add test

go/mcap/indexed_message_iterator.go

@@ -17,7 +17,7 @@ import (
 	"github.com/pierrec/lz4/v4"
 )
 
-var ErrBadOffset = errors.New("cannot seek to offset")
+var ErrBadOffset = errors.New("invalid offset")
 
 const (
 	chunkBufferGrowthMultiple = 1.2
@@ -74,23 +74,24 @@ type indexedMessageIterator struct {
 	metadataCallback func(*Metadata) error
 }
 
-func (it *indexedMessageIterator) seekTo(offset uint64) (int64, error) {
+func (it *indexedMessageIterator) seekTo(offset uint64) error {
 	if offset > uint64(math.MaxInt64) {
-		return 0, fmt.Errorf("%w: %d > int64 max", ErrBadOffset, offset)
+		return fmt.Errorf("%w: %d > int64 max", ErrBadOffset, offset)
 	}
 	signedOffset := int64(offset)
+	fmt.Printf("signedOffset: %d, size: %d\n", signedOffset, it.fileSize)
 	if signedOffset >= it.fileSize {
-		return 0, fmt.Errorf("%w: %d past file end %d", ErrBadOffset, offset, it.fileSize)
+		return fmt.Errorf("%w: %d past file end %d", ErrBadOffset, offset, it.fileSize)
 	}
-	pos, err := it.rs.Seek(signedOffset, io.SeekStart)
-	return pos, err
+	_, err := it.rs.Seek(signedOffset, io.SeekStart)
+	return err
 }
 
 // parseIndexSection parses the index section of the file and populates the
 // related fields of the structure. It must be called prior to any of the other
 // access methods.
 func (it *indexedMessageIterator) parseSummarySection() error {
-	const footerStartOffsetFromEnd = +8 + 4 + 8 + 8 // magic, plus 20 bytes footer
+	const footerStartOffsetFromEnd = 8 + 4 + 8 + 8 // magic, plus 20 bytes footer
 	footerStartPos, err := it.rs.Seek(-footerStartOffsetFromEnd, io.SeekEnd)
 	if err != nil {
 		return err
@@ -116,9 +117,9 @@ func (it *indexedMessageIterator) parseSummarySection() error {
 		it.hasReadSummarySection = true
 		return nil
 	}
-	_, err = it.seekTo(footer.SummaryStart)
+	err = it.seekTo(footer.SummaryStart)
 	if err != nil {
-		return fmt.Errorf("failed to seek to summary start")
+		return fmt.Errorf("failed to seek to summary start: %w", err)
 	}
 
 	lexer, err := NewLexer(bufio.NewReader(it.rs), &LexerOptions{
@@ -208,7 +209,7 @@ func (it *indexedMessageIterator) parseSummarySection() error {
 // loadChunk seeks to and decompresses a chunk into a chunk slot, then populates it.messageIndexes
 // with the offsets of messages in that chunk.
 func (it *indexedMessageIterator) loadChunk(chunkIndex *ChunkIndex) error {
-	_, err := it.seekTo(chunkIndex.ChunkStartOffset)
+	err := it.seekTo(chunkIndex.ChunkStartOffset)
 	if err != nil {
 		return err
 	}
@@ -397,7 +398,7 @@ func (it *indexedMessageIterator) NextInto(msg *Message) (*Schema, *Channel, *Me
 		// take care of the metadata here
 		if it.metadataCallback != nil {
 			for _, idx := range it.metadataIndexes {
-				_, err := it.seekTo(idx.Offset)
+				err := it.seekTo(idx.Offset)
 				if err != nil {
 					return nil, nil, nil, fmt.Errorf("failed to seek to metadata: %w", err)
 				}

go/mcap/reader_test.go

@@ -1126,3 +1126,29 @@ func BenchmarkReader(b *testing.B) {
 		})
 	}
 }
+
+func TestFooterOffsetErrorDetected(t *testing.T) {
+	buf := &bytes.Buffer{}
+	writer, err := NewWriter(buf, &WriterOptions{
+		Chunked:     true,
+		ChunkSize:   1024,
+		Compression: "",
+	})
+	require.NoError(t, err)
+	require.NoError(t, writer.WriteHeader(&Header{}))
+	require.NoError(t, writer.WriteChannel(&Channel{ID: 1}))
+	require.NoError(t, writer.WriteMessage(&Message{ChannelID: 1}))
+	require.NoError(t, writer.Close())
+
+	// break the footer summary offset field. This is 8 + 8 + 4 + 8 bytes from end of file.
+	mcapBytes := buf.Bytes()
+	end := len(mcapBytes)
+	fmt.Printf("end is %d\n", end)
+	binary.LittleEndian.PutUint64(mcapBytes[end-8-8-4-8:], 999999999)
+
+	reader, err := NewReader(bytes.NewReader(mcapBytes))
+	require.NoError(t, err)
+
+	_, err = reader.Info()
+	require.ErrorIs(t, err, ErrBadOffset)
+}