net/http: in Transport, don't error on non-chunked response with Trailer header

There are cases where HTTP message specifies the Trailer header
but not the Transfer-Encoding = chunked. The existing
implementation would return an error in those cases, without
returning also the message itself.
Instead, it would be preferable to let the library user decide when
the message is valid or not.
This change makes the fixTrailer() function not to return an error
and to keep the Trailer value in the Response.Header map but not
populate Response.Trailer.

Fixes #27197

Change-Id: Ic1e96791fde97f31dc5ecb8de05c8e4f49465c2c
Reviewed-on: https://go-review.googlesource.com/c/145398
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/http/request.go

@@ -53,8 +53,9 @@ var (
 	// available.
 	ErrNotSupported = &ProtocolError{"feature not supported"}
 
-	// ErrUnexpectedTrailer is returned by the Transport when a server
-	// replies with a Trailer header, but without a chunked reply.
+	// Deprecated: ErrUnexpectedTrailer is no longer returned by
+	// anything in the net/http package. Callers should not
+	// compare errors against this variable.
 	ErrUnexpectedTrailer = &ProtocolError{"trailer header without chunked transfer encoding"}
 
 	// ErrMissingBoundary is returned by Request.MultipartReader when the

src/net/http/response_test.go

@@ -157,6 +157,34 @@ var respTests = []respTest{
 		"Body here\ncontinued",
 	},
 
+	// Trailer header but no TransferEncoding
+	{
+		"HTTP/1.0 200 OK\r\n" +
+			"Trailer: Content-MD5, Content-Sources\r\n" +
+			"Content-Length: 10\r\n" +
+			"Connection: close\r\n" +
+			"\r\n" +
+			"Body here\n",
+
+		Response{
+			Status:     "200 OK",
+			StatusCode: 200,
+			Proto:      "HTTP/1.0",
+			ProtoMajor: 1,
+			ProtoMinor: 0,
+			Request:    dummyReq("GET"),
+			Header: Header{
+				"Connection":     {"close"},
+				"Content-Length": {"10"},
+				"Trailer":        []string{"Content-MD5, Content-Sources"},
+			},
+			Close:         true,
+			ContentLength: 10,
+		},
+
+		"Body here\n",
+	},
+
 	// Chunked response with Content-Length.
 	{
 		"HTTP/1.1 200 OK\r\n" +

src/net/http/transfer.go

@@ -740,6 +740,16 @@ func fixTrailer(header Header, te []string) (Header, error) {
 	if !ok {
 		return nil, nil
 	}
+	if !chunked(te) {
+		// Trailer and no chunking:
+		// this is an invalid use case for trailer header.
+		// Nevertheless, no error will be returned and we
+		// let users decide if this is a valid HTTP message.
+		// The Trailer header will be kept in Response.Header
+		// but not populate Response.Trailer.
+		// See issue #27197.
+		return nil, nil
+	}
 	header.Del("Trailer")
 
 	trailer := make(Header)
@@ -763,10 +773,6 @@ func fixTrailer(header Header, te []string) (Header, error) {
 	if len(trailer) == 0 {
 		return nil, nil
 	}
-	if !chunked(te) {
-		// Trailer and no chunking
-		return nil, ErrUnexpectedTrailer
-	}
 	return trailer, nil
 }