resolve TODO surrounding cache ttl on supplementary providers

TODO.md

@@ -403,45 +403,6 @@ Eastern rather than a rolling window. This would avoid unnecessary refetches
 during the trading day and ensure a fetch shortly after close gets fresh data.
 Probably alleviated by the cron job approach.
 
-## Cache TTL semantics on merge writes — priority LOW
-
-The `writeMerged` primitive in `cache/store.zig` rewrites `dividends.srf` /
-`splits.srf` with `expires = now + ttl` whenever it adds a new record or
-upgrades fields on an existing one. This is conceptually wrong: TTL should
-reflect "when do we expect new information from the primary provider?",
-which is a property of the conversation with that provider — not of the
-file's last-modification time. Adding a 25-year-old historical dividend
-that Tiingo just supplied tells us nothing about Polygon's freshness; we
-shouldn't bump the file's expiry as a side effect.
-
-The cleaner design:
-
-- Cache file's `#!expires=` reflects "when did Polygon (the primary) last
-  say `here's everything I have`?"
-- Tiingo merge writes preserve the existing expires, only rewriting records.
-- Only `fetchCached`'s post-Polygon-fetch write bumps expires.
-
-In practice the current behavior caused exactly one observable problem: a
-one-time TTL herd on 2026-06-04 when the new merge code's first run added
-pre-2010 Tiingo backfill across 23+ symbols in a single overnight burst,
-and they all inherited that day's clock for `expires = now + 14d`. We
-manually re-staggered (`stagger_cache_ttls.py`) and moved on.
-
-Steady-state risk: minimal. The merge primitive's "skip if nothing changed"
-branch means no-op refreshes don't bump expires. New entries from genuinely
-new dividends are spread across the calendar by the dividends themselves
-(quarterly cadence varies per ticker). Field upgrades stop firing once
-Polygon's metadata is in place.
-
-When this could matter again:
-- Adding a third source for div/splits (TTL semantics get murkier).
-- Wiping and rebuilding the server cache (one-time herd recurs).
-- A long pause in nightly refreshes followed by a backlog of merge writes.
-
-Fix would be small: thread `?expires_override` into `writeMerged` and have
-the merge path call `serializeWithMeta` with the existing expires (from the
-read) when source_hint isn't the primary.
-
 ## On-demand server-side fetch for new symbols
 
 Currently the server's SRF endpoints (`/candles`, `/dividends`, etc.) are pure

src/cache/store.zig

@@ -346,7 +346,16 @@ pub const Store = struct {
     }
 
     pub fn CacheResult(comptime T: type) type {
-        return struct { data: DataFor(T), timestamp: i64 };
+        return struct {
+            data: DataFor(T),
+            timestamp: i64,
+            /// The on-disk `#!expires=` directive, or null when the
+            /// file carried none. Surfaced so the merge path can
+            /// preserve the primary provider's freshness clock when a
+            /// secondary source supplements the file (see
+            /// `writeSupplement`). Most callers ignore it.
+            expires: ?i64 = null,
+        };
     }
 
     pub const Freshness = enum { fresh_only, any };
@@ -404,12 +413,32 @@ pub const Store = struct {
             const timestamp = it.created orelse std.Io.Timestamp.now(self.io, .real).toSeconds();
 
             const items = deserializeOptions(allocator, &it) catch return null;
-            return .{ .data = items, .timestamp = timestamp };
+            return .{ .data = items, .timestamp = timestamp, .expires = it.expires };
         }
 
