net/http: populate Response.Request when using NewFileTransport

Fixes #51562

Change-Id: Ia6fe4728b1e3e0cf3a6462be99c1044260cadf31
Reviewed-on: https://go-review.googlesource.com/c/go/+/720822
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Mark Freeman <markfreeman@google.com>

src/net/http/filetransport.go

@@ -57,7 +57,7 @@ func (t fileTransport) RoundTrip(req *Request) (resp *Response, err error) {
 	// sends our *Response on, once the *Response itself has been
 	// populated (even if the body itself is still being
 	// written to the res.Body, a pipe)
-	rw, resc := newPopulateResponseWriter()
+	rw, resc := newPopulateResponseWriter(req)
 	go func() {
 		t.fh.ServeHTTP(rw, req)
 		rw.finish()
@@ -65,7 +65,7 @@ func (t fileTransport) RoundTrip(req *Request) (resp *Response, err error) {
 	return <-resc, nil
 }
 
-func newPopulateResponseWriter() (*populateResponse, <-chan *Response) {
+func newPopulateResponseWriter(req *Request) (*populateResponse, <-chan *Response) {
 	pr, pw := io.Pipe()
 	rw := &populateResponse{
 		ch: make(chan *Response),
@@ -76,6 +76,7 @@ func newPopulateResponseWriter() (*populateResponse, <-chan *Response) {
 			Header:     make(Header),
 			Close:      true,
 			Body:       pr,
+			Request:    req,
 		},
 	}
 	return rw, rw.ch

src/net/http/filetransport_test.go

@@ -53,6 +53,9 @@ func TestFileTransport(t *testing.T) {
 		if string(slurp) != "Bar" {
 			t.Errorf("for %s, got content %q, want %q", urlstr, string(slurp), "Bar")
 		}
+		if got := res.Request.URL.String(); got != urlstr {
+			t.Errorf("for %s, Response.Request.URL = %s, want = %s", urlstr, got, urlstr)
+		}
 	}
 
 	const badURL = "file://../no-exist.txt"