitude = -0.1278 },
+        Location.isoFrom("GB"),
+    );
+
+    var ht = httpz.testing.init(.{});
+    defer ht.deinit();
+
+    ht.url("/12.94.132.170?format=1");
+    ht.param("location", "12.94.132.170");
+
+    try handleWeather(&harness.opts, ht.req, ht.res, "12.94.132.170");
+
+    try ht.expectStatus(200);
+    // Metric. Before unit selection consulted pins this rendered in Fahrenheit,
+    // because it read the database directly and the database still says US.
+    try ht.expectBody("☀️ +20°C\n");
+}
+
+test "handler: a US pin selects imperial where the database has no entry" {
+    const allocator = std.testing.allocator;
+    const MockHarness = @import("Server.zig").MockHarness;
+    const Pins = @import("../location/Pins.zig");
+    const Location = @import("../location/resolver.zig").Location;
+
+    var harness = try MockHarness.init(allocator);
+    defer harness.deinit();
+
+    // TEST-NET-3 is absent from GeoLite2, so without the pin there is no country
+    // and the client would fall back to metric despite being pinned to the US.
+    try harness.geoip.pins.put(
+        try Pins.parseCidr("203.0.113.0/24"),
+        "Seattle, Washington, United States",
+        .{ .latitude = 47.6, .longitude = -122.3 },
+        Location.isoFrom("US"),
+    );
+
+    var ht = httpz.testing.init(.{});
+    defer ht.deinit();
+
+    ht.url("/203.0.113.7?format=1");
+    ht.param("location", "203.0.113.7");
+
+    try handleWeather(&harness.opts, ht.req, ht.res, "203.0.113.7");
+
+    try ht.expectStatus(200);
+    try ht.expectBody("☀️ +68°F\n");
+}
+
+test "handler: an explicit unit request still overrides the pin's country" {
+    const allocator = std.testing.allocator;
+    const MockHarness = @import("Server.zig").MockHarness;
+    const Pins = @import("../location/Pins.zig");
+    const Location = @import("../location/resolver.zig").Location;
+
+    var harness = try MockHarness.init(allocator);
+    defer harness.deinit();
+
+    try harness.geoip.pins.put(
+        try Pins.parseCidr("203.0.113.0/24"),
+        "Seattle, Washington, United States",
+        .{ .latitude = 47.6, .longitude = -122.3 },
+        Location.isoFrom("US"),
+    );
+
+    var ht = httpz.testing.init(.{});
+    defer ht.deinit();
+
+    // `?m` has to keep winning: the country is only consulted when the caller
+    // expressed no preference.
+    ht.url("/203.0.113.7?format=1&m");
+    ht.param("location", "203.0.113.7");
+
+    try handleWeather(&harness.opts, ht.req, ht.res, "203.0.113.7");
+
+    try ht.expectStatus(200);
+    try ht.expectBody("☀️ +20°C\n");
+}

src/location/GeoCache.zig

@@ -1,5 +1,6 @@
 const std = @import("std");
 const Coordinates = @import("../Coordinates.zig");
+const Location = @import("resolver.zig").Location;
 const srf = @import("srf");
 
 const GeoCache = @This();
@@ -17,6 +18,10 @@ last_save: i64,
 pub const CachedLocation = struct {
     name: []const u8,
     coords: Coordinates,
+    /// ISO 3166-1 alpha-2 country code, upper case, when the geocoder reported
+    /// one. Cached so that `wttr pin <cidr> <place>` records the country even
+    /// when the place name is answered from here rather than from Nominatim.
+    iso_country: ?[2]u8 = null,
 };
 
 pub fn init(allocator: std.mem.Allocator, io: std.Io, cache_file: ?[]const u8) !GeoCache {
@@ -66,6 +71,7 @@ pub fn put(self: *GeoCache, query: []const u8, location: CachedLocation) !void {
     const value = CachedLocation{
         .name = try self.allocator.dupe(u8, location.name),
         .coords = location.coords,
+        .iso_country = location.iso_country,
     };
     try self.cache.put(key, value);
     self.dirty = true;
@@ -96,11 +102,17 @@ pub fn saveIfNeeded(self: *GeoCache) void {
 ///
 /// The query is stored alongside the result rather than used as an object key,
 /// because SRF records are flat field lists rather than a nested map.
+///
+/// `iso` defaults to null so files written before it existed still load: SRF
+/// fills a missing field from the Zig default and only errors when there is
+/// none. It is also omitted on write when null, so unknown countries add
+/// nothing to the file.
 const Record = struct {
     query: []const u8,
     name: []const u8,
     lat: f64,
     lon: f64,
+    iso: ?[]const u8 = null,
 };
 
 fn load(allocator: std.mem.Allocator, cache: *std.StringHashMap(CachedLocation), content: []const u8) !void {
@@ -129,6 +141,7 @@ fn load(allocator: std.mem.Allocator, cache: *std.StringHashMap(CachedLocation),
         const existing = try cache.fetchPut(cache_key, .{
             .name = name_copy,
             .coords = .{ .latitude = record.lat, .longitude = record.lon },
+            .iso_country = if (record.iso) |iso| Location.isoFrom(iso) else null,
         });
         if (existing) |old| {
             allocator.free(old.key);
@@ -159,6 +172,10 @@ fn save(self: *GeoCache, writer: *std.Io.Writer) !void {
             .name = entry.value_ptr.name,
             .lat = entry.value_ptr.coords.latitude,
             .lon = entry.value_ptr.coords.longitude,
+            // Borrowed from the map rather than copied: the map is not touched
+            // between here and the `print` below, and a by-value capture would
+            // leave the slice pointing at a dead loop temporary.
+            .iso = if (entry.value_ptr.iso_country) |*iso| iso[0..] else null,
         };
     }
 
@@ -327,3 +344,76 @@ test "save and load round-trip" {
     try std.testing.expectApproxEqAbs(@as(f64, 52.5200), berlin.?.coords.latitude, 0.0001);
     try std.testing.expectApproxEqAbs(@as(f64, 13.4050), berlin.?.coords.longitude, 0.0001);
 }
+
+test "save and load round-trip the country" {
+    const allocator = std.testing.allocator;
+    var cache1 = try GeoCache.init(allocator, std.testing.io, null);
+    defer cache1.deinit();
+
+    try cache1.put("Seattle,+Washington", .{
+        .name = "Seattle, King County, Washington, United States",
+        .coords = .{ .latitude = 47.6038321, .longitude = -122.330062 },
+        .iso_country = Location.isoFrom("us"),
+    });
+    // Kept as unknown rather than acquiring a country on the round trip.
+    try cache1.put("Nowhere", .{
+        .name = "Nowhere",
+        .coords = .{ .latitude = 1, .longitude = 2 },
+    });
+
+    var buffer: [2048]u8 = undefined;
+    var writer = std.Io.Writer.fixed(&buffer);
+    try cache1.save(&writer);
+
+    var cache2 = std.StringHashMap(CachedLocation).init(allocator);
+    defer {
+        var it = cache2.iterator();
+        while (it.next()) |entry| {
+            allocator.free(entry.key_ptr.*);
+            allocator.free(entry.value_ptr.name);
+        }
+        cache2.deinit();
+    }
+
+    try load(allocator, &cache2, buffer[0..writer.end]);
+
+    const seattle = cache2.get("Seattle,+Washington") orelse return error.TestUnexpectedResult;
+    // Normalized on the way in, so it reads back upper case even though
+    // Nominatim reports "us".
+    try std.testing.expectEqualStrings("US", &(seattle.iso_country.?));
+
+    const nowhere = cache2.get("Nowhere") orelse return error.TestUnexpectedResult;
+    try std.testing.expect(nowhere.iso_country == null);
+}
+
+test "load: a geocache written before the country field still loads" {
+    const allocator = std.testing.allocator;
+    var cache_map = std.StringHashMap(CachedLocation).init(allocator);
+    defer {
+        var it = cache_map.iterator();
+        while (it.next()) |entry| {
+            allocator.free(entry.key_ptr.*);
+            allocator.free(entry.value_ptr.name);
+        }
+        cache_map.deinit();
+    }
+
+    // The four-field shape. These entries are re-geocodable, but discarding them
+    // would mean a burst of Nominatim traffic on the first restart after upgrade.
+    const content =
+        \\#!srfv1
+        \\#!long
+        \\query::London
+        \\name::London, UK
+        \\lat:num:51.5074
+        \\lon:num:-0.1278
+        \\
+    ;
+
+    try load(allocator, &cache_map, content);
+
+    try std.testing.expectEqual(@as(usize, 1), cache_map.count());
+    const london = cache_map.get("London") orelse return error.TestUnexpectedResult;
+    try std.testing.expectEqualStrings("London, UK", london.name);
+    try std.testing.expect(london.iso_country == null);
+}

src/location/GeoIp.zig

@@ -41,6 +41,44 @@ const FallbackClient = union(enum) {
         };
     }
 
+    /// Country code for an address already in the provider's cache.
+    ///
+    /// Cache-only by design: unit selection must not be able to trigger an
+    /// outbound request, and the cache is permanent, so anything previously
+    /// resolved is still there.
+    fn cachedCountry(self: FallbackClient, ip: []const u8) ?[2]u8 {
+        const packed_ip = Ip2location.packIp(ip) orelse return null;
+        return switch (self) {
+            .ip2location => |client| client.cache.getCountry(packed_ip.key),
+            .ipwhois => |client| client.cache.getCountry(packed_ip.key),
+        };
+    }
+
+    /// Full cached answer for an address, without contacting the provider.
+    fn cachedLookup(self: FallbackClient, ip: []const u8) ?Location {
+        const packed_ip = Ip2location.packIp(ip) orelse return null;
+        return switch (self) {
+            .ip2location => |client| client.cache.get(packed_ip.key),
+            .ipwhois => |client| client.cache.get(packed_ip.key),
+        };
+    }
+
+    /// Human-readable provider name, for diagnostics and logs.
+    fn name(self: FallbackClient) []const u8 {
+        return switch (self) {
+            .ip2location => "ip2location.io",
+            .ipwhois => "ipwho.is",
+        };
+    }
+
+    /// Where this provider's permanent cache lives.
+    fn cachePath(self: FallbackClient) []const u8 {
+        return switch (self) {
+            .ip2location => |client| client.cache.path,
+            .ipwhois => |client| client.cache.path,
+        };
+    }
+
     fn deinit(self: FallbackClient, allocator: std.mem.Allocator) void {
         switch (self) {
             .ip2location => |client| {
@@ -192,10 +230,31 @@ pub fn reloadPins(self: *GeoIP) !void {
     log.info("reloaded {d} IP pin(s) from {s}", .{ self.pins.entries.items.len, self.pins_path });
 }
 
