Record one probe for two reads of a file that saw the same content

A read within IDENTITY_SETTLE_MS of a file's last write records no
identity, and a later read of the same bytes in the same frontend does.
FrontendInputTracker.record compared the two probes as JSON and marked
the snapshot unstable. A build or coverage run that read its entry
before and after that point then published no cache entry: the exact
repeat of a build lowered again, and a coverage verdict was not replayed.
Two file probes with the same digest now merge into one that keeps the
settled identity; any other difference still marks the snapshot
unstable.

packages/compiler/src/frontend/input-tracker.test.ts

@@ -316,3 +316,41 @@ test("a file changed shortly before the read is hashed on every check", async ()
   expect(frontendInputsStillMatch(snapshot)).toBe(true);
   expect(reads.mock.calls.filter(([path]) => path === source)).toHaveLength(1);
 });
+
+test("a read before and a read after a file settles record one probe", async () => {
+  const dir = await mkdtemp(join(tmpdir(), "scriptc-inputs-"));
+  scratch.push(dir);
+  const source = join(dir, "entry.ts");
+  await writeFile(source, "export const answer = 1;\n");
+  const tracker = new FrontendInputTracker();
+  // The first read comes right after the write, the second once the file
+  // has been unchanged long enough for its identity to stand for it.
+  tracker.run(() => trackedReadFile(source));
+  vi.useFakeTimers({ toFake: ["Date"] });
+  vi.setSystemTime(Date.now() + 5000);
+  try {
+    tracker.run(() => trackedReadFile(source));
+  } finally {
+    vi.useRealTimers();
+  }
+  const snapshot = tracker.snapshot();
+  expect(snapshot.stable).toBe(true);
+  const probe = snapshot.probes.find((entry) => entry.op === "file");
+  expect(probe?.op === "file" && probe.identity !== undefined).toBe(true);
+  expect(frontendInputsStillMatch(snapshot)).toBe(true);
+  // The identity still invalidates the snapshot on any write.
+  await writeFile(source, "export const answer = 2;\n");
+  expect(frontendInputsStillMatch(snapshot)).toBe(false);
+});
+
+test("two reads of a file that saw different content prevent cache publication", async () => {
+  const dir = await mkdtemp(join(tmpdir(), "scriptc-inputs-"));
+  scratch.push(dir);
+  const source = join(dir, "entry.ts");
+  await writeFile(source, "export const answer = 1;\n");
+  const tracker = new FrontendInputTracker();
+  tracker.run(() => trackedReadFile(source));
+  await writeFile(source, "export const answer = 2;\n");
+  tracker.run(() => trackedReadFile(source));
+  expect(tracker.snapshot().stable).toBe(false);
+});

packages/compiler/src/frontend/input-tracker.ts

@@ -84,6 +84,22 @@ function readWithIdentity(path: string): { text: string; identity: string | null
   return { text, identity: settled && after === identity ? identity : null };
 }
 
+/** Two reads of one file that saw the same content, as one probe: a read
+ * before the file had been unchanged for IDENTITY_SETTLE_MS records no
+ * identity, and a later read of the same bytes does. The probe keeps the
+ * identity, which the settled read proved for that content. Null when the
+ * reads differ in anything else. */
+function sameFileContent(
+  previous: FrontendInputProbe,
+  probe: FrontendInputProbe,
+): FrontendInputProbe | null {
+  if (previous.op !== "file" || probe.op !== "file" || previous.digest !== probe.digest)
+    return null;
+  if (previous.identity === undefined) return probe;
+  if (probe.identity === undefined || probe.identity === previous.identity) return previous;
+  return null;
+}
+
 function pathKind(path: string): Extract<FrontendInputProbe, { op: "kind" }>["kind"] {
   try {
     const info = statSync(path);
@@ -142,12 +158,17 @@ export class FrontendInputTracker {
     const key = `${probe.op}\0${probe.path}`;
     if (probe.op === "text" && this.probes.has(key)) return;
     const previous = this.probes.get(key);
-    if (previous !== undefined && JSON.stringify(previous) !== JSON.stringify(probe)) {
+    const merged = previous === undefined ? null : sameFileContent(previous, probe);
+    if (
+      merged === null &&
+      previous !== undefined &&
+      JSON.stringify(previous) !== JSON.stringify(probe)
+    ) {
       // An input changed while this frontend was running.  The build may
       // still finish normally, but it is not safe to publish as a cache key.
       this.stable = false;
     }
-    this.probes.set(key, probe);
+    this.probes.set(key, merged ?? probe);
     if (probe.op === "file") {
       // A successful content read supersedes earlier existence probes for the
       // same path only when they also observed a file. A missing/non-file