Account for coalesced Trailer headers

agent/utils/utils.go

@@ -473,17 +473,24 @@ func (w *streamingResponseWriter) WriteHeader(status int) {
 
 	// Initialize the response trailers.
 	w.trailer = make(http.Header)
-	for _, k := range w.Header().Values("Trailer") {
-		// Initialize trailers with empty slices for any pre-declared values.
-		//
-		// This is necessary for the httputil.ReverseProxy type to forward them correctly.
-		// See [here](https://github.com/golang/go/blob/5e3c4016a436c357a57a6f7870913c6911c6904e/src/net/http/httputil/reverseproxy.go#L509)
-		if _, ok := hopHeaders[k]; ok {
-			continue
+	for _, joined := range w.Header().Values("Trailer") {
+		for _, k := range strings.Split(joined, ",") {
+			k = strings.TrimSpace(k)
+			if k == "" {
+				continue
+			}
+			if _, ok := hopHeaders[k]; ok {
+				continue
+			}
+			// Initialize trailers with empty slices for any pre-declared values.
+			//
+			// This is necessary for the httputil.ReverseProxy type to forward them correctly.
+			// See [here](https://github.com/golang/go/blob/5e3c4016a436c357a57a6f7870913c6911c6904e/src/net/http/httputil/reverseproxy.go#L509)
+			//
+			// We manually call `CanonicalHeaderKey` to preserve the invariant that
+			// all keys in a `Header` instance must be in their canonical format.
+			w.trailer[http.CanonicalHeaderKey(k)] = []string{}
 		}
-		// We manually call `CanonicalHeaderKey` to preserve the invariant that
-		// all keys in a `Header` instance must be in their canonical format.
-		w.trailer[http.CanonicalHeaderKey(k)] = []string{}
 	}
 
 	// Filter out hop-by-hop headers.

agent/utils/utils_test.go

@@ -292,6 +292,26 @@ func TestStreamingResponseWriter(t *testing.T) {
 			},
 			WantResponseBody: "OK",
 		},
+		{
+			Description: "Coalesced trailer",
+			Request:     &http.Request{},
+			Handler: http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+				w.Header().Add("Trailer", "Foo, Bar")
+				w.Write([]byte("OK"))
+				w.Header().Add("Foo", "Baz")
+				w.Header().Add("Bar", "Baz")
+			}),
+			WantResponse: &http.Response{
+				Status:     http.StatusText(http.StatusOK),
+				StatusCode: http.StatusOK,
+				Header:     http.Header{},
+				Trailer: http.Header{
+					"Foo": []string{"Baz"},
+					"Bar": []string{"Baz"},
+				},
+			},
+			WantResponseBody: "OK",
+		},
 		{
 			Description: "Undeclared trailer",
 			Request:     &http.Request{},