Go: add support for custom compression

Adds support for bringing custom decompressors to the go lexer and
reader.

For the writer, a ResettableWriteCloser is accepted. For the reader, a
map of compression format to ResettableReader is accepted. If the reader
implements io.Closer we'll call that on file close.

go/mcap/Makefile

@@ -1,5 +1,5 @@
 test:
-	go test ./...
+	go test -cover ./...
 
 bench:
 	go test -benchmem -run=^$$ -count 5 -bench ^BenchmarkLexer/demo -memprofile mem.out -cpuprofile cpu.out

go/mcap/lexer.go

@@ -158,6 +158,7 @@ type Lexer struct {
 	maxRecordSize            int
 	maxDecompressedChunkSize int
 	attachmentCallback       func(*AttachmentReader) error
+	decompressors            map[CompressionFormat]ResettableReader
 }
 
 // Next returns the next token from the lexer as a byte array. The result will
@@ -302,6 +303,11 @@ func (l *Lexer) Close() {
 	if l.decoders.zstd != nil {
 		l.decoders.zstd.Close()
 	}
+	for _, decompressor := range l.decompressors {
+		if closer, ok := decompressor.(io.Closer); ok {
+			closer.Close()
+		}
+	}
 }
 
 type decoders struct {
@@ -414,15 +420,19 @@ func loadChunk(l *Lexer, recordLen uint64) error {
 
 	// remaining bytes in the record are the chunk data
 	lr := io.LimitReader(l.reader, int64(recordsLength))
-	switch compression {
-	case CompressionNone:
+	switch {
+	case l.decompressors[compression] != nil: // must be top
+		decoder := l.decompressors[compression]
+		decoder.Reset(lr)
+		l.reader = decoder
+	case compression == CompressionNone:
 		l.reader = lr
-	case CompressionZSTD:
+	case compression == CompressionZSTD:
 		err = l.setZSTDDecoder(lr)
 		if err != nil {
 			return err
 		}
-	case CompressionLZ4:
+	case compression == CompressionLZ4:
 		l.setLZ4Decoder(lr)
 	default:
 		return fmt.Errorf("unsupported compression: %s", string(compression))
@@ -498,13 +508,20 @@ type LexerOptions struct {
 	MaxRecordSize int
 	// AttachmentCallback is a function to execute on attachments encountered in the file.
 	AttachmentCallback func(*AttachmentReader) error
+	// Decompressors are custom decompressors. Chunks matching the supplied
+	// compression format will be decompressed with the provided
+	// ResettableReader instead of the default implementation. If the
+	// ResettableReader also implements io.Closer, Close will be called on close
+	// of the reader.
+	Decompressors map[CompressionFormat]ResettableReader
 }
 
 // NewLexer returns a new lexer for the given reader.
 func NewLexer(r io.Reader, opts ...*LexerOptions) (*Lexer, error) {
 	var maxRecordSize, maxDecompressedChunkSize int
 	var computeAttachmentCRCs, validateChunkCRCs, emitChunks, emitInvalidChunks, skipMagic bool
 	var attachmentCallback func(*AttachmentReader) error
+	var decompressors map[CompressionFormat]ResettableReader
 	if len(opts) > 0 {
 		validateChunkCRCs = opts[0].ValidateChunkCRCs
 		computeAttachmentCRCs = opts[0].ComputeAttachmentCRCs
@@ -514,13 +531,15 @@ func NewLexer(r io.Reader, opts ...*LexerOptions) (*Lexer, error) {
 		maxRecordSize = opts[0].MaxRecordSize
 		maxDecompressedChunkSize = opts[0].MaxDecompressedChunkSize
 		attachmentCallback = opts[0].AttachmentCallback
+		decompressors = opts[0].Decompressors
 	}
 	if !skipMagic {
 		err := validateMagic(r)
 		if err != nil {
 			return nil, err
 		}
 	}
+
 	return &Lexer{
 		basereader:               r,
 		reader:                   r,
@@ -532,5 +551,6 @@ func NewLexer(r io.Reader, opts ...*LexerOptions) (*Lexer, error) {
 		maxRecordSize:            maxRecordSize,
 		maxDecompressedChunkSize: maxDecompressedChunkSize,
 		attachmentCallback:       attachmentCallback,
+		decompressors:            decompressors,
 	}, nil
 }

go/mcap/lexer_test.go

@@ -11,6 +11,7 @@ import (
 	"testing"
 	"time"
 
+	"github.com/pierrec/lz4/v4"
 	"github.com/stretchr/testify/assert"
 )
 
@@ -134,6 +135,43 @@ func TestBadMagic(t *testing.T) {
 	}
 }
 
+func TestCustomDecompressor(t *testing.T) {
+	buf := file(
+		header(),
+		chunk(t, CompressionLZ4, true, channelInfo(), message(), message()),
+		chunk(t, CompressionLZ4, true, channelInfo(), message(), message()),
+		attachment(), attachment(),
+		footer(),
+	)
+	lzr := lz4.NewReader(nil)
+	blockCount := 0
+	lzr.Apply(lz4.OnBlockDoneOption(func(size int) {
+		blockCount++
+	}))
+	lexer, err := NewLexer(bytes.NewReader(buf), &LexerOptions{
+		Decompressors: map[CompressionFormat]ResettableReader{
+			CompressionLZ4: lzr,
+		},
+	})
+	assert.Nil(t, err)
+	expected := []TokenType{
+		TokenHeader,
+		TokenChannel,
+		TokenMessage,
+		TokenMessage,
+		TokenChannel,
+		TokenMessage,
+		TokenMessage,
+		TokenFooter,
+	}
+	for i, expectedTokenType := range expected {
+		tokenType, _, err := lexer.Next(nil)
+		assert.Nil(t, err)
+		assert.Equal(t, expectedTokenType, tokenType, fmt.Sprintf("mismatch element %d", i))
+	}
+	assert.Positive(t, blockCount)
+}
+
 func TestReturnsEOFOnSuccessiveCalls(t *testing.T) {
 	lexer, err := NewLexer(bytes.NewReader(file()))
 	assert.Nil(t, err)

