net/http: make timeout errors match context.DeadlineExceeded

When returning an error which implements net.Error and reports
itself as a timeout, also report it as matching context.DeadlineExceeded.
This matches the behavior of timeout errors in the net package
and elsewhere.

Fixes #50856

Change-Id: I2ca911e3677a699af27ba89b1200401baa8b3b1b
Reviewed-on: https://go-review.googlesource.com/c/go/+/567537
Reviewed-by: Jonathan Amsterdam <jba@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/net/http/client.go

@@ -725,10 +725,7 @@ func (c *Client) do(req *Request) (retres *Response, reterr error) {
 			// c.send() always closes req.Body
 			reqBodyClosed = true
 			if !deadline.IsZero() && didTimeout() {
-				err = &httpError{
-					err:     err.Error() + " (Client.Timeout exceeded while awaiting headers)",
-					timeout: true,
-				}
+				err = &timeoutError{err.Error() + " (Client.Timeout exceeded while awaiting headers)"}
 			}
 			return nil, uerr(err)
 		}
@@ -968,10 +965,7 @@ func (b *cancelTimerBody) Read(p []byte) (n int, err error) {
 		return n, err
 	}
 	if b.reqDidTimeout() {
-		err = &httpError{
-			err:     err.Error() + " (Client.Timeout or context cancellation while reading body)",
-			timeout: true,
-		}
+		err = &timeoutError{err.Error() + " (Client.Timeout or context cancellation while reading body)"}
 	}
 	return n, err
 }

src/net/http/client_test.go

@@ -1249,6 +1249,9 @@ func testClientTimeout(t *testing.T, mode testMode) {
 		} else if !ne.Timeout() {
 			t.Errorf("net.Error.Timeout = false; want true")
 		}
+		if !errors.Is(err, context.DeadlineExceeded) {
+			t.Errorf("ReadAll error = %q; expected some context.DeadlineExceeded", err)
+		}
 		if got := ne.Error(); !strings.Contains(got, "(Client.Timeout") {
 			if runtime.GOOS == "windows" && strings.HasPrefix(runtime.GOARCH, "arm") {
 				testenv.SkipFlaky(t, 43120)
@@ -1292,6 +1295,9 @@ func testClientTimeout_Headers(t *testing.T, mode testMode) {
 	if !ne.Timeout() {
 		t.Error("net.Error.Timeout = false; want true")
 	}
+	if !errors.Is(err, context.DeadlineExceeded) {
+		t.Errorf("ReadAll error = %q; expected some context.DeadlineExceeded", err)
+	}
 	if got := ne.Error(); !strings.Contains(got, "Client.Timeout exceeded") {
 		if runtime.GOOS == "windows" && strings.HasPrefix(runtime.GOARCH, "arm") {
 			testenv.SkipFlaky(t, 43120)
@@ -1992,6 +1998,9 @@ func testClientDoCanceledVsTimeout(t *testing.T, mode testMode) {
 			if g, w := ue.Err, wantErr; g != w {
 				t.Errorf("url.Error.Err = %v; want %v", g, w)
 			}
+			if got := errors.Is(err, context.DeadlineExceeded); got != wantIsTimeout {
+				t.Errorf("errors.Is(err, context.DeadlineExceeded) = %v, want %v", got, wantIsTimeout)
+			}
 		})
 	}
 }

src/net/http/transport.go

@@ -2557,16 +2557,18 @@ type writeRequest struct {
 	continueCh <-chan struct{}
 }
 
-type httpError struct {
-	err     string
-	timeout bool
+// httpTimeoutError represents a timeout.
+// It implements net.Error and wraps context.DeadlineExceeded.
+type timeoutError struct {
+	err string
 }
 
-func (e *httpError) Error() string   { return e.err }
-func (e *httpError) Timeout() bool   { return e.timeout }
-func (e *httpError) Temporary() bool { return true }
+func (e *timeoutError) Error() string     { return e.err }
+func (e *timeoutError) Timeout() bool     { return true }
+func (e *timeoutError) Temporary() bool   { return true }
+func (e *timeoutError) Is(err error) bool { return err == context.DeadlineExceeded }
 
-var errTimeout error = &httpError{err: "net/http: timeout awaiting response headers", timeout: true}
+var errTimeout error = &timeoutError{"net/http: timeout awaiting response headers"}
 
 // errRequestCanceled is set to be identical to the one from h2 to facilitate
 // testing.