Send gRPC error metadata only as HTTP trailers (#410)

In #406, I was too cavalier about `net/http.ResponseWriter`'s API to
send HTTP trailers. Because we're not calling `WriteHeader` before
between predeclaring our trailers and actually setting the trailer
values, we end up sending the same data as both HTTP headers and
trailers.

connect_ext_test.go

@@ -19,6 +19,7 @@ import (
 	"compress/flate"
 	"compress/gzip"
 	"context"
+	"encoding/binary"
 	"errors"
 	"fmt"
 	"io"
@@ -1824,6 +1825,46 @@ func TestUnflushableResponseWriter(t *testing.T) {
 	}
 }
 
+func TestGRPCErrorMetadataIsTrailersOnly(t *testing.T) {
+	t.Parallel()
+	mux := http.NewServeMux()
+	mux.Handle(pingv1connect.NewPingServiceHandler(pingServer{}))
+	server := httptest.NewUnstartedServer(mux)
+	server.EnableHTTP2 = true
+	server.StartTLS()
+	t.Cleanup(server.Close)
+
+	protoBytes, err := proto.Marshal(&pingv1.FailRequest{Code: int32(connect.CodeInternal)})
+	assert.Nil(t, err)
+	// Manually construct a gRPC prefix. Data is uncompressed, so the first byte
+	// is 0. Set the last 4 bytes to the message length.
+	var prefix [5]byte
+	binary.BigEndian.PutUint32(prefix[1:5], uint32(len(protoBytes)))
+	body := append(prefix[:], protoBytes...)
+	// Manually send off a gRPC request.
+	req, err := http.NewRequestWithContext(
+		context.Background(),
+		http.MethodPost,
+		server.URL+"/connect.ping.v1.PingService/Fail",
+		bytes.NewReader(body),
+	)
+	assert.Nil(t, err)
+	req.Header.Set("Content-Type", "application/grpc")
+	res, err := server.Client().Do(req)
+	assert.Nil(t, err)
+	assert.Equal(t, res.StatusCode, http.StatusOK)
+	assert.Equal(t, res.Header.Get("Content-Type"), "application/grpc")
+	// pingServer.Fail adds handlerHeader and handlerTrailer to the error
+	// metadata. The gRPC protocol should send all error metadata as trailers.
+	assert.Zero(t, res.Header.Get(handlerHeader))
+	assert.Zero(t, res.Header.Get(handlerTrailer))
+	_, err = io.Copy(io.Discard, res.Body)
+	assert.Nil(t, err)
+	assert.Nil(t, res.Body.Close())
+	assert.NotZero(t, res.Trailer.Get(handlerHeader))
+	assert.NotZero(t, res.Trailer.Get(handlerTrailer))
+}
+
 func TestBidiOverHTTP1(t *testing.T) {
 	t.Parallel()
 	mux := http.NewServeMux()

protocol_grpc.go

@@ -528,8 +528,11 @@ func (hc *grpcHandlerConn) Close(err error) (retErr error) {
 		// has wrapped the response writer in net/http middleware that doesn't
 		// implement http.Flusher, we must pre-declare our HTTP trailers. We can
 		// remove this when Go 1.21 ships and we drop support for Go 1.19.
-		for key, values := range mergedTrailers {
+		for key := range mergedTrailers {
 			hc.responseWriter.Header().Add("Trailer", key)
+		}
+		hc.responseWriter.WriteHeader(http.StatusOK)
+		for key, values := range mergedTrailers {
 			for _, value := range values {
 				hc.responseWriter.Header().Add(key, value)
 			}