fix(server): preserve racy edits in review diff previews (#12613)

Co-authored-by: shivam <91240327+shivamhwp@users.noreply.github.com>

Author: tris203

apps/server/src/vcs/GitVcsDriverCore.test.ts

@@ -1449,6 +1449,51 @@ it.layer(TestLayer)("GitVcsDriver core integration", (it) => {
       }),
     );
 
+    for (const [timestamp, splitIndex] of [
+      [1_700_000_000, false],
+      [1_700_000_000.9999, false],
+      [1_700_000_000, true],
+      [1_700_000_000.9999, true],
+    ] as const) {
+      it.effect(
+        `preserves same-size edits with a racy review index (${timestamp}, split: ${splitIndex})`,
+        () =>
+          Effect.gen(function* () {
+            const cwd = yield* makeTmpDir();
+            yield* initRepoWithCommit(cwd);
+            const driver = yield* GitVcsDriver.GitVcsDriver;
+            const fileSystem = yield* FileSystem.FileSystem;
+            const path = yield* Path.Path;
+            const filePath = path.join(cwd, "tracked.txt");
+            const indexPath = path.join(cwd, ".git", "index");
+            // Reproduce a same-timestamp edit without relying on filesystem clock resolution.
+            yield* git(cwd, ["config", "core.trustctime", "false"]);
+            yield* writeTextFile(cwd, "tracked.txt", "before\n");
+            yield* fileSystem.utimes(filePath, timestamp, timestamp);
+            yield* git(cwd, ["add", "tracked.txt"]);
+            yield* git(cwd, ["commit", "-m", "record racy file"]);
+            if (splitIndex) yield* git(cwd, ["update-index", "--split-index"]);
+            yield* fileSystem.utimes(indexPath, timestamp, timestamp);
+            const originalIndex = yield* fileSystem.readFile(indexPath);
+            const originalIndexMtime = (yield* fileSystem.stat(indexPath)).mtime;
+            yield* writeTextFile(cwd, "tracked.txt", "after!\n");
+            yield* fileSystem.utimes(filePath, timestamp, timestamp);
+            yield* writeTextFile(cwd, "untracked.txt", "new\n");
+
+            const preview = yield* driver.getReviewDiffPreview({ cwd });
+            const dirty = preview.sources.find((source) => source.kind === "working-tree")!;
+            assert.deepStrictEqual(dirty.files, [
+              { path: "tracked.txt", previousPath: null, additions: 1, deletions: 1 },
+              { path: "untracked.txt", previousPath: null, additions: 1, deletions: 0 },
+            ]);
+            assert.include(dirty.diff, "-before");
+            assert.include(dirty.diff, "+after!");
+            assert.deepStrictEqual(yield* fileSystem.readFile(indexPath), originalIndex);
+            assert.deepStrictEqual((yield* fileSystem.stat(indexPath)).mtime, originalIndexMtime);
+          }),
+      );
+    }
+
     it.effect("keeps complete stats for files beyond the combined patch limit", () =>
       Effect.gen(function* () {
         const cwd = yield* makeTmpDir();

apps/server/src/vcs/GitVcsDriverCore.ts

@@ -2360,7 +2360,15 @@ export const makeGitVcsDriverCore = Effect.fn("makeGitVcsDriverCore")(function*
       prefix: `t3code-review-index-${process.pid}-`,
     });
     const indexExists = yield* fileSystem.exists(indexPath);
-    if (indexExists) yield* fileSystem.copyFile(indexPath, tempIndexPath);
+    if (indexExists) {
+      const { mtime } = yield* fileSystem.stat(indexPath);
+      yield* fileSystem.copyFile(indexPath, tempIndexPath);
+      // A newer copy timestamp hides racily clean edits. Round down before Git reads or rewrites it.
+      const indexTime = Option.isSome(mtime)
+        ? Math.max(0, Math.floor((mtime.value.getTime() - 1) / 1000))
+        : 0;
+      yield* fileSystem.utimes(tempIndexPath, indexTime, indexTime);
+    }
     const env = { GIT_INDEX_FILE: tempIndexPath } satisfies NodeJS.ProcessEnv;
     const tempIndexConfig = [
       "-c",