+/// Where a resolution came from.
+///
+/// Reported by `explain` and named in the per-resolution log line, because
+/// "which source answered" is the question an operator actually has when a
+/// client is placed in the wrong city.
+pub const Source = enum {
+    pin,
+    database,
+    fallback_cache,
+    fallback_online,
+
+    pub fn label(self: Source) []const u8 {
+        return switch (self) {
+            .pin => "pin",
+            .database => "geolite2",
+            .fallback_cache => "fallback (cached)",
+            .fallback_online => "fallback (fetched)",
+        };
+    }
+};
+
 pub fn lookup(self: *GeoIP, ip: []const u8) ?Location {
-    // Try MaxMind first. The shared lock has to cover `extractCoordinates` as
-    // well as the lookup itself: `MMDB_lookup_result_s.entry` holds a pointer
-    // back to the `MMDB_s`, which `extractCoordinates` dereferences.
+    // The shared lock has to cover `readEntry` as well as the lookup itself:
+    // `MMDB_lookup_result_s.entry` holds a pointer back to the `MMDB_s`, which
+    // `readEntry` dereferences.
     const from_db: ?Location = blk: {
         self.lock.lockSharedUncancelable(self.io);
         defer self.lock.unlockShared(self.io);
@@ -212,7 +271,24 @@ pub fn lookup(self: *GeoIP, ip: []const u8) ?Location {
 
         log.debug("lookup geoip db for ip {s}. Found: {}", .{ ip, result.found_entry });
         if (!result.found_entry) break :blk null;
-        break :blk self.extractCoordinates(ip, result);
+
+        const hit = self.readEntry(ip, result) orelse break :blk null;
+
+        // Reject low-confidence entries so the online provider gets a chance.
+        // `readEntry` reports the radius rather than acting on it, so that
+        // `explain` can describe this decision without repeating it.
+        if (hit.accuracy_radius) |radius| {
+            if (radius > max_accuracy_radius_km) {
+                log.info(
+                    "GeoLite2 accuracy_radius for ip {s} is {d} km (>{d} km threshold), falling back to the online provider",
+                    .{ ip, radius, max_accuracy_radius_km },
+                );
+                hit.location.deinit();
+                break :blk null;
+            }
+        }
+
+        break :blk hit.location;
     };
     if (from_db) |coords| return coords;
 
@@ -236,26 +312,283 @@ fn lookupInternal(mmdb: *c.MMDB_s, ip: []const u8) !c.MMDB_lookup_result_s {
     return result;
 }
 
-pub fn isUSIp(self: *GeoIP, ip: []const u8) bool {
-    // `country_data` borrows from the mapped database via `result.entry`, so the
-    // whole read stays inside the shared lock.
-    self.lock.lockSharedUncancelable(self.io);
-    defer self.lock.unlockShared(self.io);
+/// A full account of how an address resolves, for `wttr explain`.
+///
+/// Reports every stage rather than just the winner, because the interesting
+/// failures are the ones where an early stage answered confidently and wrongly
+/// and later stages were therefore never consulted.
+pub const Explanation = struct {
+    allocator: std.mem.Allocator,
 
-    var result = lookupInternal(self.mmdb, ip) catch return false;
-    if (!result.found_entry) return false;
+    /// False when the argument was not an address at all. Everything else is
+    /// skipped in that case: nothing should reach a provider with a value that is
+    /// not an address.
+    parsed: bool,
+    /// How many pins are loaded, so "no pin matched" reads differently from
+    /// "there are no pins".
+    pins_loaded: usize,
+    pin: ?Pinned,
+    database: Database,
+    fallback: Fallback,
+    /// The provider that would be consulted, named even when it is not reached.
+    fallback_provider: []const u8,
+    fallback_cache_path: []const u8,
 
-    // SAFETY: The C API will initialize this on the next line
-    var country_data: c.MMDB_entry_data_s = undefined;
-    const status = c.MMDB_get_value(&result.entry, &country_data, "country", "iso_code", @as([*c]const u8, null));
+    /// The answer `lookup` would return, and where it came from.
+    result: ?Result,
+    /// The country `lookupCountry` would return, which decides units.
+    country: ?[2]u8,
 
-    if (status != c.MMDB_SUCCESS or !country_data.has_data) {
-        log.info("lookup found result, but no country available in data for ip {s}. MMDB_get_value returned {d}", .{ ip, status });
-        return false;
+    pub const Pinned = struct {
+        /// Owned text of the matching range.
+        cidr: []const u8,
+        location: Location,
+    };
+
+    pub const Result = struct {
+        location: Location,
+        source: Source,
+    };
+
+    pub const Database = union(enum) {
+        /// No entry for this address.
+        absent,
+        /// An entry whose coordinates could not be read.
+        unusable,
+        /// An entry within the confidence threshold, so `lookup` uses it.
+        accepted: DbHit,
+        /// An entry too coarse to trust, so `lookup` falls through to the
+        /// provider. This is the case the threshold exists for.
+        rejected: DbHit,
+
+        pub fn hit(self: Database) ?DbHit {
+            return switch (self) {
+                .accepted, .rejected => |h| h,
+                .absent, .unusable => null,
+            };
+        }
+    };
+
+    pub const Fallback = union(enum) {
+        /// Answered from the permanent cache.
+        cached: Location,
+        /// Fetched over the network during this run.
+        fetched: Location,
+        /// Not cached, and no online lookup was asked for.
+        not_cached,
+        /// An online lookup was asked for and did not produce an answer.
+        failed,
+
+        pub fn location(self: Fallback) ?Location {
+            return switch (self) {
+                .cached, .fetched => |loc| loc,
+                .not_cached, .failed => null,
+            };
+        }
+    };
+
+    /// Whether the provider's answer is the one `lookup` would return.
+    ///
+    /// Derived rather than stored: the report needs to show what the provider
+    /// says even when an earlier stage won, so "do we have an answer" and "is it
+    /// the answer" are separate questions.
+    pub fn fallbackWasUsed(self: Explanation) bool {
+        const r = self.result orelse return false;
+        return r.source == .fallback_cache or r.source == .fallback_online;
     }
 
-    const country_code = country_data.unnamed_0.utf8_string[0..country_data.data_size];
-    return std.mem.eql(u8, country_code, "US");
+    pub fn deinit(self: Explanation) void {
+        if (self.pin) |p| {
+            self.allocator.free(p.cidr);
+            p.location.deinit();
+        }
+        if (self.database.hit()) |h| h.location.deinit();
+        if (self.fallback.location()) |loc| loc.deinit();
+        // `result` borrows whichever stage won rather than owning a copy, so it
+        // is deliberately not freed here.
+    }
+};
+
+pub const ExplainOptions = struct {
+    /// Whether a cache miss may be resolved over the network.
+    ///
+    /// Off by default, and that default matters: the fallback cache is permanent
+    /// and append-only, so a diagnostic that fetched by default would grow the
+    /// cache and spend the provider's quota every time it ran.
+    online: bool = false,
+};
+
+/// Explains how `ip` resolves, walking every stage.
+///
+/// This re-runs the chain rather than replaying a past request. The result
+/// matches what a running server would answer as long as that server's pins and
+/// mapped database are the same as the ones on disk, which is why the caller
+/// reports both files.
+pub fn explain(self: *GeoIP, ip: []const u8, opts: ExplainOptions) !Explanation {
+    var out: Explanation = .{
+        .allocator = self.allocator,
+        .parsed = Ip2location.packIp(ip) != null,
+        .pins_loaded = 0,
+        .pin = null,
+        .database = .absent,
+        // Overwritten below for any address that parses. Left as "not cached" so
+        // an address that never reaches the provider reads as untried rather than
+        // as a miss the provider reported.
+        .fallback = .not_cached,
+        .fallback_provider = self.fallback_client.name(),
+        .fallback_cache_path = self.fallback_client.cachePath(),
+        .result = null,
+        .country = null,
+    };
+    // Reads whatever has been filled in so far, so a failure partway through
+    // does not leak the stages that already succeeded.
+    errdefer out.deinit();
+
+    {
+        self.lock.lockSharedUncancelable(self.io);
+        defer self.lock.unlockShared(self.io);
+
+        out.pins_loaded = self.pins.entries.items.len;
+
+        if (out.parsed) {
+            if (self.pins.matchFor(ip)) |entry| {
+                var buf: [64]u8 = undefined;
+                const text = try Pins.formatCidr(
+                    .{ .family = entry.family, .network = entry.network, .prefix_len = entry.prefix_len },
+                    &buf,
+                );
+                // Both copies are owned, because `entry` points into the pin set
+                // and is only valid while the lock is held.
+                const cidr = try self.allocator.dupe(u8, text);
+                errdefer self.allocator.free(cidr);
+                const name = try self.allocator.dupe(u8, entry.name);
+
+                out.pin = .{
+                    .cidr = cidr,
+                    .location = .{
+                        .allocator = self.allocator,
+                        .name = name,
+                        .coords = entry.coords,
+                        .iso_country = entry.iso_country,
+                    },
+                };
+            }
+
+            // The database is read even when a pin matched. An operator debugging
+            // a pin needs to see what it is overriding.
+            if (lookupInternal(self.mmdb, ip)) |result| {
+                if (!result.found_entry) {
+                    out.database = .absent;
+                } else if (self.readEntry(ip, result)) |hit| {
+                    const too_coarse = if (hit.accuracy_radius) |r| r > max_accuracy_radius_km else false;
+                    out.database = if (too_coarse) .{ .rejected = hit } else .{ .accepted = hit };
+                } else {
+                    out.database = .unusable;
+                }
+            } else |_| {
+                out.database = .absent;
+            }
+        }
+    }
+
+    if (!out.parsed) return out;
+
+    // The provider is consulted even when an earlier stage already answered.
+    // "What would the other source have said" is the question worth asking when
+    // the database answers confidently and wrongly, and comparing the two is the
+    // whole reason to run this. It does not affect `result`: precedence below
+    // still prefers the pin and the database.
+    //
+    // Reading the cache is free. Going to the network is not, which is why it
+    // stays behind `--online` regardless of whether the answer is needed.
+    if (self.fallback_client.cachedLookup(ip)) |cached| {
+        out.fallback = .{ .cached = cached };
+    } else if (opts.online) {
+        if (self.fallback_client.lookup(ip)) |fetched| {
+            out.fallback = .{ .fetched = fetched };
+        } else {
+            out.fallback = .failed;
+        }
+    } else {
+        out.fallback = .not_cached;
+    }
+
+    // Precedence, restated here against the collected stages so the report and
+    // `resolve` cannot disagree about who wins. The drift test below asserts
+    // this matches `lookup` for real addresses.
+    if (out.pin) |p| {
+        out.result = .{ .location = p.location, .source = .pin };
+    } else if (out.database.hit()) |h| {
+        if (out.database == .accepted) out.result = .{ .location = h.location, .source = .database };
+    }
+    if (out.result == null) {
+        switch (out.fallback) {
+            .cached => |loc| out.result = .{ .location = loc, .source = .fallback_cache },
+            .fetched => |loc| out.result = .{ .location = loc, .source = .fallback_online },
+            else => {},
+        }
+    }
+
+    out.country = self.lookupCountry(ip);
+    return out;
+}
+
+pub fn isUSIp(self: *GeoIP, ip: []const u8) bool {
+    const iso = self.lookupCountry(ip) orelse return false;
+    return std.mem.eql(u8, &iso, "US");
+}
+
+/// ISO 3166-1 alpha-2 country code for an address, for unit selection.
+///
+/// Walks the same precedence as `lookup` -- pins, then the database, then the
+/// online provider's cache -- so both an operator's pin and a previously fetched
+/// fallback answer influence which units a client sees. This previously read the
+/// database directly, which meant a pinned or fallback-resolved client could be
+/// rendered in metric while the location it was given was in the US.
+///
+/// Never performs a live fallback fetch. Units are not worth an outbound request
+/// on their own, and for a request with no explicit location the resolution in
+/// `handleWeather` has already populated the cache by the time this runs. The
+/// residual case -- an explicit location query from an address seen for the first
+/// time, whose only answer would come from the network -- gets the default units
+/// once and the right ones on every later request.
+///
+/// A matching pin with no recorded country falls through to the database rather
+/// than answering "unknown", so pins written before the country was stored keep
+/// behaving exactly as they did.
+pub fn lookupCountry(self: *GeoIP, ip: []const u8) ?[2]u8 {
+    {
+        // Covers `pins` and `mmdb` only. The cache read below is left outside for
+        // the same reason `lookup` calls the fallback outside the lock: it is not
+        // guarded by this lock on the write side either.
+        self.lock.lockSharedUncancelable(self.io);
+        defer self.lock.unlockShared(self.io);
+
+        if (self.pins.lookupCountry(ip)) |iso| return iso;
+
+        if (lookupInternal(self.mmdb, ip)) |result| {
+            if (result.found_entry) {
+                if (readIso(result)) |iso| return iso;
+            }
+        } else |_| {}
+    }
+
+    return self.fallback_client.cachedCountry(ip);
+}
+
+/// Reads `country/iso_code` from a lookup result.
+///
+/// Shared by `lookupCountry` and `extractCoordinates` so there is one place that
+/// knows where the country code lives and how it is validated.
+fn readIso(result: c.MMDB_lookup_result_s) ?[2]u8 {
+    var entry_copy = result.entry;
+
+    // SAFETY: iso_data is set by MMDB_get_value
+    var iso_data: c.MMDB_entry_data_s = undefined;
+    const status = c.MMDB_get_value(&entry_copy, &iso_data, "country", "iso_code", @as([*c]const u8, null));
+    if (status != c.MMDB_SUCCESS or !iso_data.has_data) return null;
+
+    return Location.isoFrom(iso_data.unnamed_0.utf8_string[0..iso_data.data_size]);
 }
 
