subdomains: Fix some implicit uses of "" for the root subdomain.

These are just instances that jumped out at me while working on the
subdomains code, mostly while grepping for get_subdomain call sites.
I haven't attempted a comprehensive search, and there are likely
still others left.

zerver/context_processors.py

@@ -72,7 +72,8 @@ def zulip_default_context(request):
     about_link_disabled = settings.ABOUT_LINK_DISABLED
     find_team_link_disabled = settings.FIND_TEAM_LINK_DISABLED
 
-    if settings.ROOT_DOMAIN_LANDING_PAGE and get_subdomain(request) == "":
+    if (settings.ROOT_DOMAIN_LANDING_PAGE
+            and get_subdomain(request) == Realm.SUBDOMAIN_FOR_ROOT_DOMAIN):
         register_link_disabled = True
         login_link_disabled = True
         about_link_disabled = True

zerver/lib/subdomains.py

@@ -24,7 +24,8 @@ def get_subdomain(request):
 def is_subdomain_root_or_alias(request):
     # type: (HttpRequest) -> bool
     subdomain = _extract_subdomain(request)
-    return not subdomain or subdomain in settings.ROOT_SUBDOMAIN_ALIASES
+    return (subdomain == Realm.SUBDOMAIN_FOR_ROOT_DOMAIN
+            or subdomain in settings.ROOT_SUBDOMAIN_ALIASES)
 
 def user_matches_subdomain(realm_subdomain, user_profile):
     # type: (Optional[Text], UserProfile) -> bool

zerver/middleware.py

@@ -16,7 +16,7 @@ from zerver.lib.utils import statsd
 from zerver.lib.queue import queue_json_publish
 from zerver.lib.cache import get_remote_cache_time, get_remote_cache_requests
 from zerver.lib.bugdown import get_bugdown_time, get_bugdown_requests
-from zerver.models import flush_per_request_caches, get_realm
+from zerver.models import Realm, flush_per_request_caches, get_realm
 from zerver.lib.exceptions import RateLimited
 from django.contrib.sessions.middleware import SessionMiddleware
 from django.views.csrf import csrf_failure as html_csrf_failure
@@ -378,7 +378,7 @@ class SessionHostDomainMiddleware(SessionMiddleware):
         if (not request.path.startswith("/static/") and not request.path.startswith("/api/") and
                 not request.path.startswith("/json/")):
             subdomain = get_subdomain(request)
-            if subdomain != "":
+            if subdomain != Realm.SUBDOMAIN_FOR_ROOT_DOMAIN:
                 realm = get_realm(subdomain)
                 if (realm is None):
                     return render(request, "zerver/invalid_realm.html")

zerver/views/auth.py

@@ -660,7 +660,7 @@ def get_auth_backends_data(request):
         realm = Realm.objects.get(string_id=subdomain)
     except Realm.DoesNotExist:
         # If not the root subdomain, this is an error
-        if subdomain != "":
+        if subdomain != Realm.SUBDOMAIN_FOR_ROOT_DOMAIN:
             raise JsonableError(_("Invalid subdomain"))
         # With the root subdomain, it's an error or not depending
         # whether ROOT_DOMAIN_LANDING_PAGE (which indicates whether

zerver/views/home.py

@@ -79,7 +79,7 @@ def home(request):
     # page, not the login form, on the root domain
 
     subdomain = get_subdomain(request)
-    if subdomain != "":
+    if subdomain != Realm.SUBDOMAIN_FOR_ROOT_DOMAIN:
         return home_real(request)
 
     return render(request, 'zerver/hello.html')

zerver/views/integrations.py

@@ -13,18 +13,20 @@ from zerver.decorator import has_request_variables, REQ
 from zerver.lib import bugdown
 from zerver.lib.integrations import CATEGORIES, INTEGRATIONS, HUBOT_LOZENGES
 from zerver.lib.subdomains import get_subdomain
+from zerver.models import Realm
 from zerver.templatetags.app_filters import render_markdown_path
 
 def add_api_uri_context(context, request):
     # type: (Dict[str, Any], HttpRequest) -> None
     subdomain = get_subdomain(request)
-    if subdomain or not settings.ROOT_DOMAIN_LANDING_PAGE:
+    if (subdomain != Realm.SUBDOMAIN_FOR_ROOT_DOMAIN
+            or not settings.ROOT_DOMAIN_LANDING_PAGE):
         display_subdomain = subdomain
         html_settings_links = True
     else:
         display_subdomain = 'yourZulipDomain'
         html_settings_links = False
-    if display_subdomain != "":
+    if display_subdomain != Realm.SUBDOMAIN_FOR_ROOT_DOMAIN:
         external_api_path_subdomain = '%s.%s' % (display_subdomain,
                                                  settings.EXTERNAL_API_PATH)
     else: