headblock checkpointing up to v3

pkg/chunkenc/memchunk.go

@@ -211,8 +211,10 @@ func (hb *headBlock) FromCheckpoint(b []byte) error {
 	if db.err() != nil {
 		return errors.Wrap(db.err(), "verifying headblock header")
 	}
-	if version != chunkFormatV3 {
-		return errors.New("incompatible headBlock version, only V3 is currently supported")
+	switch version {
+	case chunkFormatV1, chunkFormatV2, chunkFormatV3:
+	default:
+		return errors.Errorf("incompatible headBlock version (%v), only V1,V2,V3 is currently supported", version)
 	}
 
 	ln := db.uvarint()

pkg/chunkenc/memchunk_test.go

@@ -901,6 +901,14 @@ func TestCheckpointEncoding(t *testing.T) {
 
 	cpy, err := MemchunkFromCheckpoint(chk, head, blockSize, targetSize)
 	require.Nil(t, err)
+
+	// TODO(owen-d): remove once v3+ is the default chunk version
+	// because that is when we started serializing uncompressed size.
+	// Until then, nil them out in order to ease equality testing.
+	for i := range c.blocks {
+		c.blocks[i].uncompressedSize = 0
+	}
+
 	require.Equal(t, c, cpy)
 }