os, net: define and use os.ErrDeadlineExceeded

If an I/O operation fails because a deadline was exceeded,
return os.ErrDeadlineExceeded. We used to return poll.ErrTimeout,
an internal error, and told users to check the Timeout method.
However, there are other errors with a Timeout method that returns true,
notably syscall.ETIMEDOUT which is returned for a keep-alive timeout.
Checking errors.Is(err, os.ErrDeadlineExceeded) should permit code
to reliably tell why it failed.

This change does not affect the handling of net.Dialer.Deadline,
nor does it change the handling of net.DialContext when the context
deadline is exceeded. Those cases continue to return an error
reported as "i/o timeout" for which Timeout is true, but that error
is not os.ErrDeadlineExceeded.

Fixes #31449

Change-Id: I0323f42e944324c6f2578f00c3ac90c24fe81177
Reviewed-on: https://go-review.googlesource.com/c/go/+/228645
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Filippo Valsorda <filippo@golang.org>

1 files changed, 7 insertions, 8 deletions

diff --git a/src/net/dnsclient_unix_test.go b/src/net/dnsclient_unix_test.go
index 2ad40dfe02b..06553636eee 100644
--- a/src/net/dnsclient_unix_test.go
+++ b/src/net/dnsclient_unix_test.go
@@ -10,7 +10,6 @@ import (
 	"context"
 	"errors"
 	"fmt"
-	"internal/poll"
 	"io/ioutil"
 	"os"
 	"path"
@@ -480,7 +479,7 @@ func TestGoLookupIPWithResolverConfig(t *testing.T) {
 			break
 		default:
 			time.Sleep(10 * time.Millisecond)
-			return dnsmessage.Message{}, poll.ErrTimeout
+			return dnsmessage.Message{}, os.ErrDeadlineExceeded
 		}
 		r := dnsmessage.Message{
 			Header: dnsmessage.Header{
@@ -993,7 +992,7 @@ func TestRetryTimeout(t *testing.T) {
 		if s == "192.0.2.1:53" {
 			deadline0 = deadline
 			time.Sleep(10 * time.Millisecond)
-			return dnsmessage.Message{}, poll.ErrTimeout
+			return dnsmessage.Message{}, os.ErrDeadlineExceeded
 		}
 
 		if deadline.Equal(deadline0) {
@@ -1131,7 +1130,7 @@ func TestStrictErrorsLookupIP(t *testing.T) {
 	}
 	makeTimeout := func() error {
 		return &DNSError{
-			Err:       poll.ErrTimeout.Error(),
+			Err:       os.ErrDeadlineExceeded.Error(),
 			Name:      name,
 			Server:    server,
 			IsTimeout: true,
@@ -1247,7 +1246,7 @@ func TestStrictErrorsLookupIP(t *testing.T) {
 					Questions: q.Questions,
 				}, nil
 			case resolveTimeout:
-				return dnsmessage.Message{}, poll.ErrTimeout
+				return dnsmessage.Message{}, os.ErrDeadlineExceeded
 			default:
 				t.Fatal("Impossible resolveWhich")
 			}
@@ -1372,7 +1371,7 @@ func TestStrictErrorsLookupTXT(t *testing.T) {
 
 		switch q.Questions[0].Name.String() {
 		case searchX:
-			return dnsmessage.Message{}, poll.ErrTimeout
+			return dnsmessage.Message{}, os.ErrDeadlineExceeded
 		case searchY:
 			return mockTXTResponse(q), nil
 		default:
@@ -1387,7 +1386,7 @@ func TestStrictErrorsLookupTXT(t *testing.T) {
 		var wantRRs int
 		if strict {
 			wantErr = &DNSError{
-				Err:       poll.ErrTimeout.Error(),
+				Err:       os.ErrDeadlineExceeded.Error(),
 				Name:      name,
 				Server:    server,
 				IsTimeout: true,
@@ -1415,7 +1414,7 @@ func TestDNSGoroutineRace(t *testing.T) {
 
 	fake := fakeDNSServer{rh: func(n, s string, q dnsmessage.Message, t time.Time) (dnsmessage.Message, error) {
 		time.Sleep(10 * time.Microsecond)
-		return dnsmessage.Message{}, poll.ErrTimeout
+		return dnsmessage.Message{}, os.ErrDeadlineExceeded
 	}}
 	r := Resolver{PreferGo: true, Dial: fake.DialContext}