vfs/disk: Harden pending-window test against empty-dir race

Empty-dir disk init closes initDone almost immediately, so
TestDiskTierSignalMixedPendingReady can observe DiskTierReady=0 then
a heartbeat that already saw ready. Register a per-root init hold so
the test can keep the pending window open without slowing production.

Link: #36

vfs/disk/disk.go

@@ -45,6 +45,25 @@ type DiskFS struct {
 	initCloseOnce sync.Once
 	startupEvict  func(vfs.VFS, uint) uint // passed to New (via gc.GetGCAlgorithm); invoked as last step of bg init if over cap (no post-ctor race)
 	metrics       *metrics.Metrics
+	// initHold, if non-nil, is received on before closing initDone (test pending-window hold).
+	initHold <-chan struct{}
+}
+
+// initHolds is a per-root registry of optional init holds (root path -> <-chan struct{}).
+// Tests call RegisterInitHold before New so that instance copies the channel and waits
+// before closing initDone; production never registers, so init is unchanged.
+var initHolds sync.Map
+
+// RegisterInitHold registers a channel that DiskFS.New for this root copies onto that
+// instance. calculateSizeAndPopulateIndex receives on it before closing initDone, so
+// tests can observe the pending-attach window. Other DiskFS roots are unaffected.
+func RegisterInitHold(root string, ch <-chan struct{}) {
+	initHolds.Store(root, ch)
+}
+
+// ClearInitHold removes a previously registered hold for root.
+func ClearInitHold(root string) {
+	initHolds.Delete(root)
 }
 
 // shardPath converts a Steam cache key to a sharded directory path to reduce inode pressure
@@ -129,6 +148,12 @@ func New(root string, capacity int64, evict func(vfs.VFS, uint) uint) (*DiskFS,
 		startupEvict: evict,
 	}
 
+	if v, ok := initHolds.Load(root); ok {
+		if ch, ok := v.(<-chan struct{}); ok {
+			d.initHold = ch
+		}
+	}
+
 	d.initDone = make(chan struct{})
 	// Launch heavy population asynchronously so New returns fast (scans millions of files without blocking ctor or using O(N) temp RAM).
 	// The initDone barrier ensures first Size() and subsequent ops (including late tier attach) see fully populated + post-eviction state.
@@ -152,7 +177,7 @@ func (d *DiskFS) calculateSizeAndPopulateIndex() {
 		if r := recover(); r != nil {
 			logger.Logger.Error().Interface("recovered_panic", r).Msg("calculateSizeAndPopulateIndex panicked; ensuring initDone closed to unblock Size waiters and prevent hang")
 		}
-		d.initCloseOnce.Do(func() { close(d.initDone) })
+		d.closeInitDone()
 	}()
 
 	tstart := time.Now()
@@ -243,7 +268,19 @@ func (d *DiskFS) calculateSizeAndPopulateIndex() {
 
 	// Signal readiness: Size() and callers (late tier attach + Evict*) now see correct populated + post-eviction state.
 	// Use Once (recover path also uses it) to guarantee exactly one close even under panic.
-	d.initCloseOnce.Do(func() { close(d.initDone) })
+	d.closeInitDone()
+}
+
+// closeInitDone receives on a copied test hold (if any) then closes initDone once.
+// Both the normal end of calculateSizeAndPopulateIndex and the panic-recovery defer
+// call this so Size() waiters unblock in either path.
+func (d *DiskFS) closeInitDone() {
+	d.initCloseOnce.Do(func() {
+		if d.initHold != nil {
+			<-d.initHold
+		}
+		close(d.initDone)
+	})
 }
 
 // insertBatch populates info/LRU under lock for a bounded batch (follows maxEvictBatch pattern for short critical sections).

vfs/disk/disk_test.go

@@ -662,3 +662,57 @@ func TestDiskFS_NewMkdirError(t *testing.T) {
 		t.Errorf("expected mkdir failure error for file-as-dir, got: %v", err)
 	}
 }
+
+// TestDiskFS_InitHoldBlocksOnlyRegisteredRoot covers the per-root init hold:
+// Size() stays blocked while the hold is open, and a DiskFS on a different root
+// does not wait on that hold.
+func TestDiskFS_InitHoldBlocksOnlyRegisteredRoot(t *testing.T) {
+	td := t.TempDir()
+	hold := make(chan struct{})
+	var holdOnce sync.Once
+	closeHold := func() { holdOnce.Do(func() { close(hold) }) }
+	RegisterInitHold(td, hold)
+	t.Cleanup(func() {
+		closeHold()
+		ClearInitHold(td)
+	})
+
+	d, err := New(td, 10*1024*1024, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	blocked := make(chan struct{})
+	go func() {
+		_ = d.Size()
+		close(blocked)
+	}()
+	select {
+	case <-blocked:
+		t.Fatal("Size returned while init hold still open")
+	case <-time.After(50 * time.Millisecond):
+	}
+
+	td2 := t.TempDir()
+	d2, err := New(td2, 10*1024*1024, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	other := make(chan struct{})
+	go func() {
+		_ = d2.Size()
+		close(other)
+	}()
+	select {
+	case <-other:
+	case <-time.After(2 * time.Second):
+		t.Fatal("unrelated DiskFS Size hung; init hold leaked across roots")
+	}
+
+	closeHold()
+	select {
+	case <-blocked:
+	case <-time.After(2 * time.Second):
+		t.Fatal("Size did not return after init hold released")
+	}
+}