MySQL Galera workaround (#416)

doc/admin/config.rst

@@ -102,6 +102,10 @@ Example::
 ``user``, ``password``, ``host``, ``port``
     Connection details for the database connection. Empty by default.
 
+``galera``
+    Indicates if the database backend is a MySQL/MariaDB Galera cluster and
+    turns on some optimizations/special case handlers. Default: ``False``
+
 URLs
 ----
 

src/pretix/base/views/async.py

@@ -9,6 +9,7 @@ from django.shortcuts import redirect, render
 from django.utils.translation import ugettext as _
 
 from pretix.celery_app import app
+from pretix.helpers.database import casual_reads
 
 logger = logging.getLogger('pretix.base.async')
 
@@ -31,10 +32,11 @@ class AsyncAction:
             return JsonResponse(data)
         else:
             if res.ready():
-                if res.successful() and not isinstance(res.info, Exception):
-                    return self.success(res.info)
-                else:
-                    return self.error(res.info)
+                with casual_reads():
+                    if res.successful() and not isinstance(res.info, Exception):
+                        return self.success(res.info)
+                    else:
+                        return self.error(res.info)
             return redirect(self.get_check_url(res.id, False))
 
     def get_success_url(self, value):
@@ -64,24 +66,25 @@ class AsyncAction:
             'ready': ready
         }
         if ready:
-            if res.successful() and not isinstance(res.info, Exception):
-                smes = self.get_success_message(res.info)
-                if smes:
-                    messages.success(self.request, smes)
-                # TODO: Do not store message if the ajax client states that it will not redirect
-                # but handle the mssage itself
-                data.update({
-                    'redirect': self.get_success_url(res.info),
-                    'message': str(self.get_success_message(res.info))
-                })
-            else:
-                messages.error(self.request, self.get_error_message(res.info))
-                # TODO: Do not store message if the ajax client states that it will not redirect
-                # but handle the mssage itself
-                data.update({
-                    'redirect': self.get_error_url(),
-                    'message': str(self.get_error_message(res.info))
-                })
+            with casual_reads():
+                if res.successful() and not isinstance(res.info, Exception):
+                    smes = self.get_success_message(res.info)
+                    if smes:
+                        messages.success(self.request, smes)
+                    # TODO: Do not store message if the ajax client states that it will not redirect
+                    # but handle the mssage itself
+                    data.update({
+                        'redirect': self.get_success_url(res.info),
+                        'message': str(self.get_success_message(res.info))
+                    })
+                else:
+                    messages.error(self.request, self.get_error_message(res.info))
+                    # TODO: Do not store message if the ajax client states that it will not redirect
+                    # but handle the mssage itself
+                    data.update({
+                        'redirect': self.get_error_url(),
+                        'message': str(self.get_error_message(res.info))
+                    })
         return data
 
     def get_result(self, request):
@@ -90,10 +93,11 @@ class AsyncAction:
             return JsonResponse(self._return_ajax_result(res, timeout=0.25))
         else:
             if res.ready():
-                if res.successful() and not isinstance(res.info, Exception):
-                    return self.success(res.info)
-                else:
-                    return self.error(res.info)
+                with casual_reads():
+                    if res.successful() and not isinstance(res.info, Exception):
+                        return self.success(res.info)
+                    else:
+                        return self.error(res.info)
             return render(request, 'pretixpresale/waiting.html')
 
     def success(self, value):

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

src/pretix/settings.py

@@ -58,6 +58,7 @@ DATABASES = {
         'CONN_MAX_AGE': 0 if db_backend == 'sqlite3' else 120
     }
 }
+DATABASE_IS_GALERA = config.getboolean('database', 'galera', fallback=False)
 
 STATIC_URL = config.get('urls', 'static', fallback='/static/')