planner: Fix the issue that TiDB may dispatch duplicated tasks to TiFlash (#32813)

ref pingcap/tics#4163, close pingcap/tidb#32814

planner/core/fragment.go

@@ -127,7 +127,7 @@ func (f *Fragment) init(p PhysicalPlan) error {
 		f.TableScan = x
 	case *PhysicalExchangeReceiver:
 		// TODO: after we support partial merge, we should check whether all the target exchangeReceiver is same.
-		f.singleton = x.children[0].(*PhysicalExchangeSender).ExchangeType == tipb.ExchangeType_PassThrough
+		f.singleton = f.singleton || x.children[0].(*PhysicalExchangeSender).ExchangeType == tipb.ExchangeType_PassThrough
 		f.ExchangeReceivers = append(f.ExchangeReceivers, x)
 	case *PhysicalUnionAll:
 		return errors.New("unexpected union all detected")

planner/core/fragment_test.go

@@ -0,0 +1,53 @@
+// Copyright 2020 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,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package core
+
+import (
+	"github.com/pingcap/tipb/go-tipb"
+	"github.com/stretchr/testify/require"
+
+	"testing"
+)
+
+func TestFragmentInitSingleton(t *testing.T) {
+	r1, r2 := &PhysicalExchangeReceiver{}, &PhysicalExchangeReceiver{}
+	r1.SetChildren(&PhysicalExchangeSender{ExchangeType: tipb.ExchangeType_PassThrough})
+	r2.SetChildren(&PhysicalExchangeSender{ExchangeType: tipb.ExchangeType_Broadcast})
+	p := &PhysicalHashJoin{}
+
+	f := &Fragment{}
+	p.SetChildren(r1, r1)
+	err := f.init(p)
+	require.NoError(t, err)
+	require.Equal(t, f.singleton, true)
+
+	f = &Fragment{}
+	p.SetChildren(r1, r2)
+	err = f.init(p)
+	require.NoError(t, err)
+	require.Equal(t, f.singleton, true)
+
+	f = &Fragment{}
+	p.SetChildren(r2, r1)
+	err = f.init(p)
+	require.NoError(t, err)
+	require.Equal(t, f.singleton, true)
+
+	f = &Fragment{}
+	p.SetChildren(r2, r2)
+	err = f.init(p)
+	require.NoError(t, err)
+	require.Equal(t, f.singleton, false)
+}