go/mcap/writer.go

@@ -788,6 +788,10 @@ type WriterOptions struct {
 	// SkipMagic causes the writer to skip writing magic bytes at the start of
 	// the file. This may be useful for writing a partial section of records.
 	SkipMagic bool
+
+	// Compressor is a custom compressor. If supplied it will take precedence
+	// over the built-in ones.
+	Compressor ResettableWriteCloser
 }
 
 // Convert an MCAP compression level to the corresponding lz4.CompressionLevel.
@@ -833,27 +837,34 @@ func NewWriter(w io.Writer, opts *WriterOptions) (*Writer, error) {
 	compressed := bytes.Buffer{}
 	var compressedWriter *countingCRCWriter
 	if opts.Chunked {
-		switch opts.Compression {
-		case CompressionZSTD:
+		switch {
+		// custom compressor takes precedence.
+		case opts.Compressor != nil:
+			if opts.Compression == "" {
+				return nil, fmt.Errorf("custom compressor requires compression format")
+			}
+			opts.Compressor.Reset(&compressed)
+			compressedWriter = newCountingCRCWriter(opts.Compressor, opts.IncludeCRC)
+		case opts.Compression == CompressionZSTD:
 			level := encoderLevelFromZstd(opts.CompressionLevel)
 			zw, err := zstd.NewWriter(&compressed, zstd.WithEncoderLevel(level))
 			if err != nil {
 				return nil, err
 			}
 			compressedWriter = newCountingCRCWriter(zw, opts.IncludeCRC)
-		case CompressionLZ4:
+		case opts.Compression == CompressionLZ4:
 			level := encoderLevelFromLZ4(opts.CompressionLevel)
 			lzw := lz4.NewWriter(&compressed)
 			_ = lzw.Apply(lz4.CompressionLevelOption(level))
 			compressedWriter = newCountingCRCWriter(lzw, opts.IncludeCRC)
-		case CompressionNone:
+		case opts.Compression == CompressionNone:
 			compressedWriter = newCountingCRCWriter(bufCloser{&compressed}, opts.IncludeCRC)
 		default:
 			return nil, fmt.Errorf("unsupported compression")
 		}
-		if opts.ChunkSize == 0 {
-			opts.ChunkSize = 1024 * 1024
-		}
+	}
+	if opts.ChunkSize == 0 {
+		opts.ChunkSize = 1024 * 1024
 	}
 	return &Writer{
 		w:                        writer,

go/mcap/writer_test.go

@@ -4,9 +4,11 @@ import (
 	"bytes"
 	"crypto/md5"
 	"fmt"
+	"io"
 	"testing"
 	"time"
 
+	"github.com/pierrec/lz4/v4"
 	"github.com/stretchr/testify/assert"
 )
 
@@ -678,3 +680,64 @@ func TestWriteAttachment(t *testing.T) {
 		})
 	}
 }
+
+func assertReadable(t *testing.T, rs io.ReadSeeker) {
+	reader, err := NewReader(rs)
+	assert.Nil(t, err)
+
+	_, err = reader.Info()
+	assert.Nil(t, err)
+
+	it, err := reader.Messages()
+	assert.Nil(t, err)
+	for {
+		_, _, _, err := it.Next(nil)
+		if err != nil {
+			assert.ErrorIs(t, err, io.EOF)
+			break
+		}
+	}
+}
+
+func TestBYOCompressor(t *testing.T) {
+	buf := &bytes.Buffer{}
+	// example - custom lz4 settings
+	lzw := lz4.NewWriter(nil)
+	blockCount := 0
+	lzw.Apply(lz4.OnBlockDoneOption(func(size int) {
+		blockCount++
+	}))
+
+	writer, err := NewWriter(buf, &WriterOptions{
+		Chunked:     true,
+		ChunkSize:   1024,
+		Compressor:  lzw,
+		Compression: "lz4",
+	})
+	assert.Nil(t, err)
+
+	assert.Nil(t, writer.WriteHeader(&Header{}))
+	assert.Nil(t, writer.WriteSchema(&Schema{
+		ID:       1,
+		Name:     "schema",
+		Encoding: "ros1msg",
+		Data:     []byte{},
+	}))
+	assert.Nil(t, writer.WriteChannel(&Channel{
+		ID:              0,
+		SchemaID:        1,
+		Topic:           "/foo",
+		MessageEncoding: "ros1msg",
+	}))
+
+	for i := 0; i < 100; i++ {
+		assert.Nil(t, writer.WriteMessage(&Message{
+			ChannelID: 0,
+			Sequence:  0,
+			LogTime:   uint64(i),
+		}))
+	}
+	assert.Nil(t, writer.Close())
+	assertReadable(t, bytes.NewReader(buf.Bytes()))
+	assert.Positive(t, blockCount)
+}