syncer(dm): add MariaDB AST DDL rewriter

Author: joechenrh

dm/pkg/ddl/rewriter/rewriter.go

@@ -0,0 +1,135 @@
+// Copyright 2026 PingCAP, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package rewriter
+
+import (
+	"strings"
+
+	"github.com/pingcap/tidb/pkg/parser"
+	"github.com/pingcap/tidb/pkg/parser/ast"
+	"github.com/pingcap/tidb/pkg/parser/format"
+	_ "github.com/pingcap/tidb/pkg/types/parser_driver" // register parser driver
+)
+
+// Rule rewrites one AST node in place.
+type Rule interface {
+	Name() string
+	Apply(ast.Node) (bool, error)
+}
+
+// Rewriter applies a fixed, ordered set of AST rules.
+type Rewriter struct {
+	rules []Rule
+}
+
+// NewRewriter creates a rewriter from explicit rules.
+func NewRewriter(rules ...Rule) *Rewriter {
+	return &Rewriter{rules: append([]Rule(nil), rules...)}
+}
+
+// NewDefaultRewriter creates the default MariaDB compatibility AST rewriter.
+func NewDefaultRewriter() *Rewriter {
+	return NewRewriter(DefaultRules()...)
+}
+
+// NewRewriterForFlavor returns a default rewriter only for MariaDB upstreams.
+func NewRewriterForFlavor(flavor string) *Rewriter {
+	if !strings.EqualFold(flavor, "mariadb") {
+		return nil
+	}
+	return NewDefaultRewriter()
+}
+
+// RewriteStmt applies all rules to stmt in place.
+func (r *Rewriter) RewriteStmt(stmt ast.StmtNode) (bool, error) {
+	if r == nil || len(r.rules) == 0 || stmt == nil {
+		return false, nil
+	}
+	visitor := &rewriteVisitor{rules: r.rules}
+	stmt.Accept(visitor)
+	return visitor.changed, visitor.err
+}
+
+// RewriteSQL parses, rewrites, and restores SQL. It is mainly intended for unit tests
+// and small call sites that do not already have a parsed AST.
+func (r *Rewriter) RewriteSQL(sql string) (string, bool, error) {
+	p := parser.New()
+	stmts, _, err := p.Parse(sql, "", "")
+	if err != nil {
+		return "", false, err
+	}
+
+	changed := false
+	for _, stmt := range stmts {
+		stmtChanged, err := r.RewriteStmt(stmt)
+		if err != nil {
+			return "", false, err
+		}
+		changed = changed || stmtChanged
+	}
+	if !changed {
+		return sql, false, nil
+	}
+
+	out, err := restoreStatements(stmts)
+	if err != nil {
+		return "", false, err
+	}
+	return out, true, nil
+}
+
+type rewriteVisitor struct {
+	rules   []Rule
+	changed bool
+	err     error
+}
+
+func (v *rewriteVisitor) Enter(node ast.Node) (ast.Node, bool) {
+	if v.err != nil {
+		return node, true
+	}
+	return node, false
+}
+
+func (v *rewriteVisitor) Leave(node ast.Node) (ast.Node, bool) {
+	if v.err != nil {
+		return node, false
+	}
+	for _, rule := range v.rules {
+		changed, err := rule.Apply(node)
+		if err != nil {
+			v.err = err
+			return node, false
+		}
+		v.changed = v.changed || changed
+	}
+	return node, true
+}
+
+func restoreStatements(stmts []ast.StmtNode) (string, error) {
+	var out strings.Builder
+	for i, stmt := range stmts {
+		if i > 0 {
+			out.WriteString(";\n")
+		}
+		err := stmt.Restore(&format.RestoreCtx{
+			Flags: format.DefaultRestoreFlags | format.RestoreTiDBSpecialComment | format.RestoreStringWithoutDefaultCharset,
+			In:    &out,
+		})
+		if err != nil {
+			return "", err
+		}
+	}
+	return out.String(), nil
+}

