feat: EmbedMessage WrapV1 option & Reader#ReadEmbeddedMessage

EmbedMessage sets the second-to-leftmost bit to indicate that there is a
length-prefixed dag-cbor message object directly after the header. If that bit
is set, Reader#ReadEmbeddedMessage will decode and return that message.

Ref: ipld/js-car#89

Author: rvagg

v2/block_reader_test.go

@@ -159,6 +159,14 @@ func TestMaxHeaderLength(t *testing.T) {
 	require.EqualError(t, err, "invalid header data, length of read beyond allowable maximum")
 }
 
+func TestCanReadMessagingCar(t *testing.T) {
+	car, err := carv2.NewBlockReader(requireReaderFromPath(t, "testdata/messaging.car"))
+	require.NoError(t, err)
+	readBlock, err := car.Next()
+	require.NoError(t, err)
+	require.Equal(t, "random meaningless bytes", string(readBlock.RawData()))
+}
+
 func requireReaderFromPath(t *testing.T, path string) io.Reader {
 	f, err := os.Open(path)
 	require.NoError(t, err)

v2/car.go

@@ -44,8 +44,12 @@ type (
 	}
 )
 
-// fullyIndexedCharPos is the position of Characteristics.Hi bit that specifies whether the index is a catalog af all CIDs or not.
+// fullyIndexedCharPos is the position of Characteristics.Hi bit that specifies whether the index is
+// a catalog af all CIDs or not.
 const fullyIndexedCharPos = 7 // left-most bit
+// messageAfterHeaderCharPos is the position of Characteristics.Hi bit that specifies whether there
+// is a length-prefixed dag-cbor message object directly after the header.
+const messageAfterHeaderCharPos = 6 // second-to-left-most bit
 
 // WriteTo writes this characteristics to the given w.
 func (c Characteristics) WriteTo(w io.Writer) (n int64, err error) {
@@ -83,6 +87,22 @@ func (c *Characteristics) SetFullyIndexed(b bool) {
 	}
 }
 
+// IsMessageAfterHeader specifies whether there is a length-prefixed dag-cbor message embedded
+// directly after the CARv2 header that can optionally be decoded.
+func (c *Characteristics) IsMessageAfterHeader() bool {
+	return isBitSet(c.Hi, messageAfterHeaderCharPos)
+}
+
+// SetMessageAfterHeader sets whether there is a length-prefixed dag-cbor message embedded directly
+// after the CARv2 header.
+func (c *Characteristics) SetMessageAfterHeader(b bool) {
+	if b {
+		c.Hi = setBit(c.Hi, messageAfterHeaderCharPos)
+	} else {
+		c.Hi = unsetBit(c.Hi, messageAfterHeaderCharPos)
+	}
+}
+
 func setBit(n uint64, pos uint) uint64 {
 	n |= 1 << pos
 	return n

v2/car_test.go

@@ -205,43 +205,69 @@ func TestNewHeaderHasExpectedValues(t *testing.T) {
 	assert.Equal(t, want, got, "NewHeader got = %v, want = %v", got, want)
 }
 
