test: cover sync clobber snapshot collisions

src/config/io.clobber-snapshot.test.ts

@@ -75,6 +75,23 @@ describe("config clobber snapshots", () => {
     }
   }
 
+  function touchClobberFilesByContentOrderSync(configPath: string): void {
+    const dir = path.dirname(configPath);
+    const prefix = `${path.basename(configPath)}.clobbered.`;
+    for (const entry of fs.readdirSync(dir)) {
+      if (!entry.startsWith(prefix)) {
+        continue;
+      }
+      const targetPath = path.join(dir, entry);
+      const match = /^polluted-(\d+)\n$/.exec(fs.readFileSync(targetPath, "utf-8"));
+      if (!match) {
+        continue;
+      }
+      const touchedAt = new Date(`2026-05-03T00:00:${match[1].padStart(2, "0")}.000Z`);
+      fs.utimesSync(targetPath, touchedAt, touchedAt);
+    }
+  }
+
   it("keeps concurrent async snapshots under the per-path cap by rotating oldest files", async () => {
     await withCase(async (configPath) => {
       const warn = vi.fn();
@@ -227,4 +244,42 @@ describe("config clobber snapshots", () => {
       expect(capWarnings).toHaveLength(1);
     });
   });
+
+  it("keeps rotating same-timestamp sync snapshots after the base artifact is reused", async () => {
+    await withCase(async (configPath) => {
+      const warn = vi.fn();
+      const observedAt = "2026-05-03T00:00:00.000Z";
+
+      for (let index = 0; index < CONFIG_CLOBBER_SNAPSHOT_LIMIT; index++) {
+        persistBoundedClobberedConfigSnapshotSync({
+          deps: { fs, logger: { warn } },
+          configPath,
+          raw: `polluted-${index}\n`,
+          observedAt,
+        });
+      }
+      touchClobberFilesByContentOrderSync(configPath);
+
+      for (
+        let index = CONFIG_CLOBBER_SNAPSHOT_LIMIT;
+        index < CONFIG_CLOBBER_SNAPSHOT_LIMIT + 3;
+        index++
+      ) {
+        persistBoundedClobberedConfigSnapshotSync({
+          deps: { fs, logger: { warn } },
+          configPath,
+          raw: `polluted-${index}\n`,
+          observedAt,
+        });
+      }
+
+      const clobberFiles = await listClobberFiles(configPath);
+      expect(clobberFiles).toHaveLength(CONFIG_CLOBBER_SNAPSHOT_LIMIT);
+      const contents = await readClobberFileContents(configPath);
+      expect(contents).not.toContain("polluted-0\n");
+      expect(contents).not.toContain("polluted-1\n");
+      expect(contents).not.toContain("polluted-2\n");
+      expect(contents).toContain(`polluted-${CONFIG_CLOBBER_SNAPSHOT_LIMIT + 2}\n`);
+    });
+  });
 });

src/config/io.clobber-snapshot.ts

@@ -137,6 +137,20 @@ function compareClobberedSiblings(
   );
 }
 
+function createClobberedSiblingSnapshot(params: {
+  dir: string;
+  entry: string;
+  prefix: string;
+  mtimeMs: number;
+}): ClobberedSiblingSnapshot {
+  return {
+    name: params.entry,
+    path: path.join(params.dir, params.entry),
+    timestampKey: params.entry.slice(params.prefix.length).replace(/-\d{2}$/, ""),
+    mtimeMs: params.mtimeMs,
+  };
+}
+
 async function listClobberedSiblings(
   deps: ConfigClobberSnapshotDeps,
   dir: string,
@@ -149,14 +163,15 @@ async function listClobberedSiblings(
       if (!entry.startsWith(prefix)) {
         continue;
       }
-      const snapshotPath = path.join(dir, entry);
-      const stat = await deps.fs.promises.stat(snapshotPath).catch(() => null);
-      snapshots.push({
-        name: entry,
-        path: snapshotPath,
-        timestampKey: entry.slice(prefix.length).replace(/-\d{2}$/, ""),
-        mtimeMs: stat?.mtimeMs ?? 0,
-      });
+      const stat = await deps.fs.promises.stat(path.join(dir, entry)).catch(() => null);
+      snapshots.push(
+        createClobberedSiblingSnapshot({
+          dir,
+          entry,
+          prefix,
+          mtimeMs: stat?.mtimeMs ?? 0,
+        }),
+      );
     }
     return snapshots.toSorted(compareClobberedSiblings);
   } catch {
@@ -175,14 +190,15 @@ function listClobberedSiblingsSync(
       if (!entry.startsWith(prefix)) {
         continue;
       }
-      const snapshotPath = path.join(dir, entry);
-      const stat = deps.fs.statSync(snapshotPath, { throwIfNoEntry: false });
-      snapshots.push({
-        name: entry,
-        path: snapshotPath,
-        timestampKey: entry.slice(prefix.length).replace(/-\d{2}$/, ""),
-        mtimeMs: stat?.mtimeMs ?? 0,
-      });
+      const stat = deps.fs.statSync(path.join(dir, entry), { throwIfNoEntry: false });
+      snapshots.push(
+        createClobberedSiblingSnapshot({
+          dir,
+          entry,
+          prefix,
+          mtimeMs: stat?.mtimeMs ?? 0,
+        }),
+      );
     }
     return snapshots.toSorted(compareClobberedSiblings);
   } catch {