         return readSlice(T, self.io, allocator, data, postProcess, freshness);
     }
 
+    /// Who owns the `#!expires=` freshness clock on a merge write.
+    ///
+    /// `#!expires=` answers "when do we next expect to consult the
+    /// primary provider for this data type?" Only a fetch from that
+    /// primary may move it. A secondary source that opportunistically
+    /// supplements the file (e.g. Tiingo dividend rows arriving as a
+    /// side effect of a candle fetch) must merge its records without
+    /// resetting the clock, otherwise it masks a due primary refresh.
+    const ExpiryPolicy = enum {
+        /// Primary fetch: stamp `expires = now + ttl` (+ jitter). The
+        /// authoritative freshness signal.
+        bump,
+        /// Secondary supplement: keep the existing on-disk `expires`.
+        /// When there is no existing file yet, fall back to `bump` so
+        /// a brand-new symbol still gets an initial TTL (this is also
+        /// what keeps Tiingo-only setups, with no primary key, serving
+        /// freshly-written data instead of immediately re-missing).
+        preserve,
+    };
+
     /// Serialize data and write to cache with the given TTL.
     ///
     /// For `Dividend` and `Split`, this dispatches to `writeMerged`,
@@ -442,7 +471,7 @@ pub const Store = struct {
         source_hint: ?[]const u8,
     ) void {
         if (T == Dividend or T == Split) {
-            self.writeMerged(T, symbol, items, ttl, source_hint);
+            self.writeMerged(T, symbol, items, ttl, source_hint, .bump);
             return;
         }
         const expires = computeExpires(std.Io.Timestamp.now(self.io, .real).toSeconds(), ttl, symbol);
@@ -468,11 +497,40 @@ pub const Store = struct {
         };
     }
 
+    /// Merge supplementary `Dividend` / `Split` records from a
+    /// non-authoritative source into the cache **without** resetting
+    /// the `#!expires=` freshness clock. Use this for data that
+    /// arrives as a side effect of fetching something else (the
+    /// canonical case: Tiingo dividend/split rows piggybacking on a
+    /// candle fetch in `populateAllFromTiingo`). The records are
+    /// merged via the same sorted-union semantics as
+    /// `writeWithSource`; only the expiry handling differs — the
+    /// existing on-disk expires is preserved so the next primary
+    /// (Polygon) fetch decides freshness. If no file exists yet, an
+    /// initial TTL is established (see `ExpiryPolicy.preserve`).
+    ///
+    /// `source_hint` names the source in the `info(cache)` log lines
+    /// (e.g. `"tiingo"`).
+    pub fn writeSupplement(
+        self: *Store,
+        comptime T: type,
+        symbol: []const u8,
+        items: DataFor(T),
+        source_hint: ?[]const u8,
+    ) void {
+        comptime std.debug.assert(T == Dividend or T == Split);
+        // ttl is consulted only on the no-existing-file fallback
+        // inside `writeMerged`; derive it from the data type rather
+        // than asking the caller for a value that's usually ignored.
+        self.writeMerged(T, symbol, items, dataTypeFor(T).ttl(), source_hint, .preserve);
+    }
+
     /// Sorted-union write for `Dividend` and `Split`. Reads the
     /// existing cache file, merges `incoming` into it, sorts the
-    /// union descending by date, and writes the result. If nothing
+    /// union descending by date, and writes the result. The file is
-    /// changed, the existing file is left untouched (no mtime bump,
+    /// always rewritten (even on a no-op merge) so the `#!expires=`
-    /// no I/O).
+    /// directive is handled per `expiry`; see the note at the write
+    /// step below.
     ///
     /// Two kinds of merge happen on each incoming entry:
     ///
@@ -502,6 +560,11 @@ pub const Store = struct {
     /// `source_hint`, when present, names the source in the log
     /// lines (e.g. `"polygon"` / `"tiingo"`). Defaults to
     /// `"fetch"` when null.
+    ///
+    /// `expiry` selects whether this write owns the freshness clock
+    /// (`.bump`, the primary-fetch path via `writeWithSource`) or
+    /// preserves whatever the primary last set (`.preserve`, the
+    /// supplement path via `writeSupplement`). See `ExpiryPolicy`.
     fn writeMerged(
         self: *Store,
         comptime T: type,
@@ -509,6 +572,7 @@ pub const Store = struct {
         incoming: []const T,
         ttl: TtlSpec,
         source_hint: ?[]const u8,
+        expiry: ExpiryPolicy,
     ) void {
         comptime std.debug.assert(T == Dividend or T == Split);
 
@@ -525,6 +589,9 @@ pub const Store = struct {
         // the deinit handling — they're a pair.
         const existing_result = self.read(self.allocator, T, symbol, null, .any);
         const existing: []const T = if (existing_result) |r| r.data else &.{};
+        // Snapshot the primary's freshness clock before we rewrite, so
+        // `.preserve` writes can put it back unchanged.
+        const existing_expires: ?i64 = if (existing_result) |r| r.expires else null;
         defer if (existing_result != null) {
             if (comptime @hasDecl(T, "deinit")) {
                 for (existing) |item| item.deinit(self.allocator);
@@ -573,14 +640,16 @@ pub const Store = struct {
         }
 
