context2: add WithRetimeout

The most common reason you want to use WithoutCancellation is to call
something after the primary context has been already cancelled. However,
in most cases you should have a new timeout so things don't get stuck.

Change-Id: Id9e33363cc67e96c582c802ada5ca422b8d66ee8

Author: egonelbre

context2/nocancel.go

@@ -47,3 +47,8 @@ func (ctx noCancelContext) String() string {
 func (ctx noCancelContext) Value(key interface{}) interface{} {
 	return ctx.ctx.Value(key)
 }
+
+// WithRetimeout allows to create a new timeout for a potentially cancelled context.
+func WithRetimeout(ctx context.Context, duration time.Duration) (context.Context, context.CancelFunc) {
+	return context.WithTimeout(WithoutCancellation(ctx), duration)
+}

context2/nocancel_test.go

@@ -6,6 +6,7 @@ package context2_test
 import (
 	"context"
 	"testing"
+	"time"
 
 	"github.com/stretchr/testify/require"
 
@@ -24,3 +25,19 @@ func TestWithoutCancellation(t *testing.T) {
 	require.Equal(t, error(nil), without.Err())
 	require.Equal(t, (<-chan struct{})(nil), without.Done())
 }
+
+func TestWithRetimeout(t *testing.T) {
+	t.Parallel()
+	ctx := testcontext.New(t)
+
+	parent, cancel := context.WithCancel(ctx)
+	cancel()
+
+	start := time.Now()
+	subctx, subcancel := context2.WithRetimeout(parent, 3*time.Second)
+	<-subctx.Done()
+	finish := time.Now()
+	subcancel()
+
+	require.Greater(t, finish.Sub(start), time.Second)
+}