webhooks: Add generic exception for unexpected webhook events.

UnexpectedWebhookEventType is a generic exception that we may
now raise when we encounter a webhook event that is new or one
that we simply aren't aware of.

templates/zerver/api/webhook-walkthrough.md

@@ -535,3 +535,18 @@ be running an older version of the integration that doesn't set that header.
 
 If the requisite header is missing, this function sends a PM to the owner of the
 webhook bot, notifying them of the missing header.
+
+### Handling unexpected webhook event types
+
+Many third-party services have dozens of different event types. In some cases, we
+may choose to explicitly ignore specific events. In other cases, there may be
+events that are new or events that we don't know about. In such cases, we
+recommend raising `UnexpectedWebhookEventType` (found in
+`zerver/lib/webhooks/common.py`), like so:
+
+```
+raise UnexpectedWebhookEventType(webhook_name, event_type)
+```
+
+`webhook_name` is the name of the webhook that raises the exception. `event_type`
+is the name of the unexpected webhook event.

tools/coveragerc

@@ -11,6 +11,8 @@ exclude_lines =
     raise AssertionError
     # Don't require coverage for __str__ statements just used for printing
     def __str__[(]self[)] -> .*:
+    # Don't require coverage for errors about unsupported webhook event types
+    raise UnexpectedWebhookEventType
 
 [run]
 omit =

zerver/lib/exceptions.py

@@ -34,6 +34,7 @@ class ErrorCode(AbstractEnum):
     MISSING_HTTP_EVENT_HEADER = ()
     STREAM_DOES_NOT_EXIST = ()
     UNAUTHORIZED_PRINCIPAL = ()
+    UNEXPECTED_WEBHOOK_EVENT_TYPE = ()
     BAD_EVENT_QUEUE_ID = ()
     CSRF_FAILED = ()
     INVITATION_FAILED = ()

zerver/lib/webhooks/common.py

@@ -24,6 +24,18 @@
 # Django prefixes all custom HTTP headers with `HTTP_`
 DJANGO_HTTP_PREFIX = "HTTP_"
 
+class UnexpectedWebhookEventType(JsonableError):
+    code = ErrorCode.UNEXPECTED_WEBHOOK_EVENT_TYPE
+    data_fields = ['webhook_name', 'event_type']
+
+    def __init__(self, webhook_name: str, event_type: Optional[str]) -> None:
+        self.webhook_name = webhook_name
+        self.event_type = event_type
+
+    @staticmethod
+    def msg_format() -> str:
+        return _("The '{event_type}' event isn't currently supported by the {webhook_name} webhook")
+
 class MissingHTTPEventHeader(JsonableError):
     code = ErrorCode.MISSING_HTTP_EVENT_HEADER
     data_fields = ['header']