web: do not send content on HTTPError(204)

Change `write_error()` to not send body if
 status code in (204, 304) or (100 <= status code < 200)
 - refactored into `RequestHandler._should_not_send_content(status_code)`

Fixes #3360

tornado/test/web_test.py

@@ -1691,6 +1691,18 @@ def test_204_headers(self):
         self.assertNotIn("Transfer-Encoding", response.headers)
 
 
+class Header204viaHTTPErrorTest(SimpleHandlerTestCase):
+    class Handler(RequestHandler):
+        def get(self):
+            raise HTTPError(204)
+
+    def test_204_headers_via_httperror(self):
+        response = self.fetch("/")
+        self.assertEqual(response.code, 204)
+        self.assertNotIn("Content-Length", response.headers)
+        self.assertNotIn("Transfer-Encoding", response.headers)
+
+
 class Header304Test(SimpleHandlerTestCase):
     class Handler(RequestHandler):
         def get(self):

tornado/web.py

@@ -1222,6 +1222,10 @@ def flush(self, include_footers: bool = False) -> "Future[None]":
                 future.set_result(None)
                 return future
 
+    def _should_not_send_content(self, status_code: int) -> bool:
+        """Check if we should not send body content for given `status_code`"""
+        return status_code in (204, 304) or (100 <= status_code < 200)
+
     def finish(self, chunk: Optional[Union[str, bytes, dict]] = None) -> "Future[None]":
         """Finishes this response, ending the HTTP request.
 
@@ -1255,10 +1259,9 @@ def finish(self, chunk: Optional[Union[str, bytes, dict]] = None) -> "Future[Non
                 if self.check_etag_header():
                     self._write_buffer = []
                     self.set_status(304)
-            if self._status_code in (204, 304) or (100 <= self._status_code < 200):
-                assert not self._write_buffer, (
-                    "Cannot send body with %s" % self._status_code
-                )
+            if self._should_not_send_content(self._status_code):
+                if self._write_buffer:
+                    raise RuntimeError(f"Cannot send body with status code HTTP{self._status_code}")
                 self._clear_representation_headers()
             elif "Content-Length" not in self._headers:
                 content_length = sum(len(part) for part in self._write_buffer)
@@ -1349,7 +1352,9 @@ def write_error(self, status_code: int, **kwargs: Any) -> None:
         the "current" exception for purposes of methods like
         ``sys.exc_info()`` or ``traceback.format_exc``.
         """
-        if self.settings.get("serve_traceback") and "exc_info" in kwargs:
+        if self._should_not_send_content(status_code):
+            self.finish()
+        elif self.settings.get("serve_traceback") and "exc_info" in kwargs:
             # in debug mode, try to send a traceback
             self.set_header("Content-Type", "text/plain")
             for line in traceback.format_exception(*kwargs["exc_info"]):