-func TestCharacteristics_StoreIdentityCIDs(t *testing.T) {
-	subject := carv2.Characteristics{}
-	require.False(t, subject.IsFullyIndexed())
-
-	subject.SetFullyIndexed(true)
-	require.True(t, subject.IsFullyIndexed())
-
-	var buf bytes.Buffer
-	written, err := subject.WriteTo(&buf)
-	require.NoError(t, err)
-	require.Equal(t, int64(16), written)
-	require.Equal(t, []byte{
-		0x80, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0,
-		0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0,
-	}, buf.Bytes())
-
-	var decodedSubject carv2.Characteristics
-	read, err := decodedSubject.ReadFrom(&buf)
-	require.NoError(t, err)
-	require.Equal(t, int64(16), read)
-	require.True(t, decodedSubject.IsFullyIndexed())
-
-	buf.Reset()
-	subject.SetFullyIndexed(false)
-	require.False(t, subject.IsFullyIndexed())
-
-	written, err = subject.WriteTo(&buf)
-	require.NoError(t, err)
-	require.Equal(t, int64(16), written)
-	require.Equal(t, []byte{
-		0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0,
-		0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0,
-	}, buf.Bytes())
-
-	var decodedSubjectAgain carv2.Characteristics
-	read, err = decodedSubjectAgain.ReadFrom(&buf)
-	require.NoError(t, err)
-	require.Equal(t, int64(16), read)
-	require.False(t, decodedSubjectAgain.IsFullyIndexed())
+func TestCharacteristics(t *testing.T) {
+	tests := []struct {
+		name        string
+		isset       func(carv2.Characteristics) bool
+		set         func(*carv2.Characteristics, bool)
+		expectBytes []byte
+	}{
+		{
+			"FullyIndexed",
+			func(c carv2.Characteristics) bool { return c.IsFullyIndexed() },
+			func(c *carv2.Characteristics, s bool) { c.SetFullyIndexed(s) },
+			[]byte{
+				0x80, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0,
+				0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0,
+			},
+		},
+		{
+			"MessageAfterHeader",
+			func(c carv2.Characteristics) bool { return c.IsMessageAfterHeader() },
+			func(c *carv2.Characteristics, s bool) { c.SetMessageAfterHeader(s) },
+			[]byte{
+				0x40, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0,
+				0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0,
+			},
+		},
+	}
+	for _, tt := range tests {
+		tt := tt
+		t.Run(tt.name, func(t *testing.T) {
+			subject := carv2.Characteristics{}
+			require.False(t, tt.isset(subject))
+			tt.set(&subject, true)
+			require.True(t, tt.isset(subject))
+
+			var buf bytes.Buffer
+			written, err := subject.WriteTo(&buf)
+			require.NoError(t, err)
+			require.Equal(t, int64(16), written)
+			require.Equal(t, tt.expectBytes, buf.Bytes())
+
+			var decodedSubject carv2.Characteristics
+			read, err := decodedSubject.ReadFrom(&buf)
+			require.NoError(t, err)
+			require.Equal(t, int64(16), read)
+			require.True(t, tt.isset(decodedSubject))
+
+			buf.Reset()
+			tt.set(&subject, false)
+			require.False(t, tt.isset(subject))
+
+			written, err = subject.WriteTo(&buf)
+			require.NoError(t, err)
+			require.Equal(t, int64(16), written)
+			require.Equal(t, []byte{
+				0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0,
+				0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0,
+			}, buf.Bytes())
+
+			var decodedSubjectAgain carv2.Characteristics
+			read, err = decodedSubjectAgain.ReadFrom(&buf)
+			require.NoError(t, err)
+			require.Equal(t, int64(16), read)
+			require.False(t, tt.isset(decodedSubjectAgain))
+		})
+	}
 }

v2/options.go

@@ -4,6 +4,7 @@ import (
 	"math"
 
 	"github.com/ipld/go-car/v2/index"
+	"github.com/ipld/go-ipld-prime/datamodel"
 	"github.com/ipld/go-ipld-prime/traversal"
 	"github.com/multiformats/go-multicodec"
 
@@ -63,6 +64,8 @@ type Options struct {
 
 	MaxAllowedHeaderSize  uint64
 	MaxAllowedSectionSize uint64
+
+	EmbeddedMessage datamodel.Node
 }
 
 // ApplyOptions applies given opts and returns the resulting Options.
@@ -172,3 +175,12 @@ func MaxAllowedSectionSize(max uint64) Option {
 		o.MaxAllowedSectionSize = max
 	}
 }
+
+// EmbedMessage writes a length-prefixed dag-cbor message after the CARv2 header
+// and sets the 'MessageAfterHeader' characteristic bit is set for the resulting
+// CAR
+func EmbedMessage(msg datamodel.Node) Option {
+	return func(o *Options) {
+		o.EmbeddedMessage = msg
+	}
+}

v2/reader.go

@@ -11,6 +11,9 @@ import (
 	"github.com/ipld/go-car/v2/internal/carv1"
 	"github.com/ipld/go-car/v2/internal/carv1/util"
 	internalio "github.com/ipld/go-car/v2/internal/io"
+	ipld "github.com/ipld/go-ipld-prime"
+	"github.com/ipld/go-ipld-prime/codec/dagcbor"
+	"github.com/ipld/go-ipld-prime/datamodel"
 	"github.com/multiformats/go-multicodec"
 	"github.com/multiformats/go-multihash"
 	"github.com/multiformats/go-varint"
@@ -349,6 +352,26 @@ func (r *Reader) Inspect(validateBlockHash bool) (Stats, error) {
 	return stats, nil
 }
 