         // Note: even when nothing was added or upgraded, fall through
-        // and rewrite the file. The on-disk `#!expires=` directive
+        // and rewrite the file. On the `.bump` (primary) path the
-        // needs to be refreshed every time we successfully fetched
+        // on-disk `#!expires=` directive needs refreshing every time
-        // and merged, otherwise an aged-out file stays aged-out:
+        // we successfully fetched and merged, otherwise an aged-out
-        // every subsequent refresh pays the full provider rate-
+        // file stays aged-out: every subsequent refresh pays the full
-        // limiter cost only to discover no changes and skip the
+        // provider rate-limiter cost only to discover no changes and
-        // write, locking the cache into a permanent slow-path. The
+        // skip the write, locking the cache into a permanent slow-path.
-        // write itself is a sub-millisecond atomic rename of a tiny
+        // The write itself is a sub-millisecond atomic rename of a
-        // file, so saving it isn't worth the bookkeeping.
+        // tiny file, so saving it isn't worth the bookkeeping. On the
+        // `.preserve` (supplement) path we still rewrite to land merged
+        // records, but we put the existing expires back untouched.
 
         // Sort descending by date (newest first), matching the
         // existing on-disk convention.
@@ -588,7 +657,16 @@ pub const Store = struct {
 
         // Serialize via the same generic path as `write` for
         // non-merged types, but write the union we just built.
-        const expires = computeExpires(std.Io.Timestamp.now(self.io, .real).toSeconds(), ttl, symbol);
+        //
+        // wall-clock required: TTL math for the `.bump` path, and the
+        // no-existing-file fallback for `.preserve`.
+        const now_s = std.Io.Timestamp.now(self.io, .real).toSeconds();
+        const expires: i64 = switch (expiry) {
+            .bump => computeExpires(now_s, ttl, symbol),
+            // Keep the primary's clock. Brand-new file (no prior
+            // expires) → establish one like a first fetch would.
+            .preserve => existing_expires orelse computeExpires(now_s, ttl, symbol),
+        };
         const data_type = dataTypeFor(T);
         const srf_data = serializeWithMeta(T, self.io, self.allocator, merged.items, .{ .expires = expires }) catch |err| {
             log.warn("{s}: failed to serialize {s}: {s}", .{ symbol, @tagName(data_type), @errorName(err) });
@@ -1612,7 +1690,7 @@ pub const Store = struct {
 
         const result = items.toOwnedSlice(allocator) catch return null;
         items = .empty; // prevent defer from freeing the returned slice
-        return .{ .data = result, .timestamp = timestamp };
+        return .{ .data = result, .timestamp = timestamp, .expires = it.expires };
     }
 
     /// Generic SRF serializer: emit directives (including `#!created=`) then data records.
@@ -2028,14 +2106,17 @@ test "diskStats: empty cache is all zeros; populated counts symbols/files/bytes"
 }
 
 test "writeMerged Dividend: no-change merge still rewrites to refresh expires" {
-    // The "nothing changed" path used to skip the rewrite as an
+    // PRIMARY path (`writeWithSource` / .bump). The "nothing
-    // optimization. Problem: once the on-disk `#!expires=` aged
+    // changed" path used to skip the rewrite as an optimization.
-    // past TTL, every subsequent refresh paid the full provider
+    // Problem: once the on-disk `#!expires=` aged past TTL, every
-    // rate-limiter cost only to discover no changes and skip the
+    // subsequent refresh paid the full provider rate-limiter cost
-    // write again, locking the cache into a permanent slow-path.
+    // only to discover no changes and skip the write again, locking
+    // the cache into a permanent slow-path.
     //
     // Now we always rewrite. The write is a sub-millisecond atomic
     // rename of a tiny file; saving it isn't worth the bookkeeping.
+    // (Contrast: the supplement / .preserve path keeps the existing
+    // expires — see the writeSupplement tests below.)
     const allocator = std.testing.allocator;
     const io = std.testing.io;
     var tmp = std.testing.tmpDir(.{});
@@ -2081,6 +2162,131 @@ test "writeMerged Dividend: no-change merge still rewrites to refresh expires" {
     try std.testing.expect(new_expires - now_s > 13 * std.time.s_per_day);
 }
 
+test "writeSupplement Dividend: preserves an existing future expires" {
+    // SUPPLEMENT path (.preserve). A Tiingo-driven supplement must
+    // merge new records without moving the primary's freshness
+    // clock. Seed a known future expires, supplement a brand-new
+    // dividend, and confirm the on-disk expires is exactly the
+    // seeded value (not re-stamped to now+TTL).
+    const allocator = std.testing.allocator;
+    const io = std.testing.io;
+    var tmp = std.testing.tmpDir(.{});
+    defer tmp.cleanup();
+    const dir_path = try tmp.dir.realPathFileAlloc(io, ".", allocator);
+    defer allocator.free(dir_path);
+
+    var s = Store.init(io, allocator, dir_path);
+
+    const now_s = std.Io.Timestamp.now(io, .real).toSeconds();
+    const seed_expires = now_s + 100 * std.time.s_per_day;
+    const seed_divs = [_]Dividend{
+        .{ .ex_date = Date.fromYmd(2024, 5, 15), .amount = 0.50, .type = .regular },
+    };
+    const seed_bytes = try Store.serializeWithMeta(Dividend, io, allocator, &seed_divs, .{ .expires = seed_expires });
+    defer allocator.free(seed_bytes);
+    try s.writeRaw("TEST", .dividends, seed_bytes);
+
+    // Tiingo supplies a genuinely-new (months-earlier) dividend, so a
+    // real merge + rewrite happens.
+    var incoming = [_]Dividend{
+        .{ .ex_date = Date.fromYmd(2024, 2, 15), .amount = 0.50 },
+    };
+    s.writeSupplement(Dividend, "TEST", incoming[0..], "tiingo");
+
+    const path = try std.fs.path.join(allocator, &.{ dir_path, "TEST", "dividends.srf" });
+    defer allocator.free(path);
+    const data = try std.Io.Dir.cwd().readFileAlloc(io, path, allocator, .limited(1024 * 1024));
+    defer allocator.free(data);
+    var reader = std.Io.Reader.fixed(data);
+    const it = try srf.iterator(&reader, allocator, .{ .parse_allocator = .none });
+    defer it.deinit();
+
+    // Expires preserved exactly — the candle-driven Tiingo write did
+    // not reset Polygon's clock.
+    try std.testing.expectEqual(seed_expires, it.expires orelse return error.ExpiresMissing);
+
+    // And the merge actually landed (union grew to 2).
+    const result = s.read(s.allocator, Dividend, "TEST", null, .any) orelse return error.NoCache;
+    defer allocator.free(result.data);
+    defer for (result.data) |d| d.deinit(allocator);
+    try std.testing.expectEqual(@as(usize, 2), result.data.len);
+}
+
+test "writeSupplement Dividend: preserves an aged-out expires (does not refresh)" {
+    // Direct contrast with the .bump no-change test above: a Tiingo
+    // supplement on an already-expired file leaves the past expires
+    // in place, so the next getDividends still treats it as stale and
+    // consults the primary (Polygon). Tiingo backfilling old data is
+    // not a reason to consider the primary's data fresh.
+    const allocator = std.testing.allocator;
+    const io = std.testing.io;
+    var tmp = std.testing.tmpDir(.{});
+    defer tmp.cleanup();
+    const dir_path = try tmp.dir.realPathFileAlloc(io, ".", allocator);
+    defer allocator.free(dir_path);
+
+    var s = Store.init(io, allocator, dir_path);
+
+    const now_s = std.Io.Timestamp.now(io, .real).toSeconds();
+    const seed_expires = now_s - 30 * std.time.s_per_day;
+    const seed_divs = [_]Dividend{