 /// Maximum accuracy radius (in km) to trust from GeoLite2. Entries with a
@@ -265,25 +598,36 @@ pub fn isUSIp(self: *GeoIP, ip: []const u8) bool {
 ///
 /// Note this only catches entries GeoLite2 admits are vague. An entry can be
 /// precise *and* wrong, which is what pins exist for.
-const max_accuracy_radius_km = 200;
+pub const max_accuracy_radius_km = 200;
 
-fn extractCoordinates(self: *GeoIP, ip: []const u8, result: c.MMDB_lookup_result_s) ?Location {
+/// A database entry, read but not yet judged.
+pub const DbHit = struct {
+    location: Location,
+    /// MaxMind's own confidence signal, in km. Null when the entry carries none.
+    accuracy_radius: ?u16,
+};
+
+/// Reads an entry into a `Location`, without applying the confidence gate.
+///
+/// The gate is the caller's business: `resolve` rejects a coarse entry so the
+/// online provider gets a chance, while `explain` reports the same decision
+/// rather than acting on it. Keeping the reading here means one place knows the
+/// database's field layout.
+///
+/// Must be called with the shared lock held: the returned strings are read
+/// through `result.entry`, which points into the mapped database.
+fn readEntry(self: *GeoIP, ip: []const u8, result: c.MMDB_lookup_result_s) ?DbHit {
     if (!result.found_entry) return null;
 
     var entry_copy = result.entry;
 
-    // Check accuracy_radius first -- reject low-confidence entries so we fall
-    // back to the configured online provider instead.
     // SAFETY: accuracy_data set by MMDB_get_value
     var accuracy_data: c.MMDB_entry_data_s = undefined;
     const acc_status = c.MMDB_get_value(&entry_copy, &accuracy_data, "location", "accuracy_radius", @as([*c]const u8, null));
-    if (acc_status == c.MMDB_SUCCESS and accuracy_data.has_data) {
-        const radius = accuracy_data.unnamed_0.uint16;
-        if (radius > max_accuracy_radius_km) {
-            log.info("GeoLite2 accuracy_radius for ip {s} is {d} km (>{d} km threshold), falling back to the online provider", .{ ip, radius, max_accuracy_radius_km });
-            return null;
-        }
-    }
+    const accuracy_radius: ?u16 = if (acc_status == c.MMDB_SUCCESS and accuracy_data.has_data)
+        accuracy_data.unnamed_0.uint16
+    else
+        null;
 
     entry_copy = result.entry;
 
@@ -341,15 +685,24 @@ fn extractCoordinates(self: *GeoIP, ip: []const u8, result: c.MMDB_lookup_result
     else
         "";
 
+    // The ISO code as well as the display name: unit selection needs the country
+    // itself, and deriving it by matching the name against "United States" would
+    // break on any of the other languages the database carries.
+    const iso_country = readIso(result);
+
     const final_name = Location.buildDisplayName(self.allocator, city, subdivision, country, ip);
 
     return .{
-        .allocator = self.allocator,
-        .name = final_name,
-        .coords = .{
-            .latitude = latitude,
-            .longitude = longitude,
+        .location = .{
+            .allocator = self.allocator,
+            .name = final_name,
+            .coords = .{
+                .latitude = latitude,
+                .longitude = longitude,
+            },
+            .iso_country = iso_country,
         },
+        .accuracy_radius = accuracy_radius,
     };
 }
 
@@ -511,6 +864,7 @@ test "a pin overrides a confident but wrong database answer" {
             try Pins.parseCidr("12.94.132.0/24"),
             "San Francisco, California, United States",
             .{ .latitude = 37.7749, .longitude = -122.4194 },
+            Location.isoFrom("US"),
         );
         try pins.save(io, pins_path);
     }
@@ -553,7 +907,7 @@ test "reloadPins picks up a pin written after startup" {
     {
         var pins: Pins = .init(allocator);
         defer pins.deinit();
-        try pins.put(try Pins.parseCidr("203.0.113.0/24"), "Somewhere", .{ .latitude = 1, .longitude = 2 });
+        try pins.put(try Pins.parseCidr("203.0.113.0/24"), "Somewhere", .{ .latitude = 1, .longitude = 2 }, null);
         try pins.save(io, pins_path);
     }
 
