webhooks: Add helper to extract and validate HTTP event headers.

This is a part of our efforts to close #6213.

zerver/lib/webhooks/common.py

@@ -1,13 +1,40 @@
+from django.conf import settings
 from django.http import HttpRequest
+from django.utils.translation import ugettext as _
 from typing import Optional, Text
 
 from zerver.lib.actions import check_send_stream_message, \
-    check_send_private_message
-from zerver.lib.exceptions import StreamDoesNotExistError
+    check_send_private_message, send_rate_limited_pm_notification_to_bot_owner
+from zerver.lib.exceptions import StreamDoesNotExistError, JsonableError, \
+    ErrorCode
 from zerver.lib.request import REQ, has_request_variables
-from zerver.models import UserProfile
+from zerver.lib.send_email import FromAddress
+from zerver.models import UserProfile, get_system_bot
 
 
+MISSING_EVENT_HEADER_MESSAGE = """
+Hi there!  Your bot {bot_name} just sent an HTTP request to {request_path} that
+is missing the HTTP {header_name} header.  Because this header is how
+{integration_name} indicates the event type, this usually indicates a configuration
+issue, where you either entered the URL for a different integration, or are running
+an older version of the third-party service that doesn't provide that header.
+Contact {support_email} if you need help debugging!
+"""
+
+# Django prefixes all custom HTTP headers with `HTTP_`
+DJANGO_HTTP_PREFIX = "HTTP_"
+
+class MissingHTTPEventHeader(JsonableError):
+    code = ErrorCode.MISSING_HTTP_EVENT_HEADER
+    data_fields = ['header']
+
+    def __init__(self, header: Text) -> None:
+        self.header = header
+
+    @staticmethod
+    def msg_format() -> str:
+        return _("Missing the HTTP event header '{header}'")
+
 @has_request_variables
 def check_send_webhook_message(
         request: HttpRequest, user_profile: UserProfile,
@@ -32,3 +59,21 @@ def check_send_webhook_message(
             # stream, so we don't need to re-raise it since it clutters up
             # webhook-errors.log
             pass
+
+def validate_extract_webhook_http_header(request: HttpRequest, header: Text,
+                                         integration_name: Text) -> Text:
+    extracted_header = request.META.get(DJANGO_HTTP_PREFIX + header)
+    if extracted_header is None:
+        message_body = MISSING_EVENT_HEADER_MESSAGE.format(
+            bot_name=request.user.full_name,
+            request_path=request.path,
+            header_name=header,
+            integration_name=integration_name,
+            support_email=FromAddress.SUPPORT,
+        )
+        send_rate_limited_pm_notification_to_bot_owner(
+            request.user, request.user.realm, message_body)
+
+        raise MissingHTTPEventHeader(header)
+
+    return extracted_header