net/http: ensured that proxy errors are returned by Transport.RoundTrip.

Fixes #8755.

LGTM=bradfitz
R=bradfitz
CC=golang-codereviews, jtuley
https://golang.org/cl/136710044

src/net/http/transport.go

@@ -316,7 +316,7 @@ func (t *Transport) connectMethodForRequest(treq *transportRequest) (cm connectM
 	if t.Proxy != nil {
 		cm.proxyURL, err = t.Proxy(treq.Request)
 	}
-	return cm, nil
+	return cm, err
 }
 
 // proxyAuth returns the Proxy-Authorization header to set

src/net/http/transport_test.go

@@ -2136,6 +2136,24 @@ func TestTransportDialTLS(t *testing.T) {
 	}
 }
 
+// Test for issue 8755
+// Ensure that if a proxy returns an error, it is exposed by RoundTrip
+func TestRoundTripReturnsProxyError(t *testing.T) {
+	badProxy := func(*http.Request) (*url.URL, error) {
+		return nil, errors.New("errorMessage")
+	}
+
+	tr := &Transport{Proxy: badProxy}
+
+	req, _ := http.NewRequest("GET", "http://example.com", nil)
+
+	_, err := tr.RoundTrip(req)
+
+	if err == nil {
+		t.Error("Expected proxy error to be returned by RoundTrip")
+	}
+}
+
 func wantBody(res *http.Response, err error, want string) error {
 	if err != nil {
 		return err