@@ -564,3 +918,483 @@ test "reloadPins picks up a pin written after startup" {
     defer hit.deinit();
     try std.testing.expectEqualStrings("Somewhere", hit.name);
 }
+
+/// Shared setup for the `lookupCountry` tests.
+///
+/// Both the pins file and the fallback cache are redirected into a temp
+/// directory. The cache in particular is permanent and append-only, so a test
+/// left pointing at the configured path would write entries into the developer's
+/// real cache that nothing ever removes.
+fn testConfigIn(allocator: std.mem.Allocator, io: std.Io, dir: []const u8) !Config {
+    var config = try Config.loadForTest(allocator);
+    errdefer config.deinit(allocator);
+
+    if (@import("build_options").download_geoip) {
+        const GeoLite2 = @import("GeoLite2.zig");
+        try GeoLite2.ensureDatabase(allocator, io, config.geolite_path);
+    }
+
+    allocator.free(config.pins_file);
+    config.pins_file = try std.fmt.allocPrint(allocator, "{s}/pins", .{dir});
+    allocator.free(config.ipwhois_cache_file);
+    config.ipwhois_cache_file = try std.fmt.allocPrint(allocator, "{s}/ipwhois", .{dir});
+
+    return config;
+}
+
+test "lookupCountry: a pin decides units instead of the database" {
+    const allocator = std.testing.allocator;
+    const io = std.testing.io;
+
+    var tmp = std.testing.tmpDir(.{});
+    defer tmp.cleanup();
+    var path_buf: [std.fs.max_path_bytes]u8 = undefined;
+    const dir_len = try tmp.dir.realPath(io, &path_buf);
+    const dir = path_buf[0..dir_len];
+
+    var config = try testConfigIn(allocator, io, dir);
+    defer config.deinit(allocator);
+
+    // GeoLite2 has this AT&T address in Texas, so the database says US. Unit
+    // selection used to read the database directly, which meant a client pinned
+    // to London was still rendered in Fahrenheit.
+    const ip = "12.94.132.170";
+
+    {
+        var geoip = GeoIP.init(allocator, io, config.geolite_path, config) catch
+            return error.SkipZigTest;
+        defer geoip.deinit();
+        const unpinned = geoip.lookupCountry(ip) orelse return error.SkipZigTest;
+        if (!std.mem.eql(u8, &unpinned, "US")) return error.SkipZigTest;
+    }
+
+    {
+        var pins: Pins = .init(allocator);
+        defer pins.deinit();
+        try pins.put(try Pins.parseCidr("12.94.132.0/24"), "London, United Kingdom", .{ .latitude = 51.5, .longitude = -0.1 }, Location.isoFrom("GB"));
+        try pins.save(io, config.pins_file);
+    }
+
+    var geoip = GeoIP.init(allocator, io, config.geolite_path, config) catch
+        return error.SkipZigTest;
+    defer geoip.deinit();
+
+    const iso = geoip.lookupCountry(ip) orelse return error.TestUnexpectedResult;
+    try std.testing.expectEqualStrings("GB", &iso);
+    try std.testing.expect(!geoip.isUSIp(ip));
+}
+
+test "lookupCountry: a pin applies where the database has no entry at all" {
+    const allocator = std.testing.allocator;
+    const io = std.testing.io;
+
+    var tmp = std.testing.tmpDir(.{});
+    defer tmp.cleanup();
+    var path_buf: [std.fs.max_path_bytes]u8 = undefined;
+    const dir_len = try tmp.dir.realPath(io, &path_buf);
+    const dir = path_buf[0..dir_len];
+
+    var config = try testConfigIn(allocator, io, dir);
+    defer config.deinit(allocator);
+
+    // TEST-NET-3, reserved for documentation and absent from GeoLite2. Standing
+    // in for any address the database does not cover.
+    const ip = "203.0.113.7";
+
+    {
+        var pins: Pins = .init(allocator);
+        defer pins.deinit();
+        try pins.put(try Pins.parseCidr("203.0.113.0/24"), "Seattle, Washington, United States", .{ .latitude = 47.6, .longitude = -122.3 }, Location.isoFrom("US"));
+        try pins.save(io, config.pins_file);
+    }
+
+    var geoip = GeoIP.init(allocator, io, config.geolite_path, config) catch
+        return error.SkipZigTest;
+    defer geoip.deinit();
+
+    const iso = geoip.lookupCountry(ip) orelse return error.TestUnexpectedResult;
+    try std.testing.expectEqualStrings("US", &iso);
+    try std.testing.expect(geoip.isUSIp(ip));
+}
+
+test "lookupCountry: a pin with no country falls through to the database" {
+    const allocator = std.testing.allocator;
+    const io = std.testing.io;
+
+    var tmp = std.testing.tmpDir(.{});
+    defer tmp.cleanup();
+    var path_buf: [std.fs.max_path_bytes]u8 = undefined;
+    const dir_len = try tmp.dir.realPath(io, &path_buf);
+    const dir = path_buf[0..dir_len];
+
+    var config = try testConfigIn(allocator, io, dir);
+    defer config.deinit(allocator);
+
+    const ip = "12.94.132.170";
+
+    // Pins written before the country was recorded exist on disk. They must keep
+    // behaving as they did rather than forcing every pinned client to metric.
+    {
+        var pins: Pins = .init(allocator);
+        defer pins.deinit();
+        try pins.put(try Pins.parseCidr("12.94.132.0/24"), "Somewhere", .{ .latitude = 1, .longitude = 2 }, null);
+        try pins.save(io, config.pins_file);
+    }
+
+    var geoip = GeoIP.init(allocator, io, config.geolite_path, config) catch
+        return error.SkipZigTest;
+    defer geoip.deinit();
+
+    // The pin still decides the location...
+    const pinned = geoip.lookup(ip) orelse return error.TestUnexpectedResult;
+    defer pinned.deinit();
+    try std.testing.expectEqualStrings("Somewhere", pinned.name);
+
+    // ...while the country comes from the database, as it did before pins existed.
+    const iso = geoip.lookupCountry(ip) orelse return error.SkipZigTest;
+    try std.testing.expectEqualStrings("US", &iso);
+}
+
+test "lookupCountry: a cached fallback answer decides units" {
+    const allocator = std.testing.allocator;
+    const io = std.testing.io;
+
+    var tmp = std.testing.tmpDir(.{});
+    defer tmp.cleanup();
+    var path_buf: [std.fs.max_path_bytes]u8 = undefined;
+    const dir_len = try tmp.dir.realPath(io, &path_buf);
+    const dir = path_buf[0..dir_len];
+
+    var config = try testConfigIn(allocator, io, dir);
+    defer config.deinit(allocator);
+
+    // Absent from GeoLite2, so the only source of a country is the cache. This is
+    // the case unit selection could not see before: the fallback runs only when
+    // the database declines, and units never consulted the fallback at all.
+    const ip = "198.51.100.7";
+    const key = Ip2location.packIp(ip).?;
+
+    {
+        var cache = try Ip2location.Cache.init(allocator, io, config.ipwhois_cache_file);
+        defer cache.deinit();
+        try cache.put(key.key, key.family, .{
+            .allocator = allocator,
+            .name = "Seattle, Washington, United States",
+            .coords = .{ .latitude = 47.6, .longitude = -122.3 },
+            .iso_country = Location.isoFrom("US"),
+        });
+    }
+
+    var geoip = GeoIP.init(allocator, io, config.geolite_path, config) catch
+        return error.SkipZigTest;
+    defer geoip.deinit();
+
+    const iso = geoip.lookupCountry(ip) orelse return error.TestUnexpectedResult;
+    try std.testing.expectEqualStrings("US", &iso);
+    try std.testing.expect(geoip.isUSIp(ip));
+}
+
+test "lookupCountry: null when no source knows the address" {
+    const allocator = std.testing.allocator;
+    const io = std.testing.io;
+
+    var tmp = std.testing.tmpDir(.{});
+    defer tmp.cleanup();
+    var path_buf: [std.fs.max_path_bytes]u8 = undefined;
+    const dir_len = try tmp.dir.realPath(io, &path_buf);
+    const dir = path_buf[0..dir_len];
+
+    var config = try testConfigIn(allocator, io, dir);
+    defer config.deinit(allocator);
+
+    var geoip = GeoIP.init(allocator, io, config.geolite_path, config) catch
+        return error.SkipZigTest;
+    defer geoip.deinit();
+
+    // No pin, nothing in the database, nothing cached. Must not resolve to a
+    // country, and must not attempt a network request to find one.
+    try std.testing.expect(geoip.lookupCountry("203.0.113.7") == null);
+    try std.testing.expect(!geoip.isUSIp("203.0.113.7"));
+
+    // A malformed address must be rejected rather than reaching any provider.
+    try std.testing.expect(geoip.lookupCountry("not-an-address") == null);
+    try std.testing.expect(!geoip.isUSIp("not-an-address"));
+}
+
+test "explain agrees with lookup for every source" {
+    const allocator = std.testing.allocator;
+    const io = std.testing.io;
+
+    var tmp = std.testing.tmpDir(.{});
+    defer tmp.cleanup();
+    var path_buf: [std.fs.max_path_bytes]u8 = undefined;
+    const dir_len = try tmp.dir.realPath(io, &path_buf);
+    const dir = path_buf[0..dir_len];
+
+    var config = try testConfigIn(allocator, io, dir);
+    defer config.deinit(allocator);
+
+    // `explain` walks the chain separately from `resolve` so it can report the
+    // stages `resolve` short-circuits past. That means the two can drift, and a
+    // diagnostic that disagrees with the code it describes is worse than none.
+    // Cover one address per source.
+    {
+        var pins: Pins = .init(allocator);
+        defer pins.deinit();
+        try pins.put(try Pins.parseCidr("203.0.113.0/24"), "Pinned Place", .{ .latitude = 1.5, .longitude = 2.5 }, Location.isoFrom("US"));
+        try pins.save(io, config.pins_file);
+    }
+
+    // Seeded so the fallback-cache source is exercised without a network call.
+    const cached_ip = "198.51.100.7";
+    {
+        var cache = try Ip2location.Cache.init(allocator, io, config.ipwhois_cache_file);
+        defer cache.deinit();
+        const key = Ip2location.packIp(cached_ip).?;
+        try cache.put(key.key, key.family, .{
+            .allocator = allocator,
+            .name = "Cached Place",
+            .coords = .{ .latitude = 3.5, .longitude = 4.5 },
+            .iso_country = Location.isoFrom("US"),
+        });
+    }
+
+    var geoip = GeoIP.init(allocator, io, config.geolite_path, config) catch
+        return error.SkipZigTest;
+    defer geoip.deinit();
+
+    const cases = [_][]const u8{
+        "203.0.113.7", // pin
+        "73.158.64.1", // database
+        cached_ip, // fallback cache
+        // Null agreement is covered by an address that parses as nothing, rather
+        // than by one that is merely absent: for an absent address `lookup` is
+        // supposed to reach the provider, which a unit test cannot let it do.
+        // The absent case is asserted against `explain` alone below.
+        "not-an-address",
+    };
+
+    for (cases) |ip| {
+        const explanation = try geoip.explain(ip, .{});
+        defer explanation.deinit();
+
+        const direct = geoip.lookup(ip);
+        defer if (direct) |d| d.deinit();
+
+        if (direct) |expected| {
+            const got = explanation.result orelse {
+                std.debug.print("explain found nothing for {s}, lookup returned {s}\n", .{ ip, expected.name });
+                return error.TestUnexpectedResult;
+            };
+            try std.testing.expectEqualStrings(expected.name, got.location.name);
+            try std.testing.expectEqual(expected.coords.latitude, got.location.coords.latitude);
+            try std.testing.expectEqual(expected.coords.longitude, got.location.coords.longitude);
+            try std.testing.expectEqual(expected.iso_country, got.location.iso_country);
+        } else {
+            if (explanation.result) |got| {
+                std.debug.print("explain found {s} for {s}, lookup returned nothing\n", .{ got.location.name, ip });
+                return error.TestUnexpectedResult;
+            }
+        }
+
+        // And the units it reports have to be the ones the handler would apply.
+        try std.testing.expectEqual(geoip.lookupCountry(ip), explanation.country);
+    }
+}
+
+test "explain: reports the source each address resolved through" {
+    const allocator = std.testing.allocator;
+    const io = std.testing.io;
+
+    var tmp = std.testing.tmpDir(.{});
+    defer tmp.cleanup();
+    var path_buf: [std.fs.max_path_bytes]u8 = undefined;
+    const dir_len = try tmp.dir.realPath(io, &path_buf);
+    const dir = path_buf[0..dir_len];
+
+    var config = try testConfigIn(allocator, io, dir);
+    defer config.deinit(allocator);
+
+    {
+        var pins: Pins = .init(allocator);
+        defer pins.deinit();
+        try pins.put(try Pins.parseCidr("203.0.113.0/24"), "Pinned Place", .{ .latitude = 1.5, .longitude = 2.5 }, Location.isoFrom("US"));
+        try pins.save(io, config.pins_file);
+    }
+
+    var geoip = GeoIP.init(allocator, io, config.geolite_path, config) catch
+        return error.SkipZigTest;
+    defer geoip.deinit();
+
+    {
+        const e = try geoip.explain("203.0.113.7", .{});
+        defer e.deinit();
+        try std.testing.expectEqual(Source.pin, e.result.?.source);
+        try std.testing.expectEqualStrings("203.0.113.0/24", e.pin.?.cidr);
+        try std.testing.expectEqual(@as(usize, 1), e.pins_loaded);
+        // A pin does not stop the database from being reported: an operator
+        // checking a pin needs to see what it overrides. Here there is nothing.
+        try std.testing.expectEqual(Explanation.Database.absent, std.meta.activeTag(e.database));
+    }
+
+    {
+        const e = try geoip.explain("73.158.64.1", .{});
+        defer e.deinit();
+        try std.testing.expectEqual(Source.database, e.result.?.source);
+        try std.testing.expect(e.pin == null);
+        // The radius is the whole point of the report, so it must be populated.
+        const hit = e.database.hit() orelse return error.TestUnexpectedResult;
+        try std.testing.expect(hit.accuracy_radius != null);
+        // The provider is still reported for comparison, but must not be the
+        // answer, and must not have been fetched without --online.
+        try std.testing.expect(!e.fallbackWasUsed());
+        try std.testing.expect(std.meta.activeTag(e.fallback) != .fetched);
+    }
+
+    {
+        // Absent everywhere. Must report the miss rather than reaching the network.
+        const e = try geoip.explain("192.0.2.99", .{});
+        defer e.deinit();
+        try std.testing.expect(e.result == null);
+        try std.testing.expectEqual(Explanation.Database.absent, std.meta.activeTag(e.database));
+        try std.testing.expectEqual(Explanation.Fallback.not_cached, std.meta.activeTag(e.fallback));
+    }
+
+    {
+        // Not an address: no stage may run, because the argument would otherwise
+        // be interpolated into a provider URL.
+        const e = try geoip.explain("not-an-address", .{});
+        defer e.deinit();
+        try std.testing.expect(!e.parsed);
+        try std.testing.expect(e.result == null);
+        try std.testing.expect(e.pin == null);
+        // Not even the cache is consulted: nothing that is not an address should
+        // reach a provider or its cache key.
+        try std.testing.expectEqual(Explanation.Fallback.not_cached, std.meta.activeTag(e.fallback));
+    }
+}
+
+test "explain: a pin wins over an accepted database entry" {
+    const allocator = std.testing.allocator;
+    const io = std.testing.io;
+
+    var tmp = std.testing.tmpDir(.{});
+    defer tmp.cleanup();
+    var path_buf: [std.fs.max_path_bytes]u8 = undefined;
+    const dir_len = try tmp.dir.realPath(io, &path_buf);
+    const dir = path_buf[0..dir_len];
+
+    var config = try testConfigIn(allocator, io, dir);
+    defer config.deinit(allocator);
+
+    // The original case: GeoLite2 answers this confidently, so the report has to
+    // show both the pin that won and the entry it displaced.
+    {
+        var pins: Pins = .init(allocator);
+        defer pins.deinit();
+        try pins.put(try Pins.parseCidr("12.94.132.0/24"), "Seattle, Washington, United States", .{ .latitude = 47.6, .longitude = -122.3 }, Location.isoFrom("US"));
+        try pins.save(io, config.pins_file);
+    }
+
+    var geoip = GeoIP.init(allocator, io, config.geolite_path, config) catch
+        return error.SkipZigTest;
+    defer geoip.deinit();
+
+    const e = try geoip.explain("12.94.132.170", .{});
+    defer e.deinit();
+
+    try std.testing.expectEqual(Source.pin, e.result.?.source);
+    try std.testing.expectEqualStrings("Seattle, Washington, United States", e.result.?.location.name);
+
+    const hit = e.database.hit() orelse return error.SkipZigTest;
+    // Different answer, still reported, and reported as one the database was
+    // confident about. That combination is what tells an operator the pin is
+    // load-bearing rather than redundant.
+    try std.testing.expect(!std.mem.eql(u8, hit.location.name, "Seattle, Washington, United States"));
+    try std.testing.expectEqual(Explanation.Database.accepted, std.meta.activeTag(e.database));
+}
+
+test "explain: reports the provider's answer alongside the database that displaced it" {
+    const allocator = std.testing.allocator;
+    const io = std.testing.io;
+
+    var tmp = std.testing.tmpDir(.{});
+    defer tmp.cleanup();
+    var path_buf: [std.fs.max_path_bytes]u8 = undefined;
+    const dir_len = try tmp.dir.realPath(io, &path_buf);
+    const dir = path_buf[0..dir_len];
+
+    var config = try testConfigIn(allocator, io, dir);
+    defer config.deinit(allocator);
+
+    // An address the database answers, with a *different* provider answer seeded
+    // in the cache. `resolve` never reaches the provider for this address, but the
+    // report has to show what it would have said: when the database is confidently
+    // wrong, a side-by-side is what makes that visible.
+    const ip = "73.158.64.1";
+    {
+        var cache = try Ip2location.Cache.init(allocator, io, config.ipwhois_cache_file);
+        defer cache.deinit();
+        const key = Ip2location.packIp(ip).?;
+        try cache.put(key.key, key.family, .{
+            .allocator = allocator,
+            .name = "Provider Answer, Elsewhere",
+            .coords = .{ .latitude = 11.25, .longitude = 22.5 },
+            .iso_country = Location.isoFrom("GB"),
+        });
+    }
+
+    var geoip = GeoIP.init(allocator, io, config.geolite_path, config) catch
+        return error.SkipZigTest;
+    defer geoip.deinit();
+
+    const e = try geoip.explain(ip, .{});
+    defer e.deinit();
+
+    // The database still wins, and is still what the server would serve.
+    const db = e.database.hit() orelse return error.SkipZigTest;
+    try std.testing.expectEqual(Source.database, e.result.?.source);
+    try std.testing.expectEqualStrings(db.location.name, e.result.?.location.name);
+
+    // And the provider's answer is reported, flagged as not the one in force.
+    try std.testing.expect(!e.fallbackWasUsed());
+    const provider = e.fallback.location() orelse return error.TestUnexpectedResult;
+    try std.testing.expectEqualStrings("Provider Answer, Elsewhere", provider.name);
+    try std.testing.expectEqual(@as(f64, 11.25), provider.coords.latitude);
+    try std.testing.expectEqual(Explanation.Fallback.cached, std.meta.activeTag(e.fallback));
+
+    // Reporting it must not have changed which units the handler applies: the
+    // provider cache is only consulted for units when nothing earlier answered.
+    const iso = e.country orelse return error.SkipZigTest;
+    try std.testing.expectEqualStrings("US", &iso);
+}
+
+test "explain: does not reach the network for an unneeded provider answer" {
+    const allocator = std.testing.allocator;
+    const io = std.testing.io;
+
+    var tmp = std.testing.tmpDir(.{});
+    defer tmp.cleanup();
+    var path_buf: [std.fs.max_path_bytes]u8 = undefined;
+    const dir_len = try tmp.dir.realPath(io, &path_buf);
+    const dir = path_buf[0..dir_len];
+
+    var config = try testConfigIn(allocator, io, dir);
+    defer config.deinit(allocator);
+
+    var geoip = GeoIP.init(allocator, io, config.geolite_path, config) catch
+        return error.SkipZigTest;
+    defer geoip.deinit();
+
+    // Now that the provider is reported even when unused, the default must still
+    // never fetch. Otherwise every run of this command would spend quota and
+    // append to a cache that is never evicted.
+    const e = try geoip.explain("73.158.64.1", .{});
+    defer e.deinit();
+
+    try std.testing.expectEqual(Explanation.Fallback.not_cached, std.meta.activeTag(e.fallback));
+    // A fetch would also have been recorded in the cache file.
+    var cache = try Ip2location.Cache.init(allocator, io, config.ipwhois_cache_file);
+    defer cache.deinit();
+    try std.testing.expectEqual(@as(usize, 0), cache.entries.count());
+}

