Adds context to sqlparser errors

go/vt/sqlparser/parse_test.go

@@ -1854,6 +1854,45 @@ func TestConvert(t *testing.T) {
 	}
 }
 
+func TestPositionedErr(t *testing.T) {
+	invalidSQL := []struct {
+		input  string
+		output PositionedErr
+	}{{
+		input:  "select convert('abc' as date) from t",
+		output: PositionedErr{"syntax error at position 24 near 'as'", 24, []byte("as")},
+	}, {
+		input:  "select convert from t",
+		output: PositionedErr{"syntax error at position 20 near 'from'", 20, []byte("from")},
+	}, {
+		input:  "select cast('foo', decimal) from t",
+		output: PositionedErr{"syntax error at position 19", 19, nil},
+	}, {
+		input:  "select convert('abc', datetime(4+9)) from t",
+		output: PositionedErr{"syntax error at position 34", 34, nil},
+	}, {
+		input:  "select convert('abc', decimal(4+9)) from t",
+		output: PositionedErr{"syntax error at position 33", 33, nil},
+	}, {
+		input:  "set transaction isolation level 12345",
+		output: PositionedErr{"syntax error at position 38 near '12345'", 38, []byte("12345")},
+	}, {
+		input:  "select * from a left join b",
+		output: PositionedErr{"syntax error at position 28", 28, nil},
+	}}
+
+	for _, tcase := range invalidSQL {
+		tkn := NewStringTokenizer(tcase.input)
+		_, err := ParseNext(tkn)
+
+		if posErr, ok := err.(PositionedErr); !ok {
+			t.Errorf("%s: %v expected PositionedErr, got (%T) %v", tcase.input, err, err, tcase.output)
+		} else if posErr.Pos != tcase.output.Pos || !bytes.Equal(posErr.Near, tcase.output.Near) || err.Error() != tcase.output.Err {
+			t.Errorf("%s: %v, want: %v", tcase.input, err, tcase.output)
+		}
+	}
+}
+
 func TestSubStr(t *testing.T) {
 
 	validSQL := []struct {

go/vt/sqlparser/token.go

@@ -18,7 +18,6 @@ package sqlparser
 
 import (
 	"bytes"
-	"errors"
 	"fmt"
 	"io"
 
@@ -452,15 +451,23 @@ func (tkn *Tokenizer) Lex(lval *yySymType) int {
 	return typ
 }
 
+// PositionedErr holds context related to parser errors
+type PositionedErr struct {
+	Err  string
+	Pos  int
+	Near []byte
+}
+
+func (p PositionedErr) Error() string {
+	if p.Near != nil {
+		return fmt.Sprintf("%s at position %v near '%s'", p.Err, p.Pos, p.Near)
+	}
+	return fmt.Sprintf("%s at position %v", p.Err, p.Pos)
+}
+
 // Error is called by go yacc if there's a parsing error.
 func (tkn *Tokenizer) Error(err string) {
-	buf := &bytes2.Buffer{}
-	if tkn.lastToken != nil {
-		fmt.Fprintf(buf, "%s at position %v near '%s'", err, tkn.Position, tkn.lastToken)
-	} else {
-		fmt.Fprintf(buf, "%s at position %v", err, tkn.Position)
-	}
-	tkn.LastError = errors.New(buf.String())
+	tkn.LastError = PositionedErr{Err: err, Pos: tkn.Position, Near: tkn.lastToken}
 
 	// Try and re-sync to the next statement
 	tkn.skipStatement()