+// ReadEmbeddedMessage reads a length-prefixed dag-cbor message embedded after the CARv2 header
+// if the 'MessageAfterHeader' characteristic bit is set for this CAR and the message exists.
+func (r *Reader) ReadEmbeddedMessage() (datamodel.Node, error) {
+	if !r.Header.Characteristics.IsMessageAfterHeader() {
+		return nil, errors.New("'MessageAfterHeader' bit is not set in the characteristics for this CAR")
+	}
+
+	msgStart := int64(PragmaSize + HeaderSize)
+	gapLen := int64(r.Header.DataOffset) - msgStart
+	if gapLen <= 0 {
+		return nil, errors.New("invalid MessageAfterHeader, no space after header")
+	}
+	msgReader := io.NewSectionReader(r.r, msgStart, gapLen)
+	byts, err := util.LdRead(msgReader, false, r.opts.MaxAllowedSectionSize)
+	if err != nil {
+		return nil, err
+	}
+	return ipld.Decode(byts, dagcbor.Decode)
+}
+
 // Close closes the underlying reader if it was opened by OpenReader.
 func (r *Reader) Close() error {
 	if r.closer != nil {

v2/reader_test.go

@@ -12,6 +12,8 @@ import (
 	carv2 "github.com/ipld/go-car/v2"
 	"github.com/ipld/go-car/v2/index"
 	"github.com/ipld/go-car/v2/internal/carv1"
+	ipld "github.com/ipld/go-ipld-prime"
+	"github.com/ipld/go-ipld-prime/codec/dagjson"
 	"github.com/multiformats/go-multicodec"
 	"github.com/stretchr/testify/require"
 )
@@ -424,6 +426,31 @@ func TestInspect(t *testing.T) {
 				MinCidLength: 25,
 			},
 		},
+		// CARv2 with embedded message
+		{
+			name:        "CarWithMessage",
+			path:        "testdata/messaging.car",
+			zerLenAsEOF: true,
+			expectedStats: carv2.Stats{
+				Version: 2,
+				Header: carv2.Header{
+					Characteristics: carv2.Characteristics{Hi: 64},
+					DataOffset:      158,
+					DataSize:        120,
+				},
+				Roots:          []cid.Cid{mustCidDecode("bafkreihwkf6mtnjobdqrkiksr7qhp6tiiqywux64aylunbvmfhzeql2coa")},
+				RootsPresent:   true,
+				AvgBlockLength: 24,
+				MinBlockLength: 24,
+				MaxBlockLength: 24,
+				AvgCidLength:   36,
+				MinCidLength:   36,
+				MaxCidLength:   36,
+				BlockCount:     1,
+				CodecCounts:    map[multicodec.Code]uint64{multicodec.Raw: 1},
+				MhTypeCounts:   map[multicodec.Code]uint64{multicodec.Sha2_256: 1},
+			},
+		},
 	}
 
 	for _, tt := range tests {
@@ -560,3 +587,32 @@ func mustCidDecode(s string) cid.Cid {
 	}
 	return c
 }
+
+func TestEmbeddedMessage(t *testing.T) {
+	t.Run("has", func(t *testing.T) {
+		r, err := carv2.OpenReader("testdata/messaging.car")
+		require.NoError(t, err)
+		msg, err := r.ReadEmbeddedMessage()
+		require.NoError(t, err)
+		enc, err := ipld.Encode(msg, dagjson.Encode)
+		require.NoError(t, err)
+		require.Equal(t,
+			`{"expectedRoot":{"/":"bafkreihwkf6mtnjobdqrkiksr7qhp6tiiqywux64aylunbvmfhzeql2coa"},"sneaky":"sending a message outside of CARv1 payload"}`,
+			string(enc),
+		)
+	})
+	t.Run("hasnot-v1", func(t *testing.T) {
+		r, err := carv2.OpenReader("testdata/sample-v1.car")
+		require.NoError(t, err)
+		_, err = r.ReadEmbeddedMessage()
+		require.NotNil(t, err)
+		require.Equal(t, err.Error(), "'MessageAfterHeader' bit is not set in the characteristics for this CAR")
+	})
+	t.Run("hasnot-v2", func(t *testing.T) {
+		r, err := carv2.OpenReader("testdata/sample-wrapped-v2.car")
+		require.NoError(t, err)
+		_, err = r.ReadEmbeddedMessage()
+		require.NotNil(t, err)
+		require.Equal(t, err.Error(), "'MessageAfterHeader' bit is not set in the characteristics for this CAR")
+	})
+}