src/location/Ip2location.zig

@@ -165,6 +165,7 @@ fn fetch(self: *Self, ip_str: []const u8) !Location {
             .latitude = @floatCast(lat.float),
             .longitude = @floatCast(lon.float),
         },
+        .iso_country = Location.isoFrom(getString(obj, "country_code")),
     };
 }
 
@@ -195,11 +196,16 @@ pub const Cache = struct {
     ///
     /// The address is stored as text rather than the `u128` used for the in-memory
     /// key, so the file stays legible to whoever has to look at it.
+    ///
+    /// `iso` defaults to null so cache files written before it existed still
+    /// load: SRF fills a missing field from the Zig default. Entries cached
+    /// without a country resolve their units from the database instead.
     const Record = struct {
         ip: []const u8,
         lat: f64,
         lon: f64,
         name: []const u8,
+        iso: ?[]const u8 = null,
     };
 
     pub fn init(allocator: Allocator, io: std.Io, path: []const u8) !Cache {
@@ -292,6 +298,7 @@ pub const Cache = struct {
                 .allocator = self.allocator,
                 .name = name_copy,
                 .coords = .{ .latitude = record.lat, .longitude = record.lon },
+                .iso_country = if (record.iso) |iso| Location.isoFrom(iso) else null,
             });
             if (existing) |old| self.allocator.free(old.value.name);
         }
@@ -303,9 +310,19 @@ pub const Cache = struct {
             .allocator = self.allocator,
             .name = self.allocator.dupe(u8, entry.name) catch return null,
             .coords = entry.coords,
+            .iso_country = entry.iso_country,
         };
     }
 
+    /// Country code for a cached address, without allocating or fetching.
+    ///
+    /// Unit selection only needs the country, and doing it this way keeps that
+    /// path free of both an allocation and any chance of a network request.
+    pub fn getCountry(self: *Cache, ip: u128) ?[2]u8 {
+        const entry = self.entries.getPtr(ip) orelse return null;
+        return entry.iso_country;
+    }
+
     pub fn put(self: *Cache, ip: u128, family: u8, loc: Location) !void {
         const name_copy = try self.allocator.dupe(u8, loc.name);
         errdefer self.allocator.free(name_copy);
@@ -314,6 +331,7 @@ pub const Cache = struct {
             .allocator = self.allocator,
             .name = name_copy,
             .coords = loc.coords,
+            .iso_country = loc.iso_country,
         });
         if (existing) |old| self.allocator.free(old.value.name);
 
@@ -330,6 +348,7 @@ pub const Cache = struct {
             .lat = loc.coords.latitude,
             .lon = loc.coords.longitude,
             .name = loc.name,
+            .iso = if (loc.iso_country) |*iso| iso[0..] else null,
         }}, .{ .emit_directives = false })});
 
