fix(test:free): a browser record removed mid-read by a shutting-down daemon is absent

Run 13 (feaa28d) needed the flaky-retry for 'owned detached browser
settlement > cancel': 2 unattributed failures instead of 1. A 16-vCPU
stress run reproduced it once in 64 with the cause:
'browser ownership unavailable'. On SIGINT the cancelled daemon removes its
state file; record() saw it exist, then lstat/read hit ENOENT, which
failure() reports as an ownership error. ENOENT there now means absent.
New mode cancel-vanishing-record forces the removal between the check and
the read (pre-fix: 2 unattributed, deterministic; now 1).

scripts/test-free-shards.ts

@@ -1543,12 +1543,12 @@ function trackShardBrowser(stateDir: string, env: NodeJS.ProcessEnv) {
     }
     return true;
   };
-  const record = (file: string): any => {
+  const record = (file: string): any => { try { // a record a shutting-down daemon removes mid-read is absent
     if (!fs.existsSync(file)) return null;
     const info = fs.lstatSync(file);
     if (!info.isFile() || info.size > 65536) throw new BrowserCleanupError('unsafe browser state record');
     return JSON.parse(fs.readFileSync(file, 'utf8'));
-  };
+  } catch (error) { if ((error as NodeJS.ErrnoException).code === 'ENOENT') return null; throw error; } };
   const nativeStart = async (capture: Capture, identity: Identity): Promise<string> => {
     check(capture);
     const key = `${identity.pid}:${identity.start}`;

test/test-free-shards.test.ts

@@ -150,6 +150,19 @@ if (role === 'leaf') {
       }
       return value;
     });
+  } else if (mode === 'cancel-vanishing-record') {
+    // The daemon removes its state file on SIGINT: one existence check sees it,
+    // and it is gone before the record is read.
+    const original = fs.existsSync;
+    spy = spyOn(fs, 'existsSync').mockImplementation((filename) => {
+      if (original(file('.interrupted')) && original(file('.ready')) && !original(file('.vanished'))
+        && String(filename) === JSON.parse(fs.readFileSync(file('.ready'), 'utf8')).stateFile) {
+        fs.rmSync(String(filename), {force:true});
+        fs.writeFileSync(file('.vanished'), 'true');
+        return true;
+      }
+      return original(filename);
+    });
   } else if (mode === 'directory-remove-failure') {
     const original = fs.rmSync;
     spy = spyOn(fs, 'rmSync').mockImplementation((filename, ...args) => {
@@ -188,8 +201,8 @@ function ownershipProcessAlive(identity: { pid: number; start: string; ticks: st
 describe('test-free-shards: owned detached browser settlement', () => {
   for (const mode of ['success', 'failure', 'timeout', 'cancel', 'cancel-force', 'endpoint-mix', 'full-state-mix',
     'terminal-mix', 'chromium-mix', 'replaced-pid', 'stale-child-start', 'replaced-start', 'exit-environment-race',
-    'unavailable-environment', 'directory-remove-failure', 'cancel-cold-probes', 'settle-cold-probes', 'settle-slow-exit']) {
-    test.skipIf(process.platform === 'win32' || ((['replaced-start', 'exit-environment-race', 'unavailable-environment', 'settle-slow-exit'].includes(mode) || mode.endsWith('cold-probes')) && process.platform !== 'linux'))(mode, async () => {
+    'unavailable-environment', 'directory-remove-failure', 'cancel-cold-probes', 'settle-cold-probes', 'settle-slow-exit', 'cancel-vanishing-record']) {
+    test.skipIf(process.platform === 'win32' || ((['replaced-start', 'exit-environment-race', 'unavailable-environment', 'settle-slow-exit', 'cancel-vanishing-record'].includes(mode) || mode.endsWith('cold-probes')) && process.platform !== 'linux'))(mode, async () => {
       const directory = fs.mkdtempSync(path.join(os.tmpdir(), 'free-owned-browser-'));
       const actor = path.join(directory, 'actor.ts');
       fs.writeFileSync(actor, OWNERSHIP_ACTOR);
@@ -253,6 +266,9 @@ process.exit(code);
           'unavailable-environment', 'directory-remove-failure', 'settle-cold-probes'].includes(mode);
         expect(result.status).toBe(mode === 'timeout' ? 'timed-out' : invalid || mode === 'failure' || mode.startsWith('cancel') ? 'failed' : 'passed');
         expect(result.unattributedFailures).toBe(invalid || mode === 'timeout' || mode.startsWith('cancel') ? 1 : 0);
+        // Run 13 (feaa28d) and a 16-vCPU stress run: the cancelled daemon's state file
+        // vanished between the existence check and the read ('browser ownership unavailable').
+        if (mode === 'cancel-vanishing-record') expect(fs.existsSync(path.join(directory, 'owned.vanished'))).toBe(true);
         if (invalid) {
           expect(fs.existsSync(path.dirname(own.stateFile))).toBe(true);
           expect(fs.existsSync(path.join(directory, 'owned.interrupted'))).toBe(mode === 'directory-remove-failure');