Improve the trailing headers support

The EndBody event should only be sent after the headers as this
results in the stream being closed.

It is acceptable to send no response and only trailing headers, in
which case a default 200 status code response is sent with the
headers.

src/hypercorn/protocol/http_stream.py

@@ -194,16 +194,36 @@ async def app_send(self, message: Optional[ASGISendEvent]) -> None:
                     )
 
                 if not message.get("more_body", False):
-                    await self.send(EndBody(stream_id=self.stream_id))
-
                     if self.response.get("trailers", False):
                         self.state = ASGIHTTPState.TRAILERS
                     else:
-                        self.state = ASGIHTTPState.CLOSED
-                        await self.config.log.access(
-                            self.scope, self.response, time() - self.start_time
+                        await self._send_closed()
+            elif (
+                message["type"] == "http.response.trailers"
+                and self.scope["http_version"] in TRAILERS_VERSIONS
+                and self.state == ASGIHTTPState.REQUEST
+            ):
+                for name, value in self.scope["headers"]:
+                    if name == b"te" and value == b"trailers":
+                        headers = build_and_validate_headers(message["headers"])
+                        self.response = {
+                            "type": "http.response.start",
+                            "status": 200,
+                            "headers": headers,
+                        }
+                        await self.send(
+                            Response(
+                                stream_id=self.stream_id,
+                                headers=headers,
+                                status_code=200,
+                            )
                         )
-                        await self.send(StreamClosed(stream_id=self.stream_id))
+                        self.state = ASGIHTTPState.TRAILERS
+                        break
+
+                if not message.get("more_trailers", False):
+                    await self._send_closed()
+
             elif (
                 message["type"] == "http.response.trailers"
                 and self.scope["http_version"] in TRAILERS_VERSIONS
@@ -216,14 +236,16 @@ async def app_send(self, message: Optional[ASGISendEvent]) -> None:
                         break
 
                 if not message.get("more_trailers", False):
-                    self.state = ASGIHTTPState.CLOSED
-                    await self.config.log.access(
-                        self.scope, self.response, time() - self.start_time
-                    )
-                    await self.send(StreamClosed(stream_id=self.stream_id))
+                    await self._send_closed()
             else:
                 raise UnexpectedMessageError(self.state, message["type"])
 
+    async def _send_closed(self) -> None:
+        await self.send(EndBody(stream_id=self.stream_id))
+        self.state = ASGIHTTPState.CLOSED
+        await self.config.log.access(self.scope, self.response, time() - self.start_time)
+        await self.send(StreamClosed(stream_id=self.stream_id))
+
     async def _send_error_response(self, status_code: int) -> None:
         await self.send(
             Response(

tests/protocol/test_http_stream.py

@@ -269,8 +269,8 @@ async def test_send_trailers(stream: HTTPStream) -> None:
     assert stream.send.call_args_list == [  # type: ignore
         call(Response(stream_id=1, headers=[], status_code=200)),
         call(Body(stream_id=1, data=b"Body")),
-        call(EndBody(stream_id=1)),
         call(Trailers(stream_id=1, headers=[(b"X", b"V")])),
+        call(EndBody(stream_id=1)),
         call(StreamClosed(stream_id=1)),
     ]
 
@@ -337,7 +337,6 @@ async def test_send_app_error(stream: HTTPStream) -> None:
     "state, message_type",
     [
         (ASGIHTTPState.REQUEST, "not_a_real_type"),
-        (ASGIHTTPState.REQUEST, "http.response.trailers"),
         (ASGIHTTPState.RESPONSE, "http.response.start"),
         (ASGIHTTPState.TRAILERS, "http.response.start"),
         (ASGIHTTPState.CLOSED, "http.response.start"),