         // 0.16 has no seek+write; append at the current end offset.
@@ -446,3 +465,115 @@ test "Cache: formatKey renders both families without a port" {
     const v6 = packIp("2001:db8::1").?;
     try std.testing.expectEqualStrings("2001:db8::1", try Cache.formatKey(v6.key, v6.family, &buf));
 }
+
+test "Cache: round-trips the country through the file" {
+    const allocator = std.testing.allocator;
+    const io = std.testing.io;
+
+    var tmp = std.testing.tmpDir(.{});
+    defer tmp.cleanup();
+    var path_buf: [std.fs.max_path_bytes]u8 = undefined;
+    const dir_len = try tmp.dir.realPath(io, &path_buf);
+    const path = try std.fmt.allocPrint(allocator, "{s}/ip.cache", .{path_buf[0..dir_len]});
+    defer allocator.free(path);
+
+    const us = packIp("198.51.100.1").?;
+    const unknown = packIp("198.51.100.2").?;
+
+    {
+        var cache = try Cache.init(allocator, io, path);
+        defer cache.deinit();
+
+        try cache.put(us.key, us.family, .{
+            .allocator = allocator,
+            .name = "Seattle, Washington, United States",
+            .coords = .{ .latitude = 47.6, .longitude = -122.3 },
+            .iso_country = Location.isoFrom("US"),
+        });
+        // An entry cached with no country has to stay that way rather than
+        // acquiring one, so units fall back to the database for it.
+        try cache.put(unknown.key, unknown.family, .{
+            .allocator = allocator,
+            .name = "Nowhere",
+            .coords = .{ .latitude = 1, .longitude = 2 },
+        });
+    }
+
+    var reopened = try Cache.init(allocator, io, path);
+    defer reopened.deinit();
+
+    const got = reopened.get(us.key) orelse return error.TestUnexpectedResult;
+    defer got.deinit();
+    try std.testing.expectEqualStrings("US", &(got.iso_country.?));
+
+    const iso = reopened.getCountry(us.key) orelse return error.TestUnexpectedResult;
+    try std.testing.expectEqualStrings("US", &iso);
+
+    try std.testing.expect(reopened.getCountry(unknown.key) == null);
+}
+
+test "Cache: getCountry is null for an address that was never cached" {
+    const allocator = std.testing.allocator;
+    const io = std.testing.io;
+
+    var tmp = std.testing.tmpDir(.{});
+    defer tmp.cleanup();
+    var path_buf: [std.fs.max_path_bytes]u8 = undefined;
+    const dir_len = try tmp.dir.realPath(io, &path_buf);
+    const path = try std.fmt.allocPrint(allocator, "{s}/ip.cache", .{path_buf[0..dir_len]});
+    defer allocator.free(path);
+
+    var cache = try Cache.init(allocator, io, path);
+    defer cache.deinit();
+
+    // Must not be mistaken for "cached with no country": both read as null here,
+    // and in both cases the caller falls through, so this pins the shape rather
+    // than a distinction the caller makes.
+    try std.testing.expect(cache.getCountry(packIp("203.0.113.9").?.key) == null);
+}
+
+test "Cache: a file written before the country field still loads" {
+    const allocator = std.testing.allocator;
+    const io = std.testing.io;
+
+    var tmp = std.testing.tmpDir(.{});
+    defer tmp.cleanup();
+    var path_buf: [std.fs.max_path_bytes]u8 = undefined;
+    const dir_len = try tmp.dir.realPath(io, &path_buf);
+    const path = try std.fmt.allocPrint(allocator, "{s}/ip.cache", .{path_buf[0..dir_len]});
+    defer allocator.free(path);
+
+    // Produced by writing the four-field record, then reopened against the
+    // five-field one. This cache is permanent and never evicted, so dropping it
+    // on a schema change would mean re-fetching every address ever resolved.
+    {
+        var cache = try Cache.init(allocator, io, path);
+        defer cache.deinit();
+        const OldRecord = struct {
+            ip: []const u8,
+            lat: f64,
+            lon: f64,
+            name: []const u8,
+        };
+        var line_buf: [512]u8 = undefined;
+        const line = try std.fmt.bufPrint(&line_buf, "{f}", .{srf.fmt(OldRecord, &.{.{
+            .ip = "198.51.100.7",
+            .lat = 47.6,
+            .lon = -122.3,
+            .name = "Seattle, Washington, United States",
+        }}, .{ .emit_directives = false })});
+        const file = cache.file.?;
+        const end = try file.length(io);
+        try file.writePositionalAll(io, line, end);
+    }
+
+    var reopened = try Cache.init(allocator, io, path);
+    defer reopened.deinit();
+
+    const key = packIp("198.51.100.7").?.key;
+    const got = reopened.get(key) orelse return error.TestUnexpectedResult;
+    defer got.deinit();
+    try std.testing.expectEqualStrings("Seattle, Washington, United States", got.name);
+    try std.testing.expectEqual(@as(f64, 47.6), got.coords.latitude);
+    try std.testing.expect(got.iso_country == null);
+}

src/location/IpWhoIs.zig

@@ -65,8 +65,9 @@ fn fetch(self: *Self, ip_str: []const u8) !Location {
     var w = std.Io.Writer.fixed(&buf);
     try w.writeAll("https://ipwho.is/");
     try w.writeAll(ip_str);
-    // Request only the fields we need
-    try w.writeAll("?fields=city,region,country,latitude,longitude&output=json");
+    // Request only the fields we need. `country_code` drives unit selection,
+    // which cannot be derived from `country` without matching display text.
+    try w.writeAll("?fields=city,region,country,country_code,latitude,longitude&output=json");
 
     var response_buf: [4096]u8 = undefined;
     var writer = std.Io.Writer.fixed(&response_buf);
@@ -139,6 +140,7 @@ fn fetch(self: *Self, ip_str: []const u8) !Location {
             .latitude = lat_val,
             .longitude = lon_val,
         },
+        .iso_country = Location.isoFrom(getString(obj, "country_code")),
     };
 }
 

src/location/Pins.zig

@@ -30,6 +30,11 @@ pub const Entry = struct {
     prefix_len: u8,
     name: []const u8,
     coords: Coordinates,
+    /// ISO 3166-1 alpha-2 country code, upper case, when it was known at the
+    /// time the pin was written. Null for pins created before this was recorded,
+    /// which `GeoIp.lookupCountry` treats as "no opinion" and resolves from the
+    /// database instead of forcing a wrong answer.
+    iso_country: ?[2]u8 = null,
 };
 
 pub fn init(allocator: std.mem.Allocator) Pins {
@@ -120,7 +125,7 @@ pub fn formatCidr(cidr: Cidr, buf: []u8) ![]const u8 {
 ///
 /// Longest-prefix ordering means an exact host pin naturally wins over a range
 /// covering it, without needing a separate precedence rule.
-pub fn lookup(self: *const Pins, allocator: std.mem.Allocator, ip_str: []const u8) ?Location {
+fn bestMatch(self: *const Pins, ip_str: []const u8) ?*const Entry {
     const packed_ip = packIp(ip_str) orelse return null;
 
     var best: ?*const Entry = null;
@@ -129,17 +134,41 @@ pub fn lookup(self: *const Pins, allocator: std.mem.Allocator, ip_str: []const u
         if (maskToPrefix(packed_ip.key, e.family, e.prefix_len) != e.network) continue;
         if (best == null or e.prefix_len > best.?.prefix_len) best = e;
     }
+    return best;
+}
 
-    const entry = best orelse return null;
+/// The matching entry itself, for callers that need more than a `Location`.
+///
+/// `explain` reports the range that matched, which the `Location` does not carry.
+/// Borrowed rather than copied, so the caller must hold whatever lock guards the
+/// pin set for as long as it uses the result.
+pub fn matchFor(self: *const Pins, ip_str: []const u8) ?*const Entry {
+    return self.bestMatch(ip_str);
+}
+
+/// Longest-prefix match for `ip_str`, or null when no pin applies.
+pub fn lookup(self: *const Pins, allocator: std.mem.Allocator, ip_str: []const u8) ?Location {
+    const entry = self.bestMatch(ip_str) orelse return null;
     return .{
         .allocator = allocator,
         .name = allocator.dupe(u8, entry.name) catch return null,
         .coords = entry.coords,
+        .iso_country = entry.iso_country,
     };
 }
 
+/// Country code of the matching pin, without allocating.
+///
+/// Null both when no pin matches and when the matching pin has no recorded
+/// country, because the caller treats those the same way: fall through to the
+/// database rather than claim the country is unknown.
+pub fn lookupCountry(self: *const Pins, ip_str: []const u8) ?[2]u8 {
+    const entry = self.bestMatch(ip_str) orelse return null;
+    return entry.iso_country;
+}
+
 /// Inserts a pin, replacing any existing pin for the same network.
-pub fn put(self: *Pins, cidr: Cidr, name: []const u8, coords: Coordinates) !void {
+pub fn put(self: *Pins, cidr: Cidr, name: []const u8, coords: Coordinates, iso_country: ?[2]u8) !void {
     const name_copy = try self.allocator.dupe(u8, name);
     errdefer self.allocator.free(name_copy);
 
@@ -148,6 +177,7 @@ pub fn put(self: *Pins, cidr: Cidr, name: []const u8, coords: Coordinates) !void
             self.allocator.free(e.name);
             e.name = name_copy;
             e.coords = coords;
+            e.iso_country = iso_country;
             return;
         }
     }
@@ -158,6 +188,7 @@ pub fn put(self: *Pins, cidr: Cidr, name: []const u8, coords: Coordinates) !void
         .prefix_len = cidr.prefix_len,
         .name = name_copy,
         .coords = coords,
+        .iso_country = iso_country,
     });
 }
 
@@ -181,11 +212,15 @@ pub fn remove(self: *Pins, cidr: Cidr) bool {
 /// part for free: place names routinely contain commas ("San Francisco,
 /// California, United States"), which a comma-delimited format has to special
 /// case.
+///
+/// `iso` defaults to null so pins written before it existed still load: SRF
+/// fills a missing field from the Zig default rather than rejecting the record.
 const Record = struct {
     cidr: []const u8,
     lat: f64,
     lon: f64,
     name: []const u8,
+    iso: ?[]const u8 = null,
 };
 
 /// Loads pins from `path`. A missing file yields an empty set: having no
@@ -228,7 +263,12 @@ pub fn load(allocator: std.mem.Allocator, io: std.Io, path: []const u8) !Pins {
 
         // `put` copies the name, so it does not matter that the parsed strings
         // belong to the iterator's arena.
-        try pins.put(cidr, record.name, .{ .latitude = record.lat, .longitude = record.lon });
+        try pins.put(
+            cidr,
+            record.name,
+            .{ .latitude = record.lat, .longitude = record.lon },
+            if (record.iso) |iso| Location.isoFrom(iso) else null,
+        );
     }
 
     return pins;
@@ -255,7 +295,7 @@ pub fn save(self: *const Pins, io: std.Io, path: []const u8) !void {
         self.allocator.free(cidr_texts);
     }
 
-    for (self.entries.items, 0..) |e, i| {
+    for (self.entries.items, 0..) |*e, i| {
         var buf: [64]u8 = undefined;
         const text = try formatCidr(
             .{ .family = e.family, .network = e.network, .prefix_len = e.prefix_len },
@@ -268,6 +308,10 @@ pub fn save(self: *const Pins, io: std.Io, path: []const u8) !void {
             .lat = e.coords.latitude,
             .lon = e.coords.longitude,
             .name = e.name,
+            // Borrowed from the entry rather than copied: the list is not
+            // mutated before the `print` below, and a by-value capture would
+            // leave this pointing at a dead loop temporary.
+            .iso = if (e.iso_country) |*iso| iso[0..] else null,
         };
     }
 
@@ -337,7 +381,7 @@ test "lookup: matches an address inside the range" {
     var pins: Pins = .init(allocator);
     defer pins.deinit();
 
-    try pins.put(try parseCidr("12.94.132.0/24"), "San Francisco, California", .{ .latitude = 37.7749, .longitude = -122.4194 });
+    try pins.put(try parseCidr("12.94.132.0/24"), "San Francisco, California", .{ .latitude = 37.7749, .longitude = -122.4194 }, null);
 
     const hit = pins.lookup(allocator, "12.94.132.170") orelse return error.TestUnexpectedResult;
     defer hit.deinit();
@@ -350,7 +394,7 @@ test "lookup: ignores an address outside the range" {
     var pins: Pins = .init(allocator);
     defer pins.deinit();
 
