add h2 trailers example and fix sending

examples/addons/http-trailers.py

@@ -0,0 +1,26 @@
+"""
+This script simply prints all received HTTP Trailers.
+
+HTTP requests and responses can container trailing headers which are sent after
+the body is fully transmitted. Such trailers need to be announced in the initial
+headers by name, so the receiving endpoint can wait and read them after the
+body.
+"""
+
+from mitmproxy import http
+from mitmproxy.net.http import Headers
+
+def request(flow: http.HTTPFlow):
+    if flow.request.trailers:
+        print("HTTP Trailers detected! Request contains:", flow.request.trailers)
+
+def response(flow: http.HTTPFlow):
+    if flow.response.trailers:
+        print("HTTP Trailers detected! Response contains:", flow.response.trailers)
+
+    if flow.request.path == "/inject_trailers":
+        flow.response.headers["trailer"] = "x-my-injected-trailer-header"
+        flow.response.trailers = Headers([
+            (b"x-my-injected-trailer-header", b"foobar")
+        ])
+        print("Injected a new trailer...", flow.response.headers["trailer"])

mitmproxy/proxy/protocol/http2.py

@@ -615,7 +615,7 @@ class Http2SingleStreamLayer(httpbase._HttpTransmissionLayer, basethread.BaseThr
 
     @detect_zombie_stream
     def send_request_trailers(self, request):
-        self._send_trailers(self.server_conn, self.request_trailers)
+        self._send_trailers(self.server_conn, request.trailers)
 
     @detect_zombie_stream
     def send_request(self, request):
@@ -686,8 +686,8 @@ class Http2SingleStreamLayer(httpbase._HttpTransmissionLayer, basethread.BaseThr
         )
 
     @detect_zombie_stream
-    def send_response_trailers(self, _response):
-        self._send_trailers(self.client_conn, self.response_trailers)
+    def send_response_trailers(self, response):
+        self._send_trailers(self.client_conn, response.trailers)
 
     def _send_trailers(self, conn, trailers):
         if not trailers: