use committer timestamp

build.zig

@@ -118,21 +118,27 @@ pub fn build(b: *std.Build) void {
 }
 
 /// Produce the `build_info` module exposing `version` (derived from `git
-/// describe`) and `build_timestamp` (Unix epoch seconds at build time).
+/// describe`) and `build_timestamp` (committer timestamp of HEAD).
 /// Consumed as `@import("build_info")` from `src/version.zig`.
+///
+/// Both values are reproducible per-commit: `git describe` is stable
+/// until a new commit/tag/dirty-flag flip, and the committer timestamp
+/// comes from `git log -1 --format=%ct`. Neither varies with wall-clock
+/// time, so the Options module doesn't rebuild every invocation, which
+/// used to cascade into a full exe relink on every `zig build`.
+///
+/// Falls back to `fallbackVersion()` / `0` when git is unavailable
+/// (source tarball, pre-commit environment).
 fn buildInfoOptions(b: *std.Build) *std.Build.Module {
     const opts = b.addOptions();
 
-    // `git describe --tags --always --dirty` gives the closest tag, with a
-    // distance-+-hash suffix when HEAD isn't exactly on a tag, plus a
-    // `-dirty` marker if the working tree has uncommitted changes. Falls
-    // back to the build.zig.zon `.version` when git is unavailable.
     const version = gitDescribe(b) orelse fallbackVersion();
     opts.addOption([]const u8, "version", version);
 
-    // Seconds since the Unix epoch at build time. Rendered by `zfin version
-    // --verbose` as a human-readable date.
-    opts.addOption(i64, "build_timestamp", std.time.timestamp());
+    // Use HEAD's committer timestamp (reproducible) instead of
+    // `std.time.timestamp()` (changes every build). See comment above.
+    const timestamp = gitHeadTimestamp(b) orelse 0;
+    opts.addOption(i64, "build_timestamp", timestamp);
 
     return opts.createModule();
 }
@@ -141,7 +147,22 @@ fn buildInfoOptions(b: *std.Build) *std.Build.Module {
 /// the trimmed output. Returns null on any error (git missing, not a repo,
 /// non-zero exit).
 fn gitDescribe(b: *std.Build) ?[]const u8 {
-    var child = std.process.Child.init(&.{ "git", "describe", "--tags", "--always", "--dirty" }, b.allocator);
+    return gitCapture(b, &.{ "git", "describe", "--tags", "--always", "--dirty" });
+}
+
+/// Run `git log -1 --format=%ct HEAD` and parse the stdout as an i64
+/// (Unix seconds). Returns null on any error. Stable per-commit, which
+/// keeps the Options module cache-friendly.
+fn gitHeadTimestamp(b: *std.Build) ?i64 {
+    const text = gitCapture(b, &.{ "git", "log", "-1", "--format=%ct", "HEAD" }) orelse return null;
+    return std.fmt.parseInt(i64, text, 10) catch null;
+}
+
+/// Spawn a git subcommand in the repo root, capture stdout, trim and
+/// dupe it through `b.allocator`. Returns null on any error (git
+/// missing, non-zero exit, empty output).
+fn gitCapture(b: *std.Build, argv: []const []const u8) ?[]const u8 {
+    var child = std.process.Child.init(argv, b.allocator);
     child.cwd = b.build_root.path;
     child.stdout_behavior = .Pipe;
     child.stderr_behavior = .Ignore;

src/version.zig

@@ -2,8 +2,13 @@
 //!
 //! `version_string` is `git describe --tags --always --dirty` output (or the
 //! `.version` field from `build.zig.zon` if git is unavailable at build time).
-//! `build_timestamp` is the Unix epoch seconds at the moment the build
-//! executed. Together they identify the binary precisely.
+//! `build_timestamp` is the committer timestamp of HEAD (Unix epoch
+//! seconds), or 0 when git is unavailable. Together they identify the
+//! binary precisely.
+//!
+//! The timestamp is deliberately per-commit rather than wall-clock: it
+//! keeps the build-options module cache-stable, so repeated `zig build`
+//! invocations don't relink the exe just because the second ticked over.
 //!
 //! Consumers (the `zfin version` command, snapshot metadata writers, bug
 //! reports) should route through this module rather than importing
@@ -17,8 +22,9 @@ const build_info = @import("build_info");
 /// "1a2b3c4" (no tags yet), or a fallback from build.zig.zon.
 pub const version_string: []const u8 = build_info.version;
 
-/// Unix epoch seconds at build time. Rendered as ISO date by the
-/// `zfin version --verbose` command.
+/// Unix epoch seconds — committer timestamp of HEAD at build time, or 0
+/// when git is unavailable. Rendered as ISO date by the `zfin version
+/// --verbose` command; 0 renders as 1970-01-01 (clearly-fake sentinel).
 pub const build_timestamp: i64 = build_info.build_timestamp;
 
 /// True when `version_string` ends in `-dirty`, indicating the build was
@@ -31,9 +37,10 @@ test "version_string is non-empty" {
     try std.testing.expect(version_string.len > 0);
 }
 
-test "build_timestamp is positive" {
-    // A legitimate build always runs after 2020 (roughly 1577836800).
-    // If this fires, either the host clock is badly wrong, or the build
-    // options didn't wire through.
-    try std.testing.expect(build_timestamp > 1_577_836_800);
+test "build_timestamp is either zero (no-git fallback) or a plausible commit time" {
+    // Two acceptable shapes:
+    //   0               → git unavailable or `git log -1 %ct` failed
+    //   > 1_577_836_800 → real committer timestamp (Jan 1 2020 onward)
+    // Anything in between would indicate a broken build-info wiring.
+    try std.testing.expect(build_timestamp == 0 or build_timestamp > 1_577_836_800);
 }