add_subscriptions_backend: Add 'transaction.atomic' decorator.

This commit adds an 'transaction.atomic' decorator to the
'add_subscriptions_backend' view thus making the db operations
within the view atomic and helps to avoid race between events sent.

In tests where we make POST requests to this view, we have
wrapped the API calls with a transaction.atomic() context
manager. It helps us with NOT rolling back the entire test
transaction due to error responses.

zerver/lib/test_classes.py

@@ -32,7 +32,7 @@ from django.apps import apps
 from django.conf import settings
 from django.core.mail import EmailMessage
 from django.core.signals import got_request_exception
-from django.db import connection
+from django.db import connection, transaction
 from django.db.migrations.executor import MigrationExecutor
 from django.db.migrations.state import StateApps
 from django.db.utils import IntegrityError
@@ -1458,6 +1458,10 @@ Output:
             "invite_only": orjson.dumps(invite_only).decode(),
         }
         post_data.update(extra_post_data)
+        # We wrap the API call with a 'transaction.atomic()' context
+        # manager as it helps us with NOT rolling back the entire
+        # test transaction due to error responses.
+        with transaction.atomic():
             result = self.api_post(
                 user,
                 "/api/v1/users/me/subscriptions",

zerver/views/streams.py

@@ -533,6 +533,7 @@ RETENTION_DEFAULT: Union[str, int] = "realm_default"
 EMPTY_PRINCIPALS: Union[Sequence[str], Sequence[int]] = []
 
 
+@transaction.atomic(savepoint=False)
 @require_non_guest_user
 @has_request_variables
 def add_subscriptions_backend(