context: make DeadlineExceeded implement net.Error

It already implemented the Timeout method,
but implementing the full net.Error is more convenient.

Fixes #14238 (again).

Change-Id: Ia87f897f0f35bcb49865e2355964049227951ca6
Reviewed-on: https://go-review.googlesource.com/30370
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/context/context.go

@@ -159,9 +159,9 @@ var DeadlineExceeded error = deadlineExceededError{}
 
 type deadlineExceededError struct{}
 
-func (deadlineExceededError) Error() string { return "context deadline exceeded" }
-
-func (deadlineExceededError) Timeout() bool { return true }
+func (deadlineExceededError) Error() string   { return "context deadline exceeded" }
+func (deadlineExceededError) Timeout() bool   { return true }
+func (deadlineExceededError) Temporary() bool { return true }
 
 // An emptyCtx is never canceled, has no values, and has no deadline. It is not
 // struct{}, since vars of this type must have distinct addresses.

src/context/net_test.go

+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package context_test
+
+import (
+	"context"
+	"net"
+	"testing"
+)
+
+func TestDeadlineExceededIsNetError(t *testing.T) {
+	err, ok := context.DeadlineExceeded.(net.Error)
+	if !ok {
+		t.Fatal("DeadlineExceeded does not implement net.Error")
+	}
+	if !err.Timeout() || !err.Temporary() {
+		t.Fatalf("Timeout() = %v, Temporary() = %v, want true, true", err.Timeout(), err.Temporary())
+	}
+}