Backup: probe the target with a real write instead of access(W_OK)

On the server the backup folder is a Synology NFS share with "map all
users to admin": access(W_OK) returns EACCES for uid 1000 although
writes succeed, so the first scheduled run failed with "not usable".
Create and remove a probe file instead; test that it leaves nothing.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

backup.js

@@ -169,7 +169,11 @@ class BackupService {
     try {
       const st = await fsp.stat(this.backupRoot);
       if (!st.isDirectory()) throw new Error('not a directory');
-      await fsp.access(this.backupRoot, fs.constants.W_OK);
+      // Probe with a real write: access(W_OK) is unreliable on NFS with squashed ids (Synology
+      // "map all users to admin" denies it although writes succeed).
+      const probe = path.join(this.backupRoot, `.circus-backup-probe-${process.pid}`);
+      await fsp.writeFile(probe, '');
+      await fsp.rm(probe, { force: true });
       const names = (await fsp.readdir(this.backupRoot)).filter(name => ARCHIVE.test(name)).sort();
       this.archives = await Promise.all(names.map(async name => ({ name, size: (await fsp.stat(path.join(this.backupRoot, name))).size, at: archiveTime(name) })));
       this.targetError = null;

docs/backup.md

@@ -14,7 +14,9 @@ holds it). Implementation: [`backup.js`](../backup.js); tests: `test/backup.test
 
 The folder must already exist and be writable by uid 1000 (`create_host_path: false`:
 a missing folder makes `docker compose up` fail instead of silently creating an empty
-one on the wrong disk). Use another disk or a NAS share, never a folder inside `DATA`.
+one on the wrong disk). Use another disk or a NAS share, never a folder inside `DATA`. Writability is checked by
+creating and removing a probe file (`.circus-backup-probe-<pid>`), not with `access()`, which
+NFS shares with squashed ids (e.g. Synology "map all users to admin") answer wrongly.
 Without `BACKUP_PATH`, Compose still mounts the unused default `./backup`, so that
 folder must exist too (`node tools/prepare-compose.js` creates it). Upgrading an
 existing deployment therefore needs either `BACKUP_PATH` in `.env` or an empty

test/backup.test.js

@@ -168,6 +168,16 @@ test('backups are off without BACKUP, and a missing folder is reported, never cr
   } finally { f.cleanup(); }
 });
 
+test('the writability probe leaves nothing behind in the backup folder', async () => {
+  const f = fixture();
+  try {
+    const backup = service(f);
+    await backup.refreshTarget();
+    assert.equal(backup.publicState().targetError, null);
+    assert.deepEqual(fs.readdirSync(f.target).filter(name => name.startsWith('.circus-backup-probe')), []);
+  } finally { f.cleanup(); }
+});
+
 test('only one run at a time', async () => {
   const f = fixture();
   try {