fstree: introduce combined file versioning

We may want to have something more complex in future versions (like #2925 or
others) and we don't have a lot of safe bytes to use in place of 0x7F. So
let's have a byte for format version.

This is an incompatible change, but this code was never in production.

Signed-off-by: Roman Khimov <[email protected]>

pkg/local_object_storage/blobstor/fstree/doc.go

@@ -38,14 +38,15 @@ can not collide with protobuf-encoded or ZSTD-compressed data. Files using
 this prefix can contain an unspecified number of objects (configured via
 [WithCombinedCountLimit]) that all follow the same serialization pattern:
   - the first byte is magic 0x7F described above
+  - one byte version of the subsequent structure (currently zero only)
   - 32-byte OID of the next object then
   - 4-byte BE integer length of the next object
   - followed by protobuf-encoded or ZSTD-compressed object data (of the length
     specified in the previous field)
 
 Overall the structure is like this:
 
-	[0x7F [OID A] [uint32 size]][object A][0x7F [OID B] [uint32 size]][object B]...
+	[0x7F 0x00 [OID A] [uint32 size]][object A][0x7F 0x00 [OID B] [uint32 size]][object B]...
 
 Even though this file contains several objects it has hard links for all of
 them in the FS tree, so finding a file containing some object doesn't require

pkg/local_object_storage/blobstor/fstree/fstree.go

@@ -75,7 +75,7 @@ const (
 	combinedLenSize = 4
 
 	// combinedIDOff is the offset from the start of the combined prefix to OID.
-	combinedIDOff = 1
+	combinedIDOff = 2
 
 	// combinedLengthOff is the offset from the start of the combined prefix to object length.
 	combinedLengthOff = combinedIDOff + oid.Size
@@ -365,6 +365,16 @@ func getRawObjectBytes(id oid.ID, p string) ([]byte, error) {
 	return data, nil
 }
 
+// parseCombinedPrefix checks the given array for combined data prefix and
+// returns a subslice with OID and object length if so (nil and 0 otherwise).
+func parseCombinedPrefix(p [combinedDataOff]byte) ([]byte, uint32) {
+	if p[0] != combinedPrefix || p[1] != 0 { // Only version 0 is supported now.
+		return nil, 0
+	}
+	return p[combinedIDOff:combinedLengthOff],
+		binary.BigEndian.Uint32(p[combinedLengthOff:combinedDataOff])
+}
+
 func extractCombinedObject(id oid.ID, f *os.File) ([]byte, error) {
 	var (
 		comBuf     [combinedDataOff]byte
@@ -383,7 +393,8 @@ func extractCombinedObject(id oid.ID, f *os.File) ([]byte, error) {
 			}
 			return nil, err
 		}
-		if comBuf[0] != combinedPrefix {
+		thisOID, l := parseCombinedPrefix(comBuf)
+		if thisOID == nil {
 			st, err := f.Stat()
 			if err != nil {
 				return nil, err
@@ -401,8 +412,7 @@ func extractCombinedObject(id oid.ID, f *os.File) ([]byte, error) {
 			return data, nil
 		}
 		isCombined = true
-		var l = binary.BigEndian.Uint32(comBuf[combinedLengthOff:combinedDataOff])
-		if bytes.Equal(comBuf[combinedIDOff:combinedLengthOff], id[:]) {
+		if bytes.Equal(thisOID, id[:]) {
 			data = make([]byte, l)
 			_, err = io.ReadFull(f, data)
 			if err != nil {