http2: add support for Transport reading trailers from the server

Tests in the main go repo.

Updates golang/go#13557

Change-Id: Ide7be9e24b603008faf9f3ea55e936b86ea1c42b
Reviewed-on: https://go-review.googlesource.com/17930
Reviewed-by: Blake Mizerany <blake.mizerany@gmail.com>

http2/transport.go

@@ -159,6 +159,13 @@ type clientStream struct {
 
 	peerReset chan struct{} // closed on peer reset
 	resetErr  error         // populated before peerReset is closed
+
+	// owned by clientConnReadLoop:
+	headersDone  bool // got HEADERS w/ END_HEADERS
+	trailersDone bool // got second HEADERS frame w/ END_HEADERS
+
+	trailer    http.Header // accumulated trailers
+	resTrailer http.Header // client's Response.Trailer
 }
 
 // awaitRequestCancel runs in its own goroutine and waits for the user's
@@ -918,18 +925,42 @@ func (rl *clientConnReadLoop) processHeaderBlockFragment(frag []byte, streamID u
 		// server got our RST_STREAM.
 		return nil
 	}
+	if cs.headersDone {
+		rl.hdec.SetEmitFunc(cs.onNewTrailerField)
+	} else {
+		rl.hdec.SetEmitFunc(rl.onNewHeaderField)
+	}
 	_, err := rl.hdec.Write(frag)
 	if err != nil {
-		return err
+		return ConnectionError(ErrCodeCompression)
 	}
 	if !headersEnded {
 		rl.continueStreamID = cs.ID
 		return nil
 	}
-
 	// HEADERS (or CONTINUATION) are now over.
 	rl.continueStreamID = 0
 
+	if !cs.headersDone {
+		cs.headersDone = true
+	} else {
+		// We're dealing with trailers. (and specifically the
+		// final frame of headers)
+		if cs.trailersDone {
+			// Too many HEADERS frames for this stream.
+			return ConnectionError(ErrCodeProtocol)
+		}
+		cs.trailersDone = true
+		if !streamEnded {
+			// We expect that any header block fragment
+			// frame for trailers with END_HEADERS also
+			// has END_STREAM.
+			return ConnectionError(ErrCodeProtocol)
+		}
+		rl.endStream(cs)
+		return nil
+	}
+
 	if rl.reqMalformed != nil {
 		cs.resc <- resAndError{err: rl.reqMalformed}
 		rl.cc.writeStreamReset(cs.ID, ErrCodeProtocol, rl.reqMalformed)
@@ -970,6 +1001,7 @@ func (rl *clientConnReadLoop) processHeaderBlockFragment(frag []byte, streamID u
 		}
 	}
 
+	cs.resTrailer = res.Trailer
 	rl.activeRes[cs.ID] = cs
 	cs.resc <- resAndError{res: res}
 	rl.nextRes = nil // unused now; will be reset next HEADERS frame
@@ -1076,12 +1108,24 @@ func (rl *clientConnReadLoop) processData(f *DataFrame) error {
 	}
 
 	if f.StreamEnded() {
-		cs.bufPipe.CloseWithError(io.EOF)
-		delete(rl.activeRes, cs.ID)
+		rl.endStream(cs)
 	}
 	return nil
 }
 
+func (rl *clientConnReadLoop) endStream(cs *clientStream) {
+	// TODO: check that any declared content-length matches, like
+	// server.go's (*stream).endStream method.
+	cs.bufPipe.closeWithErrorAndCode(io.EOF, cs.copyTrailers)
+	delete(rl.activeRes, cs.ID)
+}
+
+func (cs *clientStream) copyTrailers() {
+	for k, vv := range cs.trailer {
+		cs.resTrailer[k] = vv
+	}
+}
+
 func (rl *clientConnReadLoop) processGoAway(f *GoAwayFrame) error {
 	cc := rl.cc
 	cc.t.connPool().MarkDead(cc)
@@ -1203,6 +1247,7 @@ func (rl *clientConnReadLoop) onNewHeaderField(f hpack.HeaderField) {
 	if VerboseLogs {
 		cc.logf("Header field: %+v", f)
 	}
+	// TODO: enforce max header list size like server.
 	isPseudo := strings.HasPrefix(f.Name, ":")
 	if isPseudo {
 		if rl.sawRegHeader {
@@ -1226,7 +1271,38 @@ func (rl *clientConnReadLoop) onNewHeaderField(f hpack.HeaderField) {
 		}
 	} else {
 		rl.sawRegHeader = true
-		rl.nextRes.Header.Add(http.CanonicalHeaderKey(f.Name), f.Value)
+		key := http.CanonicalHeaderKey(f.Name)
+		if key == "Trailer" {
+			t := rl.nextRes.Trailer
+			if t == nil {
+				t = make(http.Header)
+				rl.nextRes.Trailer = t
+			}
+			foreachHeaderElement(f.Value, func(v string) {
+				t[http.CanonicalHeaderKey(v)] = nil
+			})
+		} else {
+			rl.nextRes.Header.Add(key, f.Value)
+		}
+	}
+}
+
+func (cs *clientStream) onNewTrailerField(f hpack.HeaderField) {
+	isPseudo := strings.HasPrefix(f.Name, ":")
+	if isPseudo {
+		// TODO: Bogus. report an error later when we close their body.
+		// drop for now.
+		return
+	}
+	key := http.CanonicalHeaderKey(f.Name)
+	if _, ok := cs.resTrailer[key]; ok {
+		if cs.trailer == nil {
+			cs.trailer = make(http.Header)
+		}
+		const tooBig = 1000 // TODO: arbitrary; use max header list size limits
+		if cur := cs.trailer[key]; len(cur) < tooBig {
+			cs.trailer[key] = append(cur, f.Value)
+		}
 	}
 }