push notifs: Add a diagnostic in API of whether push notifs enabled.

When the answer is False, this will allow the mobile app to show a
warning that push notifications will not work and the server admin
should set them up.

Based partly on Kunal's PR #7810.  Provides the necessary backend API
for zulip/zulip-mobile#1507.

zerver/lib/push_notifications.py

@@ -69,6 +69,10 @@ def get_apns_client() -> APNsClient:
         _apns_client_initialized = True
     return _apns_client
 
+def apns_enabled() -> bool:
+    client = get_apns_client()
+    return client is not None
+
 def modernize_apns_payload(data: Dict[str, Any]) -> Dict[str, Any]:
     '''Take a payload in an unknown Zulip version's format, and return in current format.'''
     # TODO this isn't super robust as is -- if a buggy remote server
@@ -148,6 +152,9 @@ if settings.ANDROID_GCM_API_KEY:  # nocoverage
 else:
     gcm = None
 
+def gcm_enabled() -> bool:  # nocoverage
+    return gcm is not None
+
 def send_android_push_notification_to_user(user_profile: UserProfile, data: Dict[str, Any]) -> None:
     devices = list(PushDeviceToken.objects.filter(user=user_profile,
                                                   kind=PushDeviceToken.GCM))
@@ -387,6 +394,22 @@ def remove_push_device_token(user_profile: UserProfile, token_str: bytes, kind:
 # Push notifications in general
 #
 
+def push_notifications_enabled() -> bool:
+    '''True just if this server has configured a way to send push notifications.'''
+    if (uses_notification_bouncer()
+            and settings.ZULIP_ORG_KEY is not None
+            and settings.ZULIP_ORG_ID is not None):  # nocoverage
+        # We have the needed configuration to send push notifications through
+        # the bouncer.  Better yet would be to confirm that this config actually
+        # works -- e.g., that we have ever successfully sent to the bouncer --
+        # but this is a good start.
+        return True
+    if apns_enabled() and gcm_enabled():  # nocoverage
+        # We have the needed configuration to send through APNs and GCM directly
+        # (i.e., we are the bouncer, presumably.)  Again, assume it actually works.
+        return True
+    return False
+
 def get_alert_from_message(message: Message) -> Text:
     """
     Determine what alert string to display based on the missed messages.

zerver/tests/test_auth_backends.py

@@ -1326,6 +1326,7 @@ class FetchAuthBackends(ZulipTestCase):
                 ('require_email_format_usernames', check_bool),
                 ('realm_uri', check_string),
                 ('zulip_version', check_string),
+                ('push_notifications_enabled', check_bool),
                 ('msg', check_string),
                 ('result', check_string),
             ] + extra_fields)

zerver/tests/test_push_notifications.py

@@ -566,6 +566,8 @@ class TestAPNs(PushNotificationTest):
             self.user_profile.id, devices, payload_data)
 
     def test_get_apns_client(self) -> None:
+        import zerver.lib.push_notifications
+        zerver.lib.push_notifications._apns_client_initialized = False
         with self.settings(APNS_CERT_FILE='/foo.pem'), \
                 mock.patch('zerver.lib.push_notifications.APNsClient') as mock_client:
             client = get_apns_client()

zerver/views/auth.py

@@ -25,6 +25,7 @@ from zerver.context_processors import zulip_default_context, get_realm_from_requ
 from zerver.forms import HomepageForm, OurAuthenticationForm, \
     WRONG_SUBDOMAIN_ERROR, ZulipPasswordResetForm
 from zerver.lib.mobile_auth_otp import is_valid_otp, otp_encrypt_api_key
+from zerver.lib.push_notifications import push_notifications_enabled
 from zerver.lib.request import REQ, has_request_variables, JsonableError
 from zerver.lib.response import json_success, json_error
 from zerver.lib.subdomains import get_subdomain, is_subdomain_root_or_alias
@@ -713,6 +714,7 @@ def api_get_server_settings(request: HttpRequest) -> HttpResponse:
     result = dict(
         authentication_methods=get_auth_backends_data(request),
         zulip_version=ZULIP_VERSION,
+        push_notifications_enabled=push_notifications_enabled(),
     )
     context = zulip_default_context(request)
     # IMPORTANT NOTE: