net/url: make *url.Error implement net.Error

Fixes #12866

net/http.Client returns some errors wrapped in a *url.Error. To avoid
the requirement to unwrap these errors to determine if the cause was
temporary or a timeout, make *url.Error implement net.Error directly.

Change-Id: I1ba84ecc7ad5147a40f056ff1254e60290152408
Reviewed-on: https://go-review.googlesource.com/15672Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>

src/net/http/client_test.go

@@ -983,13 +983,12 @@ func TestClientTimeout_Headers(t *testing.T) {
 	if err == nil {
 		t.Fatal("got response from Get; expected error")
 	}
-	ue, ok := err.(*url.Error)
-	if !ok {
+	if _, ok := err.(*url.Error); !ok {
 		t.Fatalf("Got error of type %T; want *url.Error", err)
 	}
-	ne, ok := ue.Err.(net.Error)
+	ne, ok := err.(net.Error)
 	if !ok {
-		t.Fatalf("Got url.Error.Err of type %T; want some net.Error", err)
+		t.Fatalf("Got error of type %T; want some net.Error", err)
 	}
 	if !ne.Timeout() {
 		t.Error("net.Error.Timeout = false; want true")

src/net/url/url.go

@@ -24,6 +24,24 @@ type Error struct {
 
 func (e *Error) Error() string { return e.Op + " " + e.URL + ": " + e.Err.Error() }
 
+type timeout interface {
+	Timeout() bool
+}
+
+func (e *Error) Timeout() bool {
+	t, ok := e.Err.(timeout)
+	return ok && t.Timeout()
+}
+
+type temporary interface {
+	Temporary() bool
+}
+
+func (e *Error) Temporary() bool {
+	t, ok := e.Err.(temporary)
+	return ok && t.Temporary()
+}
+
 func ishex(c byte) bool {
 	switch {
 	case '0' <= c && c <= '9':

src/net/url/url_test.go

@@ -6,6 +6,8 @@ package url
 
 import (
 	"fmt"
+	"io"
+	"net"
 	"reflect"
 	"strings"
 	"testing"
@@ -1229,3 +1231,84 @@ func TestShouldEscape(t *testing.T) {
 		}
 	}
 }
+
+type timeoutError struct {
+	timeout bool
+}
+
+func (e *timeoutError) Error() string { return "timeout error" }
+func (e *timeoutError) Timeout() bool { return e.timeout }
+
+type temporaryError struct {
+	temporary bool
+}
+
+func (e *temporaryError) Error() string   { return "temporary error" }
+func (e *temporaryError) Temporary() bool { return e.temporary }
+
+type timeoutTemporaryError struct {
+	timeoutError
+	temporaryError
+}
+
+func (e *timeoutTemporaryError) Error() string { return "timeout/temporary error" }
+
+var netErrorTests = []struct {
+	err       error
+	timeout   bool
+	temporary bool
+}{{
+	err:       &Error{"Get", "http://google.com/", &timeoutError{timeout: true}},
+	timeout:   true,
+	temporary: false,
+}, {
+	err:       &Error{"Get", "http://google.com/", &timeoutError{timeout: false}},
+	timeout:   false,
+	temporary: false,
+}, {
+	err:       &Error{"Get", "http://google.com/", &temporaryError{temporary: true}},
+	timeout:   false,
+	temporary: true,
+}, {
+	err:       &Error{"Get", "http://google.com/", &temporaryError{temporary: false}},
+	timeout:   false,
+	temporary: false,
+}, {
+	err:       &Error{"Get", "http://google.com/", &timeoutTemporaryError{timeoutError{timeout: true}, temporaryError{temporary: true}}},
+	timeout:   true,
+	temporary: true,
+}, {
+	err:       &Error{"Get", "http://google.com/", &timeoutTemporaryError{timeoutError{timeout: false}, temporaryError{temporary: true}}},
+	timeout:   false,
+	temporary: true,
+}, {
+	err:       &Error{"Get", "http://google.com/", &timeoutTemporaryError{timeoutError{timeout: true}, temporaryError{temporary: false}}},
+	timeout:   true,
+	temporary: false,
+}, {
+	err:       &Error{"Get", "http://google.com/", &timeoutTemporaryError{timeoutError{timeout: false}, temporaryError{temporary: false}}},
+	timeout:   false,
+	temporary: false,
+}, {
+	err:       &Error{"Get", "http://google.com/", io.EOF},
+	timeout:   false,
+	temporary: false,
+}}
+
+// Test that url.Error implements net.Error and that it forwards
+func TestURLErrorImplementsNetError(t *testing.T) {
+	for i, tt := range netErrorTests {
+		err, ok := tt.err.(net.Error)
+		if !ok {
+			t.Errorf("%d: %T does not implement net.Error", i+1, tt.err)
+			continue
+		}
+		if err.Timeout() != tt.timeout {
+			t.Errorf("%d: err.Timeout(): want %v, have %v", i+1, tt.timeout, err.Timeout())
+			continue
+		}
+		if err.Temporary() != tt.temporary {
+			t.Errorf("%d: err.Temporary(): want %v, have %v", i+1, tt.temporary, err.Temporary())
+		}
+	}
+}