+        .{ .ex_date = Date.fromYmd(2024, 5, 15), .amount = 0.50, .type = .regular },
+    };
+    const seed_bytes = try Store.serializeWithMeta(Dividend, io, allocator, &seed_divs, .{ .expires = seed_expires });
+    defer allocator.free(seed_bytes);
+    try s.writeRaw("TEST", .dividends, seed_bytes);
+
+    var incoming = [_]Dividend{
+        .{ .ex_date = Date.fromYmd(2024, 2, 15), .amount = 0.50 },
+    };
+    s.writeSupplement(Dividend, "TEST", incoming[0..], "tiingo");
+
+    const path = try std.fs.path.join(allocator, &.{ dir_path, "TEST", "dividends.srf" });
+    defer allocator.free(path);
+    const data = try std.Io.Dir.cwd().readFileAlloc(io, path, allocator, .limited(1024 * 1024));
+    defer allocator.free(data);
+    var reader = std.Io.Reader.fixed(data);
+    const it = try srf.iterator(&reader, allocator, .{ .parse_allocator = .none });
+    defer it.deinit();
+
+    // Still the seeded past value — NOT bumped to now+TTL.
+    try std.testing.expectEqual(seed_expires, it.expires orelse return error.ExpiresMissing);
+}
+
+test "writeSupplement Dividend: no existing file establishes an initial TTL" {
+    // No primary clock exists yet, so there's nothing to preserve.
+    // A supplement to a brand-new symbol behaves like a first fetch:
+    // it stamps a normal jittered TTL so the freshly-written data is
+    // actually served (important for setups with no primary key).
+    const allocator = std.testing.allocator;
+    const io = std.testing.io;
+    var tmp = std.testing.tmpDir(.{});
+    defer tmp.cleanup();
+    const dir_path = try tmp.dir.realPathFileAlloc(io, ".", allocator);
+    defer allocator.free(dir_path);
+
+    var s = Store.init(io, allocator, dir_path);
+
+    const now_s = std.Io.Timestamp.now(io, .real).toSeconds();
+    var incoming = [_]Dividend{
+        .{ .ex_date = Date.fromYmd(2024, 5, 15), .amount = 0.50 },
+    };
+    s.writeSupplement(Dividend, "TEST", incoming[0..], "tiingo");
+
+    const path = try std.fs.path.join(allocator, &.{ dir_path, "TEST", "dividends.srf" });
+    defer allocator.free(path);
+    const data = try std.Io.Dir.cwd().readFileAlloc(io, path, allocator, .limited(1024 * 1024));
+    defer allocator.free(data);
+    var reader = std.Io.Reader.fixed(data);
+    const it = try srf.iterator(&reader, allocator, .{ .parse_allocator = .none });
+    defer it.deinit();
+
+    // ~14d TTL with ±11% jitter; comfortably bracket the band.
+    const new_expires = it.expires orelse return error.ExpiresMissing;
+    try std.testing.expect(new_expires - now_s > 13 * std.time.s_per_day);
+    try std.testing.expect(new_expires - now_s < 16 * std.time.s_per_day);
+}
+
 test "writeMerged Dividend: field-level upgrade fills nulls (Tiingo-then-Polygon)" {
     // Simulates the order that happens during a server refresh:
     // Tiingo writes its sparse view first (via populateAllFromTiingo
@@ -2101,7 +2307,7 @@ test "writeMerged Dividend: field-level upgrade fills nulls (Tiingo-then-Polygon
     var tiingo_view = [_]Dividend{
         .{ .ex_date = Date.fromYmd(2024, 5, 15), .amount = 0.50 },
     };
-    s.writeWithSource(Dividend, "TEST", tiingo_view[0..], .{ .seconds = Ttl.dividends }, "tiingo");
+    s.writeSupplement(Dividend, "TEST", tiingo_view[0..], "tiingo");
 
