Add context watchdog for batch send cancellation

* batch: context watchdog

* fix context cancellation

conn_batch.go

@@ -178,7 +178,14 @@ func (b *batch) Column(idx int) driver.BatchColumn {
 }
 
 func (b *batch) Send() (err error) {
+	stopCW := contextWatchdog(b.ctx, func() {
+		// close TCP connection on context cancel. There is no other way simple way to interrupt underlying operations.
+		// as verified in the test, this is safe to do and cleanups resources later on
+		_ = b.conn.conn.Close()
+	})
+
 	defer func() {
+		stopCW()
 		b.sent = true
 		b.release(err)
 	}()
@@ -192,6 +199,12 @@ func (b *batch) Send() (err error) {
 	}
 	if b.block.Rows() != 0 {
 		if err = b.conn.sendData(b.block, ""); err != nil {
+			// there might be an error caused by context cancellation
+			// in this case we should return context error instead of net.OpError
+			if ctxErr := b.ctx.Err(); ctxErr != nil {
+				return ctxErr
+			}
+
 			return err
 		}
 	}

context_watchdog.go

@@ -0,0 +1,47 @@
+// Licensed to ClickHouse, Inc. under one or more contributor
+// license agreements. See the NOTICE file distributed with
+// this work for additional information regarding copyright
+// ownership. ClickHouse, Inc. licenses this file to you 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 clickhouse
+
+import "context"
+
+// contextWatchdog is a helper function to run a callback when the context is done.
+// it has a cancellation function to prevent the callback from running.
+// Useful for interrupting some logic when the context is done,
+// but you want to not bother about context cancellation if your logic is already done.
+// Example:
+// stopCW := contextWatchdog(ctx, func() { /* do something */ })
+// // do something else
+// defer stopCW()
+func contextWatchdog(ctx context.Context, callback func()) (cancel func()) {
+	exit := make(chan struct{})
+
+	go func() {
+		for {
+			select {
+			case <-exit:
+				return
+			case <-ctx.Done():
+				callback()
+			}
+		}
+	}()
+
+	return func() {
+		exit <- struct{}{}
+	}
+}

tests/batch_test.go

@@ -0,0 +1,52 @@
+// Licensed to ClickHouse, Inc. under one or more contributor
+// license agreements. See the NOTICE file distributed with
+// this work for additional information regarding copyright
+// ownership. ClickHouse, Inc. licenses this file to you 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 tests
+
+import (
+	"github.com/ClickHouse/clickhouse-go/v2"
+	"github.com/stretchr/testify/require"
+	"golang.org/x/net/context"
+	"testing"
+	"time"
+)
+
+func TestBatchContextCancellation(t *testing.T) {
+	te, err := GetTestEnvironment(testSet)
+	require.NoError(t, err)
+	opts := ClientOptionsFromEnv(te, clickhouse.Settings{})
+	opts.MaxOpenConns = 1
+	conn, err := GetConnectionWithOptions(&opts)
+	require.NoError(t, err)
+
+	ctx, cancel := context.WithTimeout(context.Background(), time.Second)
+	defer cancel()
+
+	require.NoError(t, conn.Exec(context.Background(), "create table if not exists test_batch_cancellation (x String) engine=Memory"))
+	defer conn.Exec(context.Background(), "drop table if exists test_batch_cancellation")
+
+	b, err := conn.PrepareBatch(ctx, "insert into test_batch_cancellation")
+	require.NoError(t, err)
+	for i := 0; i < 1_000_000; i++ {
+		require.NoError(t, b.Append("value"))
+	}
+
+	require.Equal(t, context.DeadlineExceeded, b.Send())
+
+	// assert if connection is properly released after context cancellation
+	require.NoError(t, conn.Exec(context.Background(), "SELECT 1"))
+}