MySQL Galera workaround (#416)

src/pretix/helpers/database.py

@@ -1,6 +1,7 @@
 import contextlib
 
-from django.db import transaction
+from django.conf import settings
+from django.db import connection, transaction
 
 
 class DummyRollbackException(Exception):
@@ -26,3 +27,40 @@ def rolledback_transaction():
         pass
     else:
         raise Exception('Invalid state, should have rolled back.')
+
+
+if 'mysql' in settings.DATABASES['default']['ENGINE'] and settings.DATABASE_IS_GALERA:
+
+    @contextlib.contextmanager
+    def casual_reads():
+        """
+        When pretix runs with a MySQL galera cluster as a database backend, we can run into the
+        following problem:
+
+        * A celery thread starts a transaction, creates an object and commits the transaction.
+          It then returns the object ID into celery's result store (e.g. redis)
+
+        * A web thread pulls the object ID from the result store, but cannot access the object
+          yet as the transaction is not yet committed everywhere.
+
+        This sets the wsrep_sync_wait variable to deal with this problem.
+
+        See also:
+
+        * https://mariadb.com/kb/en/mariadb/galera-cluster-system-variables/#wsrep_sync_wait
+
+        * https://www.percona.com/doc/percona-xtradb-cluster/5.6/wsrep-system-index.html#wsrep_sync_wait
+        """
+        with connection.cursor() as cursor:
+            cursor.execute("SET @wsrep_sync_wait_orig = @@wsrep_sync_wait;")
+            cursor.execute("SET SESSION wsrep_sync_wait = GREATEST(@wsrep_sync_wait_orig, 1);")
+            try:
+                yield
+            finally:
+                cursor.execute("SET SESSION wsrep_sync_wait = @wsrep_sync_wait_orig;")
+
+else:
+
+    @contextlib.contextmanager
+    def casual_reads():
+        yield