v2/testdata/messaging.car

@@ -11,6 +11,9 @@ import (
 	"github.com/ipld/go-car/v2/index"
 	"github.com/ipld/go-car/v2/internal/carv1"
 	internalio "github.com/ipld/go-car/v2/internal/io"
+	ipld "github.com/ipld/go-ipld-prime"
+	"github.com/ipld/go-ipld-prime/codec/dagcbor"
+	"github.com/multiformats/go-varint"
 )
 
 // ErrAlreadyV1 signals that the given payload is already in CARv1 format.
@@ -46,21 +49,27 @@ func WrapV1File(srcPath, dstPath string) error {
 	return nil
 }
 
-// WrapV1 takes a CARv1 file and wraps it as a CARv2 file with an index.
+// WrapV1 takes a CARv1 file and wraps it as a CARv2 file with an index (unless
+// the WithoutIndex option is supplied).
 // The resulting CARv2 file's inner CARv1 payload is left unmodified,
-// and does not use any padding before the innner CARv1 or index.
+// and does not use any padding before the inner CARv1 or index.
+// The EmbedMessage option may be used to insert an additional message after the
+// CARv2 header.
 func WrapV1(src io.ReadSeeker, dst io.Writer, opts ...Option) error {
 	// TODO: verify src is indeed a CARv1 to prevent misuse.
 	// GenerateIndex should probably be in charge of that.
 
 	o := ApplyOptions(opts...)
-	idx, err := index.New(o.IndexCodec)
-	if err != nil {
-		return err
-	}
-
-	if err := LoadIndex(idx, src, opts...); err != nil {
-		return err
+	var idx index.Index
+	var err error
+	if o.IndexCodec != index.CarIndexNone {
+		idx, err = index.New(o.IndexCodec)
+		if err != nil {
+			return err
+		}
+		if err := LoadIndex(idx, src, opts...); err != nil {
+			return err
+		}
 	}
 
 	// Use Seek to learn the size of the CARv1 before reading it.
@@ -74,18 +83,44 @@ func WrapV1(src io.ReadSeeker, dst io.Writer, opts ...Option) error {
 
 	// Similar to the writer API, write all components of a CARv2 to the
 	// destination file: Pragma, Header, CARv1, Index.
-	v2Header := NewHeader(uint64(v1Size))
 	if _, err := dst.Write(Pragma); err != nil {
 		return err
 	}
-	if _, err := v2Header.WriteTo(dst); err != nil {
-		return err
+	v2Header := NewHeader(uint64(v1Size))
+	if o.IndexCodec == index.CarIndexNone {
+		v2Header.IndexOffset = 0
+	}
+
+	if o.EmbeddedMessage != nil {
+		v2Header.Characteristics.SetMessageAfterHeader(true)
+		msgBytes, err := ipld.Encode(o.EmbeddedMessage, dagcbor.Encode)
+		if err != nil {
+			return err
+		}
+		lenBuf := make([]byte, 8)
+		lenLen := varint.PutUvarint(lenBuf, uint64(len(msgBytes)))
+		v2Header.DataOffset += uint64(lenLen + len(msgBytes))
+		if _, err := v2Header.WriteTo(dst); err != nil {
+			return err
+		}
+		if _, err = dst.Write(lenBuf[:lenLen]); err != nil {
+			return err
+		}
+		if _, err = dst.Write(msgBytes); err != nil {
+			return err
+		}
+	} else {
+		if _, err := v2Header.WriteTo(dst); err != nil {
+			return err
+		}
 	}
 	if _, err := io.Copy(dst, src); err != nil {
 		return err
 	}
-	if _, err := index.WriteTo(idx, dst); err != nil {
-		return err
+	if idx != nil {
+		if _, err := index.WriteTo(idx, dst); err != nil {
+			return err
+		}
 	}
 
 	return nil