Add error transformer to vtgate executor

This makes it for other components to plug in logic to modify errors
before they are returned to the client. By default the NullTransformer
is used, which does nothing.

Signed-off-by: Rafer Hazen <[email protected]>

go/vt/vtgate/executor.go

@@ -78,6 +78,8 @@ var (
 	queriesRoutedByTable    = stats.NewCountersWithMultiLabels("QueriesRoutedByTable", "Queries routed from vtgate to vttablet by plan type, keyspace and table", []string{"Plan", "Keyspace", "Table"})
 
 	exceedMemoryRowsLogger = logutil.NewThrottledLogger("ExceedMemoryRows", 1*time.Minute)
+
+	errorTransform errorTransformer = nullErrorTransformer{}
 )
 
 const (
@@ -246,7 +248,10 @@ func (e *Executor) Execute(ctx context.Context, mysqlCtx vtgateservice.MySQLConn
 
 	logStats.SaveEndTime()
 	e.queryLogger.Send(logStats)
+
+	err = errorTransform.TransformError(err)
 	err = vterrors.TruncateError(err, truncateErrorLen)
+
 	return result, err
 }
 
@@ -385,8 +390,11 @@ func (e *Executor) StreamExecute(
 
 	logStats.SaveEndTime()
 	e.queryLogger.Send(logStats)
-	return vterrors.TruncateError(err, truncateErrorLen)
 
+	err = errorTransform.TransformError(err)
+	err = vterrors.TruncateError(err, truncateErrorLen)
+
+	return err
 }
 
 func canReturnRows(stmtType sqlparser.StatementType) bool {
@@ -1352,7 +1360,11 @@ func (e *Executor) Prepare(ctx context.Context, method string, safeSession *Safe
 		logStats.SaveEndTime()
 		e.queryLogger.Send(logStats)
 	}
-	return fld, vterrors.TruncateError(err, truncateErrorLen)
+
+	err = errorTransform.TransformError(err)
+	err = vterrors.TruncateError(err, truncateErrorLen)
+
+	return fld, err
 }
 
 func (e *Executor) prepare(ctx context.Context, safeSession *SafeSession, sql string, bindVars map[string]*querypb.BindVariable, logStats *logstats.LogStats) ([]*querypb.Field, error) {
@@ -1606,3 +1618,14 @@ func (e *Executor) Close() {
 func (e *Executor) environment() *vtenv.Environment {
 	return e.env
 }
+
+type (
+	errorTransformer interface {
+		TransformError(err error) error
+	}
+	nullErrorTransformer struct{}
+)
+
+func (nullErrorTransformer) TransformError(err error) error {
+	return err
+}