-    try pins.put(try parseCidr("12.94.132.0/24"), "San Francisco", .{ .latitude = 37.7749, .longitude = -122.4194 });
+    try pins.put(try parseCidr("12.94.132.0/24"), "San Francisco", .{ .latitude = 37.7749, .longitude = -122.4194 }, null);
 
     try std.testing.expect(pins.lookup(allocator, "12.94.133.1") == null);
 }
@@ -361,8 +405,8 @@ test "lookup: longest prefix wins over a broader range" {
     defer pins.deinit();
 
     // Deliberately inserted broad-first so the result cannot come from ordering.
-    try pins.put(try parseCidr("12.0.0.0/8"), "Broad", .{ .latitude = 1, .longitude = 1 });
-    try pins.put(try parseCidr("12.94.132.0/24"), "Specific", .{ .latitude = 2, .longitude = 2 });
+    try pins.put(try parseCidr("12.0.0.0/8"), "Broad", .{ .latitude = 1, .longitude = 1 }, null);
+    try pins.put(try parseCidr("12.94.132.0/24"), "Specific", .{ .latitude = 2, .longitude = 2 }, null);
 
     const hit = pins.lookup(allocator, "12.94.132.170") orelse return error.TestUnexpectedResult;
     defer hit.deinit();
@@ -374,8 +418,8 @@ test "lookup: an exact host pin beats a range containing it" {
     var pins: Pins = .init(allocator);
     defer pins.deinit();
 
-    try pins.put(try parseCidr("12.94.132.170"), "Host", .{ .latitude = 2, .longitude = 2 });
-    try pins.put(try parseCidr("12.94.132.0/24"), "Range", .{ .latitude = 1, .longitude = 1 });
+    try pins.put(try parseCidr("12.94.132.170"), "Host", .{ .latitude = 2, .longitude = 2 }, null);
+    try pins.put(try parseCidr("12.94.132.0/24"), "Range", .{ .latitude = 1, .longitude = 1 }, null);
 
     const hit = pins.lookup(allocator, "12.94.132.170") orelse return error.TestUnexpectedResult;
     defer hit.deinit();
@@ -387,7 +431,7 @@ test "lookup: families do not cross-match" {
     var pins: Pins = .init(allocator);
     defer pins.deinit();
 
-    try pins.put(try parseCidr("0.0.0.0/0"), "All IPv4", .{ .latitude = 1, .longitude = 1 });
+    try pins.put(try parseCidr("0.0.0.0/0"), "All IPv4", .{ .latitude = 1, .longitude = 1 }, null);
 
     // An IPv4 /0 must not swallow IPv6 clients.
     try std.testing.expect(pins.lookup(allocator, "2001:db8::1") == null);
@@ -401,8 +445,8 @@ test "put: replaces an existing pin for the same network" {
     defer pins.deinit();
 
     const cidr = try parseCidr("12.94.132.0/24");
-    try pins.put(cidr, "Old", .{ .latitude = 1, .longitude = 1 });
-    try pins.put(cidr, "New", .{ .latitude = 2, .longitude = 2 });
+    try pins.put(cidr, "Old", .{ .latitude = 1, .longitude = 1 }, null);
+    try pins.put(cidr, "New", .{ .latitude = 2, .longitude = 2 }, null);
 
     try std.testing.expectEqual(@as(usize, 1), pins.entries.items.len);
     const hit = pins.lookup(allocator, "12.94.132.5") orelse return error.TestUnexpectedResult;
@@ -416,7 +460,7 @@ test "remove: reports whether a pin was present" {
     defer pins.deinit();
 
     const cidr = try parseCidr("12.94.132.0/24");
-    try pins.put(cidr, "SF", .{ .latitude = 1, .longitude = 1 });
+    try pins.put(cidr, "SF", .{ .latitude = 1, .longitude = 1 }, null);
 
     try std.testing.expect(pins.remove(cidr));
     try std.testing.expect(!pins.remove(cidr));
@@ -449,8 +493,8 @@ test "save then load round-trips, including commas in the name" {
     {
         var pins: Pins = .init(allocator);
         defer pins.deinit();
-        try pins.put(try parseCidr("12.94.132.0/24"), "San Francisco, California, United States", .{ .latitude = 37.7749, .longitude = -122.4194 });
-        try pins.put(try parseCidr("2001:db8::/32"), "Test, Place", .{ .latitude = -1.5, .longitude = 2.25 });
+        try pins.put(try parseCidr("12.94.132.0/24"), "San Francisco, California, United States", .{ .latitude = 37.7749, .longitude = -122.4194 }, null);
+        try pins.put(try parseCidr("2001:db8::/32"), "Test, Place", .{ .latitude = -1.5, .longitude = 2.25 }, null);
         try pins.save(io, path);
     }
 
@@ -549,7 +593,7 @@ test "save writes SRF that an operator can read" {
 
     var pins: Pins = .init(allocator);
     defer pins.deinit();
-    try pins.put(try parseCidr("12.94.132.0/24"), "San Francisco, California", .{ .latitude = 37.7749, .longitude = -122.4194 });
+    try pins.put(try parseCidr("12.94.132.0/24"), "San Francisco, California", .{ .latitude = 37.7749, .longitude = -122.4194 }, null);
     try pins.save(io, path);
 
     const content = try std.Io.Dir.cwd().readFileAlloc(io, path, allocator, .limited(64 * 1024));
@@ -563,3 +607,195 @@ test "save writes SRF that an operator can read" {
     try std.testing.expect(std.mem.indexOf(u8, content, "cidr::12.94.132.0/24") != null);
     try std.testing.expect(std.mem.indexOf(u8, content, "San Francisco, California") != null);
 }
+
+test "lookupCountry: returns the country of the matching pin" {
+    const allocator = std.testing.allocator;
+    var pins: Pins = .init(allocator);
+    defer pins.deinit();
+
+    try pins.put(
+        try parseCidr("12.94.132.0/24"),
+        "San Francisco, California, United States",
+        .{ .latitude = 37.7749, .longitude = -122.4194 },
+        Location.isoFrom("US"),
+    );
+
+    const iso = pins.lookupCountry("12.94.132.170") orelse return error.TestUnexpectedResult;
+    try std.testing.expectEqualStrings("US", &iso);
+}
+
+test "lookupCountry: null for a pin with no recorded country" {
+    const allocator = std.testing.allocator;
+    var pins: Pins = .init(allocator);
+    defer pins.deinit();
+
+    // Pins written before the country was stored must read as "no opinion" so
+    // the caller can fall through to the database rather than assume a country.
+    try pins.put(try parseCidr("12.94.132.0/24"), "Somewhere", .{ .latitude = 1, .longitude = 2 }, null);
+
+    try std.testing.expect(pins.lookupCountry("12.94.132.170") == null);
+}
+
+test "lookupCountry: null when no pin matches" {
+    const allocator = std.testing.allocator;
+    var pins: Pins = .init(allocator);
+    defer pins.deinit();
+
+    try pins.put(try parseCidr("12.94.132.0/24"), "SF", .{ .latitude = 1, .longitude = 2 }, Location.isoFrom("US"));
+
+    try std.testing.expect(pins.lookupCountry("8.8.8.8") == null);
+}
+
+test "lookupCountry: longest prefix decides the country too" {
+    const allocator = std.testing.allocator;
+    var pins: Pins = .init(allocator);
+    defer pins.deinit();
+
+    try pins.put(try parseCidr("12.0.0.0/8"), "Broad", .{ .latitude = 1, .longitude = 1 }, Location.isoFrom("GB"));
+    try pins.put(try parseCidr("12.94.132.0/24"), "Specific", .{ .latitude = 2, .longitude = 2 }, Location.isoFrom("US"));
+
+    const iso = pins.lookupCountry("12.94.132.170") orelse return error.TestUnexpectedResult;
+    try std.testing.expectEqualStrings("US", &iso);
+}
+
+test "put: replacing a pin also replaces its country" {
+    const allocator = std.testing.allocator;
+    var pins: Pins = .init(allocator);
+    defer pins.deinit();
+
+    const cidr = try parseCidr("12.94.132.0/24");
+    try pins.put(cidr, "London", .{ .latitude = 1, .longitude = 1 }, Location.isoFrom("GB"));
+    try pins.put(cidr, "Seattle", .{ .latitude = 2, .longitude = 2 }, Location.isoFrom("US"));
+
+    try std.testing.expectEqual(@as(usize, 1), pins.entries.items.len);
+    const iso = pins.lookupCountry("12.94.132.5") orelse return error.TestUnexpectedResult;
+    try std.testing.expectEqualStrings("US", &iso);
+}
+
+test "save then load round-trips the country" {
+    const allocator = std.testing.allocator;
+    const io = std.testing.io;
+
+    var tmp = std.testing.tmpDir(.{});
+    defer tmp.cleanup();
+    var path_buf: [std.fs.max_path_bytes]u8 = undefined;
+    const dir_len = try tmp.dir.realPath(io, &path_buf);
+    const path = try std.fmt.allocPrint(allocator, "{s}/pins", .{path_buf[0..dir_len]});
+    defer allocator.free(path);
+
+    {
+        var pins: Pins = .init(allocator);
+        defer pins.deinit();
+        try pins.put(try parseCidr("12.94.132.0/24"), "Seattle", .{ .latitude = 47.6, .longitude = -122.3 }, Location.isoFrom("US"));
+        // A pin with no country has to survive the round trip as "no country"
+        // rather than becoming a bogus one.
+        try pins.put(try parseCidr("10.0.0.0/8"), "Unknown", .{ .latitude = 1, .longitude = 2 }, null);
+        try pins.save(io, path);
+    }
+
+    var loaded = try load(allocator, io, path);
+    defer loaded.deinit();
+
+    const us = loaded.lookupCountry("12.94.132.1") orelse return error.TestUnexpectedResult;
+    try std.testing.expectEqualStrings("US", &us);
+    try std.testing.expect(loaded.lookupCountry("10.1.2.3") == null);
+}
+
+test "save omits the country field entirely when it is unknown" {
+    const allocator = std.testing.allocator;
+    const io = std.testing.io;
+
+    var tmp = std.testing.tmpDir(.{});
+    defer tmp.cleanup();
+    var path_buf: [std.fs.max_path_bytes]u8 = undefined;
+    const dir_len = try tmp.dir.realPath(io, &path_buf);
+    const path = try std.fmt.allocPrint(allocator, "{s}/pins", .{path_buf[0..dir_len]});
+    defer allocator.free(path);
+
+    var pins: Pins = .init(allocator);
+    defer pins.deinit();
+    try pins.put(try parseCidr("10.0.0.0/8"), "Unknown", .{ .latitude = 1, .longitude = 2 }, null);
+    try pins.save(io, path);
+
+    const content = try std.Io.Dir.cwd().readFileAlloc(io, path, allocator, .limited(64 * 1024));
+    defer allocator.free(content);
+
+    // An unknown country should add nothing to a file an operator reads, rather
+    // than an empty or "null" field they have to interpret.
+    try std.testing.expect(std.mem.indexOf(u8, content, "iso") == null);
+}
+
+test "load: a pins file written before the country field still loads" {
+    const allocator = std.testing.allocator;
+    const io = std.testing.io;
+
+    var tmp = std.testing.tmpDir(.{});
+    defer tmp.cleanup();
+    var path_buf: [std.fs.max_path_bytes]u8 = undefined;
+    const dir_len = try tmp.dir.realPath(io, &path_buf);
+    const path = try std.fmt.allocPrint(allocator, "{s}/pins", .{path_buf[0..dir_len]});
+    defer allocator.free(path);
+
+    // Exactly the shape `save` produced before `iso` existed. Operators have
+    // these on disk, so failing to read them would silently drop every override.
+    try std.Io.Dir.cwd().writeFile(io, .{
+        .sub_path = path,
+        .data =
+        \\#!srfv1
+        \\#!long
+        \\cidr::70.102.70.100/32
+        \\lat:num:47.6038321
+        \\lon:num:-122.330062
+        \\name::Seattle, King County, Washington, United States
+        \\
+        ,
+    });
+
+    var pins = try load(allocator, io, path);
+    defer pins.deinit();
+
+    try std.testing.expectEqual(@as(usize, 1), pins.entries.items.len);
+    const hit = pins.lookup(allocator, "70.102.70.100") orelse return error.TestUnexpectedResult;
+    defer hit.deinit();
+    try std.testing.expectEqualStrings("Seattle, King County, Washington, United States", hit.name);
+    // No country recorded, so the caller must fall through rather than guess.
+    try std.testing.expect(hit.iso_country == null);
+    try std.testing.expect(pins.lookupCountry("70.102.70.100") == null);
+}
+
+test "load: an unparseable country is dropped, keeping the rest of the pin" {
+    const allocator = std.testing.allocator;
+    const io = std.testing.io;
+
+    var tmp = std.testing.tmpDir(.{});
+    defer tmp.cleanup();
+    var path_buf: [std.fs.max_path_bytes]u8 = undefined;
+    const dir_len = try tmp.dir.realPath(io, &path_buf);
+    const path = try std.fmt.allocPrint(allocator, "{s}/pins", .{path_buf[0..dir_len]});
+    defer allocator.free(path);
+
+    // Hand-edited files are expected here, so a bad country must not cost the
+    // operator the location they actually cared about.
+    try std.Io.Dir.cwd().writeFile(io, .{
+        .sub_path = path,
+        .data =
+        \\#!srfv1
+        \\#!long
+        \\cidr::10.0.0.0/8
+        \\lat:num:1
+        \\lon:num:2
+        \\name::Somewhere
+        \\iso::United States
+        \\
+        ,
+    });
+
+    var pins = try load(allocator, io, path);
+    defer pins.deinit();
+
+    try std.testing.expectEqual(@as(usize, 1), pins.entries.items.len);
+    try std.testing.expect(pins.lookupCountry("10.1.2.3") == null);
+    const hit = pins.lookup(allocator, "10.1.2.3") orelse return error.TestUnexpectedResult;
+    defer hit.deinit();
+    try std.testing.expectEqualStrings("Somewhere", hit.name);
+}