dm/pkg/ddl/rewriter/rewriter_test.go

@@ -0,0 +1,160 @@
+// Copyright 2026 PingCAP, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package rewriter
+
+import (
+	"strings"
+	"testing"
+
+	"github.com/pingcap/tidb/pkg/parser"
+	"github.com/pingcap/tidb/pkg/parser/ast"
+	"github.com/stretchr/testify/require"
+)
+
+func TestRewriteSQLRemovesFunctionDefaultOnVarchar(t *testing.T) {
+	rewriter := NewDefaultRewriter()
+
+	out, changed, err := rewriter.RewriteSQL("CREATE TABLE t(t VARCHAR(100) DEFAULT current_timestamp());")
+	require.NoError(t, err)
+	require.True(t, changed)
+	require.NotContains(t, strings.ToLower(out), "default")
+
+	stmt := parseCreateTable(t, out)
+	col := findColumn(stmt, "t")
+	require.NotNil(t, col)
+	require.False(t, hasColumnOption(col, ast.ColumnOptionDefaultValue))
+}
+
+func TestRewriteSQLKeepsTimeFunctionDefaultOnTimeColumn(t *testing.T) {
+	rewriter := NewDefaultRewriter()
+
+	out, changed, err := rewriter.RewriteSQL("CREATE TABLE t(ts TIMESTAMP DEFAULT current_timestamp());")
+	require.NoError(t, err)
+	require.False(t, changed)
+	require.Contains(t, strings.ToLower(out), "default current_timestamp")
+}
+
+func TestRewriteSQLDefaultRules(t *testing.T) {
+	rewriter := NewDefaultRewriter()
+	input := `CREATE TABLE t (
+  id INT(11),
+  txt TEXT DEFAULT 'x',
+  v VARCHAR(800),
+  j JSON,
+  g JSON GENERATED ALWAYS AS (JSON_EXTRACT(j, '$.a')) VIRTUAL,
+  zero_ts TIMESTAMP DEFAULT '0000-00-00 00:00:00',
+  CHECK (json_valid(j)),
+  KEY idx_txt (txt),
+  KEY idx_v (v)
+) DEFAULT CHARSET=latin1 COLLATE=latin1_swedish_ci;`
+
+	out, changed, err := rewriter.RewriteSQL(input)
+	require.NoError(t, err)
+	require.True(t, changed)
+
+	stmt := parseCreateTable(t, out)
+	require.Equal(t, "utf8mb4", findTableOption(stmt, ast.TableOptionCharset))
+	require.Equal(t, "utf8mb4_0900_ai_ci", findTableOption(stmt, ast.TableOptionCollate))
+	require.Equal(t, -1, findColumn(stmt, "id").Tp.GetFlen())
+	require.Equal(t, 768, findColumn(stmt, "v").Tp.GetFlen())
+	require.False(t, hasColumnOption(findColumn(stmt, "txt"), ast.ColumnOptionDefaultValue))
+	require.False(t, hasColumnOption(findColumn(stmt, "g"), ast.ColumnOptionGenerated))
+	require.False(t, hasColumnOption(findColumn(stmt, "zero_ts"), ast.ColumnOptionDefaultValue))
+	require.False(t, hasJSONValidCheck(stmt))
+
+	idxTxt := findConstraint(stmt, "idx_txt")
+	require.NotNil(t, idxTxt)
+	require.Equal(t, 255, idxTxt.Keys[0].Length)
+	idxV := findConstraint(stmt, "idx_v")
+	require.NotNil(t, idxV)
+	require.Equal(t, 768, idxV.Keys[0].Length)
+}
+
+func TestRewriteSQLSkipsExpressionIndexPrefix(t *testing.T) {
+	rewriter := NewDefaultRewriter()
+
+	_, _, err := rewriter.RewriteSQL("CREATE TABLE t(name VARCHAR(32), KEY idx_expr ((LOWER(name))));")
+	require.NoError(t, err)
+}
+
+func TestRewriteSQLRemovesParenthesizedJSONGeneratedColumn(t *testing.T) {
+	rewriter := NewDefaultRewriter()
+
+	out, changed, err := rewriter.RewriteSQL(
+		"CREATE TABLE t(j JSON, g JSON GENERATED ALWAYS AS ((JSON_EXTRACT(j, '$.a'))) VIRTUAL);",
+	)
+	require.NoError(t, err)
+	require.True(t, changed)
+	require.False(t, hasColumnOption(findColumn(parseCreateTable(t, out), "g"), ast.ColumnOptionGenerated))
+}
+
+func TestNewRewriterForFlavor(t *testing.T) {
+	require.NotNil(t, NewRewriterForFlavor("mariadb"))
+	require.NotNil(t, NewRewriterForFlavor("MariaDB"))
+	require.Nil(t, NewRewriterForFlavor("mysql"))
+}
+
+func parseCreateTable(t *testing.T, sql string) *ast.CreateTableStmt {
+	t.Helper()
+	stmt, err := parser.New().ParseOneStmt(sql, "", "")
+	require.NoError(t, err)
+	create, ok := stmt.(*ast.CreateTableStmt)
+	require.True(t, ok)
+	return create
+}
+
+func findColumn(stmt *ast.CreateTableStmt, name string) *ast.ColumnDef {
+	for _, col := range stmt.Cols {
+		if strings.EqualFold(col.Name.Name.O, name) {
+			return col
+		}
+	}
+	return nil
+}
+
+func hasColumnOption(col *ast.ColumnDef, optionType ast.ColumnOptionType) bool {
+	for _, opt := range col.Options {
+		if opt.Tp == optionType {
+			return true
+		}
+	}
+	return false
+}
+
+func hasJSONValidCheck(stmt *ast.CreateTableStmt) bool {
+	for _, cons := range stmt.Constraints {
+		if cons.Tp == ast.ConstraintCheck && isJSONValidExpr(cons.Expr) {
+			return true
+		}
+	}
+	return false
+}
+
+func findConstraint(stmt *ast.CreateTableStmt, name string) *ast.Constraint {
+	for _, cons := range stmt.Constraints {
+		if strings.EqualFold(cons.Name, name) {
+			return cons
+		}
+	}
+	return nil
+}
+
+func findTableOption(stmt *ast.CreateTableStmt, optionType ast.TableOptionType) string {
+	for _, opt := range stmt.Options {
+		if opt.Tp == optionType {
+			return strings.ToLower(opt.StrValue)
+		}
+	}
+	return ""
+}