compatibility: Add a compatibility check to api_get_server_settings.

This should make it convenient for the mobile app to present errors of
the form "Your Zulip app is not new enough for this Zulip server".

zerver/openapi/zulip.yaml

@@ -1705,6 +1705,11 @@ paths:
                       type: boolean
                       description: Whether mobile/push notifications are
                         enabled.
+                    is_incompatible:
+                      type: boolean
+                      description: Whether the Zulip client that has sent a
+                        request to this endpoint is deemed incompatible with
+                        the server.
                     email_auth_enabled:
                       type: boolean
                       description: Setting for allowing users authenticate with

zerver/tests/test_auth_backends.py

@@ -1528,6 +1528,7 @@ class FetchAuthBackends(ZulipTestCase):
                     ('password', check_bool),
                 ])),
                 ('email_auth_enabled', check_bool),
+                ('is_incompatible', check_bool),
                 ('require_email_format_usernames', check_bool),
                 ('realm_uri', check_string),
                 ('zulip_version', check_string),
@@ -1537,15 +1538,18 @@ class FetchAuthBackends(ZulipTestCase):
             ] + extra_fields)
             self.assert_on_error(checker("data", result.json()))
 
-        result = self.client_get("/api/v1/server_settings", subdomain="")
+        result = self.client_get("/api/v1/server_settings", subdomain="", HTTP_USER_AGENT="")
+        check_result(result)
+
+        result = self.client_get("/api/v1/server_settings", subdomain="", HTTP_USER_AGENT="ZulipInvalid")
+        self.assertTrue(result.json()["is_incompatible"])
+
+        with self.settings(ROOT_DOMAIN_LANDING_PAGE=False):
+            result = self.client_get("/api/v1/server_settings", subdomain="", HTTP_USER_AGENT="")
         check_result(result)
 
         with self.settings(ROOT_DOMAIN_LANDING_PAGE=False):
-            result = self.client_get("/api/v1/server_settings", subdomain="")
-        check_result(result)
-
-        with self.settings(ROOT_DOMAIN_LANDING_PAGE=False):
-            result = self.client_get("/api/v1/server_settings", subdomain="zulip")
+            result = self.client_get("/api/v1/server_settings", subdomain="zulip", HTTP_USER_AGENT="")
         check_result(result, [
             ('realm_name', check_string),
             ('realm_description', check_string),

zerver/views/auth.py

@@ -31,6 +31,7 @@ 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
+from zerver.lib.user_agent import parse_user_agent
 from zerver.lib.users import get_api_key
 from zerver.lib.validator import validate_login_email
 from zerver.models import PreregistrationUser, UserProfile, remote_user_to_email, Realm, \
@@ -843,6 +844,10 @@ def api_get_auth_backends(request: HttpRequest) -> HttpResponse:
     auth_backends['zulip_version'] = ZULIP_VERSION
     return json_success(auth_backends)
 
+def check_server_incompatibility(request: HttpRequest) -> bool:
+    user_agent = parse_user_agent(request.META["HTTP_USER_AGENT"])
+    return user_agent['name'] == "ZulipInvalid"
+
 @require_GET
 @csrf_exempt
 def api_get_server_settings(request: HttpRequest) -> HttpResponse:
@@ -850,6 +855,7 @@ def api_get_server_settings(request: HttpRequest) -> HttpResponse:
         authentication_methods=get_auth_backends_data(request),
         zulip_version=ZULIP_VERSION,
         push_notifications_enabled=push_notifications_enabled(),
+        is_incompatible=check_server_incompatibility(request),
     )
     context = zulip_default_context(request)
     # IMPORTANT NOTE:

zproject/urls.py

@@ -590,7 +590,8 @@ urls += [
 urls += [
     # This json format view used by the mobile apps lists which
     # authentication backends the server allows as well as details
-    # like the requested subdomains'd realm icon (if known).
+    # like the requested subdomains'd realm icon (if known) and
+    # server-specific compatibility.
     url(r'^api/v1/server_settings', zerver.views.auth.api_get_server_settings),
     # This is a deprecated old version of api/v1/server_settings that only returns auth backends.
     url(r'^api/v1/get_auth_backends', zerver.views.auth.api_get_auth_backends,