webhooks/jira: Handle anomalous payloads properly.

We recently ran into a payload in production that didn't contain
an event type at all. A payload where we can't figure out the event
type is quite rare. Instead of letting these payloads run amok, we
should raise a more informative exception for such unusual payloads.
If we encounter too many of these, then we can choose to conduct a
deeper investigation on a case-by-case basis.

With some changes by Tim Abbott.

zproject/computed_settings.py

@@ -717,6 +717,7 @@ DIGEST_LOG_PATH = zulip_path("/var/log/zulip/digest.log")
 ANALYTICS_LOG_PATH = zulip_path("/var/log/zulip/analytics.log")
 ANALYTICS_LOCK_DIR = zulip_path("/home/zulip/deployments/analytics-lock-dir")
 WEBHOOK_LOG_PATH = zulip_path("/var/log/zulip/webhooks_errors.log")
+WEBHOOK_ANOMALOUS_PAYLOADS_LOG_PATH = zulip_path("/var/log/zulip/webhooks_anomalous_payloads.log")
 WEBHOOK_UNSUPPORTED_EVENTS_LOG_PATH = zulip_path("/var/log/zulip/webhooks_unsupported_events.log")
 SOFT_DEACTIVATION_LOG_PATH = zulip_path("/var/log/zulip/soft_deactivation.log")
 TRACEMALLOC_DUMP_DIR = zulip_path("/var/log/zulip/tracemalloc")
@@ -849,6 +850,12 @@ LOGGING: Dict[str, Any] = {
             "formatter": "webhook_request_data",
             "filename": WEBHOOK_UNSUPPORTED_EVENTS_LOG_PATH,
         },
+        "webhook_anomalous_file": {
+            "level": "DEBUG",
+            "class": "logging.handlers.WatchedFileHandler",
+            "formatter": "webhook_request_data",
+            "filename": WEBHOOK_ANOMALOUS_PAYLOADS_LOG_PATH,
+        },
     },
     "loggers": {
         # The Python logging module uses a hierarchy of logger names for config:
@@ -1003,6 +1010,11 @@ LOGGING: Dict[str, Any] = {
             "handlers": ["webhook_unsupported_file"],
             "propagate": False,
         },
+        "zulip.zerver.webhooks.anomalous": {
+            "level": "DEBUG",
+            "handlers": ["webhook_anomalous_file"],
+            "propagate": False,
+        },
     },
 }