net/http, net/http/httptest: make http2's TrailerPrefix work for http1

Go's http1 implementation originally had a mechanism to send HTTP
trailers based on pre-declaring the trailer keys whose values you'd
later let after the header was written.

http2 copied the same mechanism, but it was found to be unsufficient
for gRPC's wire protocol. A second trailer mechanism was added later
(but only to http2) for handlers that want to send a trailer without
knowing in advance they'd need to.

Copy the same mechanism back to http1 and document it.

Fixes #15754

Change-Id: I8c40d55e28b0e5b7087d3d1a904a392c56ee1f9b
Reviewed-on: https://go-review.googlesource.com/32479
Reviewed-by: Ian Lance Taylor <iant@golang.org>

2 files changed, 13 insertions, 0 deletions

diff --git a/src/net/http/httptest/recorder.go b/src/net/http/httptest/recorder.go
index 24653031bd..5f1aa6af47 100644
--- a/src/net/http/httptest/recorder.go
+++ b/src/net/http/httptest/recorder.go
@@ -203,6 +203,17 @@ func (rw *ResponseRecorder) Result() *http.Response {
 			res.Trailer[k] = vv2
 		}
 	}
+	for k, vv := range rw.HeaderMap {
+		if !strings.HasPrefix(k, http.TrailerPrefix) {
+			continue
+		}
+		if res.Trailer == nil {
+			res.Trailer = make(http.Header)
+		}
+		for _, v := range vv {
+			res.Trailer.Add(strings.TrimPrefix(k, http.TrailerPrefix), v)
+		}
+	}
 	return res
 }
 
diff --git a/src/net/http/httptest/recorder_test.go b/src/net/http/httptest/recorder_test.go
index ff9b9911a8..9afba4e556 100644
--- a/src/net/http/httptest/recorder_test.go
+++ b/src/net/http/httptest/recorder_test.go
@@ -207,6 +207,7 @@ func TestRecorder(t *testing.T) {
 				w.Header().Set("Trailer-A", "valuea")
 				w.Header().Set("Trailer-C", "valuec")
 				w.Header().Set("Trailer-NotDeclared", "should be omitted")
+				w.Header().Set("Trailer:Trailer-D", "with prefix")
 			},
 			check(
 				hasStatus(200),
@@ -216,6 +217,7 @@ func TestRecorder(t *testing.T) {
 				hasTrailer("Trailer-A", "valuea"),
 				hasTrailer("Trailer-C", "valuec"),
 				hasNotTrailers("Non-Trailer", "Trailer-B", "Trailer-NotDeclared"),
+				hasTrailer("Trailer-D", "with prefix"),
 			),
 		},
 		{