fix(cosift): wrap Pebble Close in sync.Once for idempotency (closes PILOT-190)

internal/store/pebble.go

@@ -67,6 +67,13 @@ type PebbleStore struct {
 	// where the previous one stopped, wrapping at the end.
 	frontierCursorMu sync.Mutex
 	frontierCursor   []byte
+
+	// PILOT-190: pebble.DB.Close() panics if called twice. Wrap teardown
+	// in sync.Once so repeated Close() calls (e.g. from layered cleanups
+	// or signal handlers) are idempotent. closeErr caches the first
+	// Close result so every caller sees the same outcome.
+	closeOnce sync.Once
+	closeErr  error
 }
 
 const (
@@ -188,7 +195,16 @@ func OpenPebble(path string) (*PebbleStore, error) {
 }
 
 // Close flushes and closes the underlying Pebble DB.
-func (p *PebbleStore) Close() error { return p.db.Close() }
+//
+// Idempotent: pebble.DB.Close() panics if called twice (PILOT-190), so
+// the teardown is wrapped in sync.Once. Subsequent calls are no-ops and
+// return the same error as the first call.
+func (p *PebbleStore) Close() error {
+	p.closeOnce.Do(func() {
+		p.closeErr = p.db.Close()
+	})
+	return p.closeErr
+}
 
 // Checkpoint creates a consistent, point-in-time snapshot of the DB at destDir
 // via hard-links to live SSTs. Cheap (no data copy) and safe to tar afterwards

internal/store/pebble_test.go

@@ -661,3 +661,32 @@ func TestPebbleRecrawlURL(t *testing.T) {
 		t.Errorf("missing URL: want ErrNotFound, got %v", err)
 	}
 }
+
+// TestPebbleStore_CloseIsIdempotent — regression for PILOT-190.
+// pebble.DB.Close() panics if called twice. PebbleStore wraps it in
+// sync.Once so repeated Close() calls are safe and return the same
+// error. Opens a store, calls Close() three times, asserts no panic
+// and that every call returns the same value as the first.
+func TestPebbleStore_CloseIsIdempotent(t *testing.T) {
+	// Skip newPebbleStore — its t.Cleanup() would also call Close()
+	// and we want to control invocation count explicitly.
+	dir := filepath.Join(t.TempDir(), "pebble")
+	p, err := OpenPebble(dir)
+	if err != nil {
+		t.Fatalf("OpenPebble: %v", err)
+	}
+
+	defer func() {
+		if r := recover(); r != nil {
+			t.Fatalf("Close() panicked on repeated invocation: %v", r)
+		}
+	}()
+
+	first := p.Close()
+	for i := 2; i <= 3; i++ {
+		got := p.Close()
+		if got != first {
+			t.Fatalf("Close() call #%d returned %v; want %v (cached first result)", i, got, first)
+		}
+	}
+}