push_notifications: Lazily import APNS libraries.

The APNS client libraries (especially the hyper.http20 one) were
determined via profiling to take significant time during the import
process, so we move them to be lazily imported in order to optimize
the overall Zulip import process.  This save up to about 100ms in
import time.

These libraries are only used in certain Django processes inside
zulipchat.com, and so are unnecessary both in development as well as
for self-hosted Zulip servers.

zerver/lib/push_notifications.py

@@ -12,14 +12,11 @@ import random
 
 from typing import Any, Dict, List, Optional, SupportsInt, Tuple, Type, Union
 
-from apns2.client import APNsClient
-from apns2.payload import Payload as APNsPayload
 from django.conf import settings
 from django.db import IntegrityError, transaction
 from django.utils.timezone import now as timezone_now
 from django.utils.translation import ugettext as _
 from gcm import GCM
-from hyper.http20.exceptions import HTTP20Error
 import requests
 import urllib
 import ujson
@@ -55,10 +52,13 @@ def hex_to_b64(data: str) -> bytes:
 # Sending to APNs, for iOS
 #
 
-_apns_client = None  # type: Optional[APNsClient]
+_apns_client = None  # type: Optional[Any]
 _apns_client_initialized = False
 
-def get_apns_client() -> APNsClient:
+def get_apns_client() -> Any:
+    # We lazily do this import as part of optimizing Zulip's base
+    # import time.
+    from apns2.client import APNsClient
     global _apns_client, _apns_client_initialized
     if not _apns_client_initialized:
         # NB if called concurrently, this will make excess connections.
@@ -104,7 +104,15 @@ APNS_MAX_RETRIES = 3
 @statsd_increment("apple_push_notification")
 def send_apple_push_notification(user_id: int, devices: List[DeviceToken],
                                  payload_data: Dict[str, Any], remote: bool=False) -> None:
-    client = get_apns_client()
+    # We lazily do the APNS imports as part of optimizing Zulip's base
+    # import time; since these are only needed in the push
+    # notification queue worker, it's best to only import them in the
+    # code that needs them.
+    from apns2.payload import Payload as APNsPayload
+    from apns2.client import APNsClient
+    from hyper.http20.exceptions import HTTP20Error
+
+    client = get_apns_client()  # type: APNsClient
     if client is None:
         logging.warning("APNs: Dropping a notification because nothing configured.  "
                         "Set PUSH_NOTIFICATION_BOUNCER_URL (or APNS_CERT_FILE).")