executor: fix data race in GetDirtyTable()

executor/union_scan.go

@@ -15,6 +15,7 @@ package executor
 
 import (
 	"context"
+	"sync"
 
 	"github.com/pingcap/errors"
 	"github.com/pingcap/parser/model"
@@ -28,12 +29,15 @@ import (
 // DirtyDB stores uncommitted write operations for a transaction.
 // It is stored and retrieved by context.Value and context.SetValue method.
 type DirtyDB struct {
+	sync.Mutex
+
 	// tables is a map whose key is tableID.
 	tables map[int64]*DirtyTable
 }
 
 // GetDirtyTable gets the DirtyTable by id from the DirtyDB.
 func (udb *DirtyDB) GetDirtyTable(tid int64) *DirtyTable {
+	udb.Lock()
 	dt, ok := udb.tables[tid]
 	if !ok {
 		dt = &DirtyTable{
@@ -43,6 +47,7 @@ func (udb *DirtyDB) GetDirtyTable(tid int64) *DirtyTable {
 		}
 		udb.tables[tid] = dt
 	}
+	udb.Unlock()
 	return dt
 }