fix(vfs): fail fast when write buffer initialization fails (#974)

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

vfs.go

@@ -591,7 +591,7 @@ func (f *VFSFile) Open() error {
 
 		// Initialize write buffer file for durability (discards any existing buffer)
 		if err := f.initWriteBuffer(); err != nil {
-			f.logger.Warn("failed to initialize write buffer", "error", err)
+			return fmt.Errorf("initialize write buffer: %w", err)
 		}
 	}
 
@@ -1256,11 +1256,8 @@ func (f *VFSFile) createLTXFromDirty() io.Reader {
 
 // initWriteBuffer initializes the write buffer file for durability.
 // Any existing buffer content is discarded since unsync'd changes are lost on restart.
+// This function is only called when writeEnabled is true, which guarantees bufferPath is set.
 func (f *VFSFile) initWriteBuffer() error {
-	if f.bufferPath == "" {
-		return nil // No buffer configured
-	}
-
 	// Ensure parent directory exists
 	if err := os.MkdirAll(filepath.Dir(f.bufferPath), 0755); err != nil {
 		return fmt.Errorf("create buffer directory: %w", err)
@@ -1282,10 +1279,6 @@ func (f *VFSFile) initWriteBuffer() error {
 // Otherwise, it appends to the end of the file.
 // Must be called with f.mu held.
 func (f *VFSFile) writeToBuffer(pgno uint32, data []byte) error {
-	if f.bufferFile == nil {
-		return nil // No buffer configured
-	}
-
 	var writeOffset int64
 	if existingOff, ok := f.dirty[pgno]; ok {
 		// Page already exists - overwrite at same offset
@@ -1309,10 +1302,6 @@ func (f *VFSFile) writeToBuffer(pgno uint32, data []byte) error {
 
 // clearWriteBuffer clears and resets the write buffer after successful sync.
 func (f *VFSFile) clearWriteBuffer() error {
-	if f.bufferFile == nil {
-		return nil
-	}
-
 	// Truncate file to zero
 	if err := f.bufferFile.Truncate(0); err != nil {
 		return fmt.Errorf("truncate buffer: %w", err)

vfs_write_test.go

@@ -229,11 +229,15 @@ func TestVFSFile_WriteEnabled(t *testing.T) {
 	createTestLTXFile(t, client, 1, pageSize, 1, map[uint32][]byte{1: initialPage})
 
 	// Create VFSFile directly with write enabled
+	tmpDir := t.TempDir()
+	bufferPath := tmpDir + "/write-buffer"
+
 	logger := slog.Default()
 	f := NewVFSFile(client, "test.db", logger)
 	f.writeEnabled = true
 	f.dirty = make(map[uint32]int64)
 	f.syncInterval = 0
+	f.bufferPath = bufferPath
 
 	if err := f.Open(); err != nil {
 		t.Fatal(err)
@@ -639,6 +643,54 @@ func TestVFSFile_WriteBufferClearAfterSync(t *testing.T) {
 	}
 }
 
+func TestVFSFile_OpenFailsWithInvalidBufferPath(t *testing.T) {
+	client := newWriteTestReplicaClient()
+
+	pageSize := uint32(4096)
+	initialPage := make([]byte, pageSize)
+	createTestLTXFile(t, client, 1, pageSize, 1, map[uint32][]byte{1: initialPage})
+
+	logger := slog.Default()
+	f := NewVFSFile(client, "test.db", logger)
+	f.writeEnabled = true
+	f.dirty = make(map[uint32]int64)
+	f.syncInterval = 0
+	f.bufferPath = "/nonexistent/path/that/cannot/be/created/buffer"
+
+	err := f.Open()
+	if err == nil {
+		f.Close()
+		t.Fatal("expected Open to fail with invalid buffer path")
+	}
+}
+
+func TestVFSFile_BufferFileAlwaysCreatedWhenWriteEnabled(t *testing.T) {
+	client := newWriteTestReplicaClient()
+
+	pageSize := uint32(4096)
+	initialPage := make([]byte, pageSize)
+	createTestLTXFile(t, client, 1, pageSize, 1, map[uint32][]byte{1: initialPage})
+
+	tmpDir := t.TempDir()
+	bufferPath := tmpDir + "/write-buffer"
+
+	logger := slog.Default()
+	f := NewVFSFile(client, "test.db", logger)
+	f.writeEnabled = true
+	f.dirty = make(map[uint32]int64)
+	f.syncInterval = 0
+	f.bufferPath = bufferPath
+
+	if err := f.Open(); err != nil {
+		t.Fatal(err)
+	}
+	defer f.Close()
+
+	if f.bufferFile == nil {
+		t.Fatal("bufferFile should never be nil when writeEnabled is true")
+	}
+}
+
 func TestVFSFile_OpenNewDatabase(t *testing.T) {
 	// Test opening a VFSFile with write mode enabled when no LTX files exist (new database)
 	client := newWriteTestReplicaClient()