src/location/resolver.zig

@@ -11,11 +11,38 @@ pub const Location = struct {
     name: []const u8,
     coords: Coordinates,
     allocator: std.mem.Allocator,
+    /// ISO 3166-1 alpha-2 country code, upper case, when the producing source
+    /// knew it. Drives unit selection, which needs the country rather than the
+    /// display name.
+    ///
+    /// Deliberately a fixed-size array rather than a slice: a country code is
+    /// always two characters, so storing it inline keeps `deinit` a single free
+    /// and leaves every existing construction site valid via the default.
+    iso_country: ?[2]u8 = null,
 
     pub fn deinit(self: Location) void {
         self.allocator.free(self.name);
     }
 
+    /// Normalizes a country code from a provider into the stored form.
+    ///
+    /// Providers disagree on case -- GeoLite2 and ipwho.is return "US" while
+    /// Nominatim returns "us" -- so everything is upper-cased on the way in and
+    /// comparisons downstream can be plain equality. Anything that is not two
+    /// ASCII letters is rejected rather than stored, so a provider returning ""
+    /// or "N/A" reads back as "unknown" instead of as a country.
+    pub fn isoFrom(text: []const u8) ?[2]u8 {
+        if (text.len != 2) return null;
+        if (!std.ascii.isAlphabetic(text[0]) or !std.ascii.isAlphabetic(text[1])) return null;
+        return .{ std.ascii.toUpper(text[0]), std.ascii.toUpper(text[1]) };
+    }
+
+    /// Whether this location is in the United States, for unit selection.
+    pub fn isUS(self: Location) bool {
+        const iso = self.iso_country orelse return false;
+        return std.mem.eql(u8, &iso, "US");
+    }
+
     /// Build a display name from city, subdivision (state/province), and country
     /// Returns allocated string that must be freed by caller
     pub fn buildDisplayName(allocator: std.mem.Allocator, city: []const u8, subdivision: []const u8, country: []const u8, fallback: []const u8) []const u8 {
@@ -161,15 +188,18 @@ pub const Resolver = struct {
                 .allocator = self.allocator,
                 .name = try self.allocator.dupe(u8, cached.name),
                 .coords = cached.coords,
+                .iso_country = cached.iso_country,
             };
         }
 
         log.info("Calling nominatim (OpenStreetMap) to resolve place name {s} to coordinates", .{name});
         if (@import("builtin").is_test) return error.GeocodingUnavailableInUnitTest;
-        // Call Nominatim API
+        // Call Nominatim API. `addressdetails=1` is requested for
+        // `address.country_code`, which is what lets a pin created from a place
+        // name record its country for unit selection.
         const url = try std.fmt.allocPrint(
             self.allocator,
-            "https://nominatim.openstreetmap.org/search?q={s}&format=json&limit=1",
+            "https://nominatim.openstreetmap.org/search?q={s}&format=json&limit=1&addressdetails=1",
             .{name},
         );
         defer self.allocator.free(url);
@@ -218,6 +248,17 @@ pub const Resolver = struct {
         const lon = try std.fmt.parseFloat(f64, first.object.get("lon").?.string);
         log.info("nominatim resolved place name {s} to {}, {}", .{ name, lat, lon });
 
+        // Nominatim nests the country code under `address` and returns it lower
+        // case; `isoFrom` normalizes. Absent for results that carry no address
+        // block, which is why this is optional rather than required.
+        const iso_country: ?[2]u8 = blk: {
+            const address = first.object.get("address") orelse break :blk null;
+            if (address != .object) break :blk null;
+            const code = address.object.get("country_code") orelse break :blk null;
+            if (code != .string) break :blk null;
+            break :blk Location.isoFrom(code.string);
+        };
+
         // Cache the result
         try self.geocache.put(name, .{
             .name = display_name,
@@ -225,6 +266,7 @@ pub const Resolver = struct {
                 .latitude = lat,
                 .longitude = lon,
             },
+            .iso_country = iso_country,
         });
 
         return .{
@@ -234,6 +276,7 @@ pub const Resolver = struct {
                 .latitude = lat,
                 .longitude = lon,
             },
+            .iso_country = iso_country,
         };
     }
 
@@ -441,3 +484,50 @@ test "buildDisplayName: city only (no state or country)" {
     defer allocator.free(name);
     try std.testing.expectEqualStrings("Paris", name);
 }
+
+test "isoFrom: upper-cases so providers of either convention agree" {
+    // Nominatim returns "us", GeoLite2 and ipwho.is return "US". Without
+    // normalization the same country would compare unequal depending on source.
+    try std.testing.expectEqualStrings("US", &(Location.isoFrom("us").?));
+    try std.testing.expectEqualStrings("US", &(Location.isoFrom("US").?));
+    try std.testing.expectEqualStrings("GB", &(Location.isoFrom("gB").?));
+}
+
+test "isoFrom: rejects anything that is not two letters" {
+    // Providers return "" for unknown and occasionally a placeholder. Storing
+    // those would make an unknown country read back as a real one.
+    try std.testing.expect(Location.isoFrom("") == null);
+    try std.testing.expect(Location.isoFrom("U") == null);
+    try std.testing.expect(Location.isoFrom("USA") == null);
+    try std.testing.expect(Location.isoFrom("12") == null);
+    try std.testing.expect(Location.isoFrom("N/A") == null);
+    try std.testing.expect(Location.isoFrom("-") == null);
+}
+
+test "isUS: only true for a known US country code" {
+    const allocator = std.testing.allocator;
+    const base: Location = .{ .allocator = allocator, .name = "", .coords = .{ .latitude = 0, .longitude = 0 } };
+
+    // An unknown country must not read as US: that would flip every client with
+    // no country data to imperial.
+    try std.testing.expect(!base.isUS());
+
+    var us = base;
+    us.iso_country = Location.isoFrom("us");
+    try std.testing.expect(us.isUS());
+
+    var gb = base;
+    gb.iso_country = Location.isoFrom("GB");
+    try std.testing.expect(!gb.isUS());
+}
+
+test "Location: iso_country defaults to unknown" {
+    // The default is what keeps every existing construction site valid, so it is
+    // worth pinning down rather than leaving implied.
+    const loc: Location = .{
+        .allocator = std.testing.allocator,
+        .name = "",
+        .coords = .{ .latitude = 0, .longitude = 0 },
+    };
+    try std.testing.expect(loc.iso_country == null);
+}

src/main.zig

@@ -12,6 +12,7 @@ const GeoLite2 = @import("location/GeoLite2.zig");
 const Refresher = @import("location/Refresher.zig");
 const Signals = @import("Signals.zig");
 const pin_cmd = @import("cli/pin.zig");
+const explain_cmd = @import("cli/explain.zig");
 const version = @import("build_options").version;
 
 /// Zig 0.16 entry point: the runtime supplies allocators, the `Io`
@@ -40,6 +41,7 @@ pub fn main(init: std.process.Init) !u8 {
     if (std.mem.eql(u8, cmd, "pin")) return pin_cmd.runPin(allocator, io, cfg, args[2..]);
     if (std.mem.eql(u8, cmd, "unpin")) return pin_cmd.runUnpin(allocator, io, cfg, args[2..]);
     if (std.mem.eql(u8, cmd, "pins")) return pin_cmd.runList(allocator, io, cfg);
+    if (std.mem.eql(u8, cmd, "explain")) return explain_cmd.runExplain(allocator, io, cfg, args[2..]);
     if (std.mem.eql(u8, cmd, "help") or std.mem.eql(u8, cmd, "--help") or std.mem.eql(u8, cmd, "-h"))
         return pin_cmd.printUsage(io, args[0]);
 
@@ -193,4 +195,5 @@ test {
     _ = @import("location/Pins.zig");
     _ = @import("Signals.zig");
     _ = @import("cli/pin.zig");
+    _ = @import("cli/explain.zig");
 }