Do not parse MySQL server packets (#9422)

lib/srv/db/access_test.go

@@ -275,6 +275,26 @@ func TestAccessMySQLChangeUser(t *testing.T) {
 	require.Error(t, err)
 }
 
+// TestAccessMySQLServerPacket verifies some edge-cases related to reading
+// wire packets sent by the MySQL server.
+func TestAccessMySQLServerPacket(t *testing.T) {
+	ctx := context.Background()
+	testCtx := setupTestContext(ctx, t, withSelfHostedMySQL("mysql"))
+	go testCtx.startHandlingConnections()
+
+	// Create user/role with access permissions.
+	testCtx.createUserAndRole(ctx, t, "alice", "admin", []string{"alice"}, []string{types.Wildcard})
+
+	// Connect to the database as this user.
+	mysqlConn, err := testCtx.mysqlClient("alice", "mysql", "alice")
+	require.NoError(t, err)
+
+	// Execute "show tables" command which will make the test server to reply
+	// in a way that previously would cause our packet parsing logic to fail.
+	_, err = mysqlConn.Execute("show tables")
+	require.NoError(t, err)
+}
+
 type testModules struct {
 	modules.Modules
 }

lib/srv/db/mysql/engine.go

@@ -227,7 +227,7 @@ func (e *Engine) receiveFromServer(serverConn, clientConn net.Conn, serverErrCh
 		close(serverErrCh)
 	}()
 	for {
-		packet, err := protocol.ParsePacket(serverConn)
+		packet, _, err := protocol.ReadPacket(serverConn)
 		if err != nil {
 			if utils.IsOKNetworkError(err) {
 				log.Debug("Server connection closed.")
@@ -237,7 +237,7 @@ func (e *Engine) receiveFromServer(serverConn, clientConn net.Conn, serverErrCh
 			serverErrCh <- err
 			return
 		}
-		_, err = protocol.WritePacket(packet.Bytes(), clientConn)
+		_, err = protocol.WritePacket(packet, clientConn)
 		if err != nil {
 			log.WithError(err).Error("Failed to write client packet.")
 			serverErrCh <- err

lib/srv/db/mysql/test.go

@@ -198,6 +198,24 @@ type testHandler struct {
 func (h *testHandler) HandleQuery(query string) (*mysql.Result, error) {
 	h.log.Debugf("Received query %q.", query)
 	atomic.AddUint32(&h.queryCount, 1)
+	// When getting a "show tables" query, construct the response in a way
+	// which previously caused server packets parsing logic to fail.
+	if query == "show tables" {
+		resultSet, err := mysql.BuildSimpleTextResultset(
+			[]string{"Tables_in_test"},
+			[][]interface{}{
+				// In raw bytes, this table name starts with 0x11 which used to
+				// cause server packet parsing issues since it clashed with
+				// COM_CHANGE_USER packet type.
+				{"metadata_md_table"},
+			})
+		if err != nil {
+			return nil, trace.Wrap(err)
+		}
+		return &mysql.Result{
+			Resultset: resultSet,
+		}, nil
+	}
 	return TestQueryResponse, nil
 }