     // Polygon second: rich — pay_date, record_date, type, currency.
     var polygon_view = [_]Dividend{
@@ -2376,7 +2582,7 @@ test "writeMerged Dividend: near-match dedup catches last-biz-day vs calendar-en
             .amount = 0.003422654,
         },
     };
-    s.writeWithSource(Dividend, "FDRXX", tiingo_view[0..], .{ .seconds = Ttl.dividends }, "tiingo");
+    s.writeSupplement(Dividend, "FDRXX", tiingo_view[0..], "tiingo");
 
     const result = s.read(s.allocator, Dividend, "FDRXX", null, .any) orelse return error.NoCache;
     defer allocator.free(result.data);
@@ -2482,7 +2688,7 @@ test "writeMerged Dividend: near-match dedup tolerates Tiingo amount rounding" {
     var tiingo_view = [_]Dividend{
         .{ .ex_date = Date.fromYmd(2024, 8, 30), .amount = 0.04 },
     };
-    s.writeWithSource(Dividend, "FAGIX", tiingo_view[0..], .{ .seconds = Ttl.dividends }, "tiingo");
+    s.writeSupplement(Dividend, "FAGIX", tiingo_view[0..], "tiingo");
 
     const result = s.read(s.allocator, Dividend, "FAGIX", null, .any) orelse return error.NoCache;
     defer allocator.free(result.data);
@@ -2544,7 +2750,7 @@ test "writeMerged Split: SPYM-style supplementary entry added" {
     var tiingo_view = [_]Split{
         .{ .date = Date.fromYmd(2017, 10, 16), .numerator = 4, .denominator = 1 },
     };
-    s.write(Split, "SPYM", tiingo_view[0..], .{ .seconds = Ttl.splits });
+    s.writeSupplement(Split, "SPYM", tiingo_view[0..], "tiingo");
 
     const result = s.read(s.allocator, Split, "SPYM", null, .any) orelse return error.NoCache;
     defer allocator.free(result.data);
@@ -2577,7 +2783,7 @@ test "writeMerged Split: forward-looking Polygon entry preserved across Tiingo r
     var tiingo_view = [_]Split{
         .{ .date = Date.fromYmd(2020, 8, 31), .numerator = 4, .denominator = 1 },
     };
-    s.write(Split, "TEST", tiingo_view[0..], .{ .seconds = Ttl.splits });
+    s.writeSupplement(Split, "TEST", tiingo_view[0..], "tiingo");
 
     const result = s.read(s.allocator, Split, "TEST", null, .any) orelse return error.NoCache;
     defer allocator.free(result.data);

src/service.zig

@@ -532,11 +532,12 @@ pub const DataService = struct {
                 // 2026-06-15 ex_date), which Tiingo's price-series
                 // response does not. Tiingo opportunistically
                 // supplements the cache via `populateAllFromTiingo`
-                // when candle fetches happen — that path uses the
+                // when candle fetches happen — that path uses
-                // sorted-union write semantics in
+                // `cache.Store.writeSupplement`, whose sorted-union
-                // `cache.Store.writeMerged`, so Polygon's entries
+                // merge lets Polygon's and Tiingo's entries coexist in
-                // and Tiingo's entries coexist in `dividends.srf`
+                // `dividends.srf` without overwriting each other, and
-                // without overwriting each other.
+                // which preserves the `#!expires=` clock so a Tiingo
+                // candle fetch never masks a due Polygon refresh.
                 var pg = try self.getProvider(Polygon);
                 return pg.fetchDividends(self.allocator, symbol, null, null);
             },
@@ -596,12 +597,14 @@ pub const DataService = struct {
         if (triple.candles.len > 0) {
             s.cacheCandles(symbol, triple.candles, .tiingo, 0);
         }
-        // Dividends and splits use the merge write path so Tiingo's
+        // Dividends and splits use the supplement write path: Tiingo's
-        // view supplements rather than replaces existing (typically
+        // view merges into existing (typically Polygon-sourced) records
-        // Polygon-sourced) records. New entries are logged with
+        // without resetting the `#!expires=` freshness clock, since
-        // "tiingo" attribution.
+        // Polygon — not this opportunistic candle-driven write — owns
-        s.writeWithSource(Dividend, symbol, triple.dividends, cache.DataType.dividends.ttl(), "tiingo");
+        // when div/split data is next due for a primary refresh. New
-        s.writeWithSource(Split, symbol, triple.splits, cache.DataType.splits.ttl(), "tiingo");
+        // entries are logged with "tiingo" attribution.
+        s.writeSupplement(Dividend, symbol, triple.dividends, "tiingo");
+        s.writeSupplement(Split, symbol, triple.splits, "tiingo");
 
         return triple;
     }