headerfs: fail gracefully on write

This commit adds recovery mechanism from
failures that may happen during headers I/O write

Author: mohamedawnallah

headerfs/file.go

@@ -3,6 +3,7 @@ package headerfs
 import (
 	"bytes"
 	"fmt"
+	"io"
 
 	"github.com/btcsuite/btcd/chaincfg/chainhash"
 	"github.com/btcsuite/btcd/wire"
@@ -16,10 +17,31 @@ type ErrHeaderNotFound struct {
 
 // appendRaw appends a new raw header to the end of the flat file.
 func (h *headerStore) appendRaw(header []byte) error {
-	if _, err := h.file.Write(header); err != nil {
+	// Get current file position before writing. We'll use this position to
+	// revert to if the write fails partially.
+	currentPos, err := h.file.Seek(0, io.SeekCurrent)
+	if err != nil {
 		return err
 	}
 
+	n, err := h.file.Write(header)
+	if err != nil {
+		// If we wrote some bytes but not all (partial write),
+		// truncate the file back to its original size to maintain consistency.
+		// This removes the partial/corrupt header.
+		if n > 0 {
+			truncErr := h.file.Truncate(currentPos)
+			if truncErr != nil {
+				return fmt.Errorf("failed to write header type %s: partial "+
+					"write (%d bytes), write error: %w, truncate "+
+					"error: %v", h.indexType, n, err, truncErr)
+			}
+		}
+
+		return fmt.Errorf("failed to write header type %s: write "+
+			"error: %w", h.indexType, err)
+	}
+
 	return nil
 }
 

headerfs/file_test.go

@@ -0,0 +1,224 @@
+package headerfs
+
+import (
+	"bytes"
+	"errors"
+	"fmt"
+	"io"
+	"os"
+	"strings"
+	"testing"
+)
+
+// TestAppendRow verifies that headerStore.appendRaw correctly appends data to
+// the file, handles full and partial write errors, and properly recovers from
+// failures.
+func TestAppendRow(t *testing.T) {
+	tests := []struct {
+		name          string
+		initialData   []byte
+		headerToWrite []byte
+		writeFn       func([]byte, File) (int, error)
+		truncFn       func(int64, File) error
+		expected      []byte
+		wantErr       bool
+		errMsg        string
+	}{
+		{
+			name:          "NormalWrite ValidHeader DataAppendedSuccessfully",
+			initialData:   []byte{0x01, 0x02, 0x03},
+			headerToWrite: []byte{0x04, 0x05, 0x06},
+			expected:      []byte{0x01, 0x02, 0x03, 0x04, 0x05, 0x06},
+		},
+		{
+			name:          "WriteError ZeroBytesWritten OriginalDataPreserved",
+			initialData:   []byte{0x01, 0x02, 0x03},
+			headerToWrite: []byte{0x04, 0x05, 0x06},
+			writeFn: func(p []byte, _ File) (int, error) {
+				return 0, errors.New("simulated write failure")
+			},
+			expected: []byte{0x01, 0x02, 0x03},
+			wantErr:  true,
+			errMsg:   "simulated write failure",
+		},
+		{
+			name:          "PartialWrite WriteErrorMidway RollsBackToOriginal",
+			initialData:   []byte{0x01, 0x02, 0x03},
+			headerToWrite: []byte{0x04, 0x05, 0x06},
+			writeFn: func(p []byte, file File) (int, error) {
+				// Mock a partial write - write the first two bytes.
+				n, err := file.Write(p[:2])
+				if err != nil {
+					return n, err
+				}
+
+				return n, errors.New("simulated partial write failure")
+			},
+			expected: []byte{0x01, 0x02, 0x03},
+			wantErr:  true,
+			errMsg:   "simulated partial write failure",
+		},
+		{
+			name:          "PartialWrite TruncateFailure ReportsCompoundError",
+			initialData:   []byte{0x01, 0x02, 0x03},
+			headerToWrite: []byte{0x04, 0x05, 0x06},
+			writeFn: func(p []byte, file File) (int, error) {
+				// Mock a partial write - write just the first byte.
+				n, err := file.Write(p[:1])
+				if err != nil {
+					return n, err
+				}
+
+				return n, errors.New("simulated partial write failure")
+			},
+			truncFn: func(size int64, _ File) error {
+				return errors.New("simulated truncate failure")
+			},
+			expected: []byte{0x01, 0x02, 0x03, 0x04},
+			wantErr:  true,
+			errMsg: fmt.Sprintf("failed to write header type %s: partial "+
+				"write (1 bytes), write error: simulated partial write "+
+				"failure, truncate error: simulated truncate failure", Block),
+		},
+		{
+			name:          "PartialWrite TruncateFailureMiddle Unrecovered",
+			initialData:   []byte{0x01, 0x02, 0x03},
+			headerToWrite: []byte{0x04, 0x05, 0x06},
+			writeFn: func(p []byte, file File) (int, error) {
+				// Mock a partial write - write the first two bytes.
+				n, err := file.Write(p[:2])
+				if err != nil {
+					return n, err
+				}
+
+				return n, errors.New("simulated partial write failure")
+			},
+			truncFn: func(size int64, file File) error {
+				// Simulate an incomplete truncation: shrink the file by just
+				// one byte, leaving part of the partial write data in place
+				// (i.e., not fully removing the partially written header from
+				// the end of the file).
+				err := file.Truncate(4)
+				if err != nil {
+					return err
+				}
+
+				return errors.New("simulated truncate failure")
+			},
+			expected: []byte{0x01, 0x02, 0x03, 0x04},
+			wantErr:  true,
+			errMsg: fmt.Sprintf("failed to write header type %s: partial "+
+				"write (2 bytes), write error: simulated partial write "+
+				"failure, truncate error: simulated truncate failure", Block),
+		},
+		{
+			name:          "NormalWrite ValidHeader DataAppendedSuccessfully",
+			initialData:   []byte{},
+			headerToWrite: []byte{0x01, 0x02, 0x03},
+			expected:      []byte{0x01, 0x02, 0x03},
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			// Create a temporary file for testing.
+			tmpFile, err := os.CreateTemp(t.TempDir(), "header_store_test")
+			if err != nil {
+				t.Fatalf("Failed to create temp file: %v", err)
+			}
+			defer os.Remove(tmpFile.Name())
+
+			// Write initial data.
+			if _, err := tmpFile.Write(test.initialData); err != nil {
+				t.Fatalf("Failed to write initial data: %v", err)
+			}
+
+			// Reset the file position to the end of initial data.
+			_, err = tmpFile.Seek(int64(len(test.initialData)), io.SeekStart)
+			if err != nil {
+				t.Fatalf("Failed to seek: %v", err)
+			}
+
+			// Create a mock file that wraps the real file.
+			mockFile := &mockFile{
+				File:    tmpFile,
+				writeFn: test.writeFn,
+				truncFn: test.truncFn,
+			}
+
+			// Create a header store with our mock file.
+			h := &headerStore{
+				file:        mockFile,
+				headerIndex: &headerIndex{indexType: Block},
+			}
+
+			// Call the function being tested.
+			err = h.appendRaw(test.headerToWrite)
+			if err == nil && test.wantErr {
+				t.Fatal("expected an error, but got none")
+			}
+			if err != nil && !test.wantErr {
+				t.Fatalf("unexpected error: %v", err)
+			}
+			if err != nil && test.wantErr &&
+				!strings.Contains(err.Error(), test.errMsg) {
+
+				t.Errorf("expected error message %q to be "+
+					"in %q", test.errMsg, err.Error())
+			}
+
+			// Reset file position to start for reading.
+			if _, err := tmpFile.Seek(0, io.SeekStart); err != nil {
+				t.Fatalf("Failed to seek to start: %v", err)
+			}
+
+			// Read the file contents.
+			actualData, err := io.ReadAll(tmpFile)
+			if err != nil {
+				t.Fatalf("Failed to read file: %v", err)
+			}
+
+			// Compare expected vs. actual file contents.
+			if !bytes.Equal(actualData, test.expected) {
+				t.Fatalf("Expected file data: %v, "+
+					"got: %v", test.expected, actualData)
+			}
+		})
+	}
+}
+
+// mockFile wraps a real file but allows us to override the Write, Sync, and
+// Truncate methods.
+type mockFile struct {
+	*os.File
+	writeFn func([]byte, File) (int, error)
+	syncFn  func() error
+	truncFn func(int64, File) error
+}
+
+// Write implements the Write method for FileInterface.
+func (m *mockFile) Write(p []byte) (int, error) {
+	if m.writeFn != nil {
+		return m.writeFn(p, m.File)
+	}
+	return m.File.Write(p)
+}
+
+// Sync implements the Sync method for FileInterface.
+func (m *mockFile) Sync() error {
+	if m.syncFn != nil {
+		return m.syncFn()
+	}
+	return m.File.Sync()
+}
+
+// Truncate implements the Truncate method for FileInterface.
+func (m *mockFile) Truncate(size int64) error {
+	if m.truncFn != nil {
+		return m.truncFn(size, m.File)
+	}
+	return m.File.Truncate(size)
+}
+
+// Ensure mockFile implements necessary interfaces.
+var _ io.Writer = &mockFile{}