worker: Split into separate files.

This makes each worker faster to start up.

zerver/worker/email_mirror.py

@@ -0,0 +1,47 @@
+# Documented in https://zulip.readthedocs.io/en/latest/subsystems/queuing.html
+import base64
+import email
+import email.policy
+import logging
+from email.message import EmailMessage
+from typing import Any, Mapping
+
+from typing_extensions import override
+
+from zerver.lib.email_mirror import (
+    decode_stream_email_address,
+    is_missed_message_address,
+    rate_limit_mirror_by_realm,
+)
+from zerver.lib.email_mirror import process_message as mirror_email
+from zerver.lib.exceptions import RateLimitedError
+from zerver.worker.base import QueueProcessingWorker, assign_queue
+
+logger = logging.getLogger(__name__)
+
+
+@assign_queue("email_mirror")
+class MirrorWorker(QueueProcessingWorker):
+    @override
+    def consume(self, event: Mapping[str, Any]) -> None:
+        rcpt_to = event["rcpt_to"]
+        msg = email.message_from_bytes(
+            base64.b64decode(event["msg_base64"]),
+            policy=email.policy.default,
+        )
+        assert isinstance(msg, EmailMessage)  # https://github.com/python/typeshed/issues/2417
+        if not is_missed_message_address(rcpt_to):
+            # Missed message addresses are one-time use, so we don't need
+            # to worry about emails to them resulting in message spam.
+            recipient_realm = decode_stream_email_address(rcpt_to)[0].realm
+            try:
+                rate_limit_mirror_by_realm(recipient_realm)
+            except RateLimitedError:
+                logger.warning(
+                    "MirrorWorker: Rejecting an email from: %s to realm: %s - rate limited.",
+                    msg["From"],
+                    recipient_realm.subdomain,
+                )
+                return
+
+        mirror_email(msg, rcpt_to=rcpt_to)