fix 'invalid compressed sequence' error, issue #871 (#872)

Co-authored-by: dvilaverde <dvilaverde@adobe.com>

Author: dvilaverde

client/client_test.go

@@ -89,6 +89,17 @@ func (s *clientTestSuite) TestConn_Ping() {
 	require.NoError(s.T(), err)
 }
 
+func (s *clientTestSuite) TestConn_Compress() {
+	addr := fmt.Sprintf("%s:%s", *test_util.MysqlHost, s.port)
+	conn, err := Connect(addr, *testUser, *testPassword, "", func(conn *Conn) {
+		conn.SetCapability(mysql.CLIENT_COMPRESS)
+	})
+	require.NoError(s.T(), err)
+
+	_, err = conn.Execute("SELECT VERSION()")
+	require.NoError(s.T(), err)
+}
+
 func (s *clientTestSuite) TestConn_SetCapability() {
 	caps := []uint32{
 		mysql.CLIENT_LONG_PASSWORD,

packet/conn.go

@@ -67,6 +67,8 @@ type Conn struct {
 	compressedHeader [7]byte
 
 	compressedReader io.Reader
+
+	compressedReaderActive bool
 }
 
 func NewConn(conn net.Conn) *Conn {
@@ -106,12 +108,19 @@ func (c *Conn) ReadPacketReuseMem(dst []byte) ([]byte, error) {
 	}()
 
 	if c.Compression != MYSQL_COMPRESS_NONE {
-		if c.compressedReader == nil {
+		// it's possible that we're using compression but the server response with a compressed
+		// packet with uncompressed length of 0. In this case we leave compressedReader nil. The
+		// compressedReaderActive flag is important to track the state of the reader, allowing
+		// for the compressedReader to be reset after a packet write. Without this flag, when a
+		// compressed packet with uncompressed length of 0 is read, the compressedReader would
+		// be nil, and we'd incorrectly attempt to read the next packet as compressed.
+		if !c.compressedReaderActive {
 			var err error
 			c.compressedReader, err = c.newCompressedPacketReader()
 			if err != nil {
 				return nil, err
 			}
+			c.compressedReaderActive = true
 		}
 	}
 
@@ -312,6 +321,7 @@ func (c *Conn) WritePacket(data []byte) error {
 			return errors.Wrapf(ErrBadConn, "Write failed. only %v bytes written, while %v expected", n, len(data))
 		}
 		c.compressedReader = nil
+		c.compressedReaderActive = false
 	default:
 		return errors.Wrapf(ErrBadConn, "Write failed. Unsuppored compression algorithm set")
 	}