webhooks/common: Allow suppressing exceptions for missing headers.

This is useful in cases where an event type HTTP header isn't
crucial for inferring the payload's type.

zerver/lib/webhooks/common.py

@@ -98,9 +98,10 @@ def check_send_webhook_message(
             pass
 
 def validate_extract_webhook_http_header(request: HttpRequest, header: str,
-                                         integration_name: str) -> str:
+                                         integration_name: str,
+                                         fatal: Optional[bool]=True) -> Optional[str]:
     extracted_header = request.META.get(DJANGO_HTTP_PREFIX + header)
-    if extracted_header is None:
+    if extracted_header is None and fatal:
         message_body = MISSING_EVENT_HEADER_MESSAGE.format(
             bot_name=request.user.full_name,
             request_path=request.path,

zerver/webhooks/bitbucket2/view.py

@@ -142,6 +142,7 @@ def get_type(request: HttpRequest, payload: Dict[str, Any]) -> str:
         # Note that we only need the HTTP header to determine pullrequest events.
         # We rely on the payload itself to determine the other ones.
         event_key = validate_extract_webhook_http_header(request, "X_EVENT_KEY", "BitBucket")
+        assert event_key is not None
         action = re.match('pullrequest:(?P<action>.*)$', event_key)
         if action:
             action_group = action.group('action')

zerver/webhooks/reviewboard/view.py

@@ -168,6 +168,7 @@ def api_reviewboard_webhook(
 ) -> HttpResponse:
     event_type = validate_extract_webhook_http_header(
         request, 'X_REVIEWBOARD_EVENT', 'ReviewBoard')
+    assert event_type is not None
 
     body_function = RB_MESSAGE_FUNCTIONS.get(event_type)
     if body_function is not None: