fix(main): enumerate git-common worktrees every tick to catch coarse-FS add/remove

The worktrees-dir readdir was gated on the dir's mtime:ctime:ino:size signature, so on
a coarse-mtime/FAT filesystem a same-granule add+remove (all four fields collide) went
undetected until the ~30s index backstop. A single readdir of a small dir is negligible
next to the per-entry structural stats that already run every tick, so always enumerate —
the listing is the authoritative add/remove signal. The expensive per-entry index read
stays gated on each entry's own dir signature; onFullScan now reflects the ungated
index-metadata backstop fan-out (the real periodic cost) rather than the readdir.

Co-authored-by: Orca <help@stably.ai>

src/main/ipc/worktree-git-common-polling.ts

@@ -132,21 +132,20 @@ async function snapshotGitCommon(
     dirSignature(worktreesDir),
     includePrimary ? snapshotPrimaryCheckoutSignatures(commonDirPath) : new Map<string, string>()
   ])
-  const shouldReadWorktreesDir =
-    forceFullScan || !previous || previous.worktreesDirSignature !== worktreesDirSignature
+  // Why: enumerate the worktrees dir EVERY tick rather than gating the readdir on its stat signature.
+  // A single readdir of a small dir is negligible next to the per-entry structural stats that already
+  // run each tick, and the signature gate could miss a same-granule add+remove on a coarse-mtime/FAT
+  // filesystem (its size/mtime/ino/ctime all collide), leaving a linked worktree add/remove undetected
+  // until the ~30s index backstop (#9882 review). The listing is the authoritative add/remove signal.
   let entryPaths: string[]
-  if (shouldReadWorktreesDir) {
-    try {
-      const entries = await readdir(worktreesDir, { withFileTypes: true })
-      entryPaths = entries
-        .filter((entry) => entry.isDirectory())
-        .map((entry) => join(worktreesDir, entry.name))
-    } catch {
-      // Missing worktrees dir is normal for repos without linked worktrees.
-      entryPaths = []
-    }
-  } else {
-    entryPaths = [...previous.entries.keys()]
+  try {
+    const entries = await readdir(worktreesDir, { withFileTypes: true })
+    entryPaths = entries
+      .filter((entry) => entry.isDirectory())
+      .map((entry) => join(worktreesDir, entry.name))
+  } catch {
+    // Missing worktrees dir is normal for repos without linked worktrees.
+    entryPaths = []
   }
 
   const entries = new Map<string, GitCommonEntrySnapshot>()
@@ -156,13 +155,14 @@ async function snapshotGitCommon(
       entries.set(entryPath, await snapshotGitCommonEntry(entryPath, previousEntry, forceFullScan))
     })
   )
-  // Why: structural leaves are re-stat'd every tick now, so the only gated work is the worktrees-dir
-  // readdir; onFullScan reflects that enumeration running (vs a gated skip when the dir is unchanged).
+  // Why: the expensive per-entry `index` read stays gated on each entry's own dir signature; onFullScan
+  // now reflects an ungated index-metadata backstop fan-out (forceFullScan) — the real periodic cost —
+  // rather than the always-run worktrees-dir readdir.
   return {
     worktreesDirSignature,
     entries,
     primarySignatures,
-    didFullScan: shouldReadWorktreesDir
+    didFullScan: forceFullScan
   }
 }
 

src/main/ipc/worktree-git-common-watch.test.ts

@@ -304,7 +304,10 @@ describe('worktree git-common polling gate (non-darwin)', () => {
     cleanups.push(() => watch.unsubscribe())
   }
 
-  it('skips worktrees readdir and entry metadata fan-out on idle ticks', async () => {
+  it('skips the ungated index-metadata backstop on idle ticks', async () => {
+    // Why: idle ticks still re-stat structural leaves and list the (small) worktrees dir cheaply, but the
+    // heavier ungated per-entry index fan-out (onFullScan) must NOT run until the backstop — and no
+    // spurious events are emitted while nothing changes.
     const commonDir = await makePollingCommonDir()
     const entry = join(commonDir, 'worktrees', 'idle')
     await mkdir(join(entry, 'logs'), { recursive: true })
@@ -320,7 +323,10 @@ describe('worktree git-common polling gate (non-darwin)', () => {
     expect(received.flat()).toHaveLength(0)
   })
 
-  it('fans out when linked worktrees are added and removed', async () => {
+  it('detects linked worktree add and remove from the every-tick readdir', async () => {
+    // Why: the worktrees-dir listing runs every tick (not gated on its stat signature), so an add/remove
+    // surfaces within one poll interval even on a coarse-mtime filesystem whose dir signature would not
+    // move — without waiting on the index backstop (onFullScan).
     const commonDir = await makePollingCommonDir()
     const received: WorktreeBasePollEvent[][] = []
     const fullScans = vi.fn()
@@ -331,20 +337,17 @@ describe('worktree git-common polling gate (non-darwin)', () => {
     await vi.waitFor(() => {
       expect(received.flat()).toContainEqual({ type: 'create', path: entry })
     })
-    const scansAfterAdd = fullScans.mock.calls.length
-    expect(scansAfterAdd).toBeGreaterThan(0)
 
     await rm(entry, { recursive: true })
     await vi.waitFor(() => {
       expect(received.flat()).toContainEqual({ type: 'delete', path: entry })
     })
-    expect(fullScans.mock.calls.length).toBeGreaterThan(scansAfterAdd)
   })
 
-  it('detects an in-place structural (HEAD) write on a known entry every tick, without a readdir', async () => {
+  it('detects an in-place structural (HEAD) write on a known entry every tick, without the index backstop', async () => {
     // Why: a raw HEAD/gitdir/config.worktree rewrite does not bump the entry-dir mtime, so the
     // structural leaves are re-stat'd every tick (never gated) — the change surfaces within one tick
-    // and does NOT require the gated worktrees-dir readdir (onFullScan).
+    // and does NOT require the ungated index-metadata backstop (onFullScan).
     const commonDir = await makePollingCommonDir()
     const entry = join(commonDir, 'worktrees', 'structural')
     await mkdir(entry)