Run exporters in repeatable read by default (Z#23173095) (#5500)

* Run exporters in repeatable read by default (Z#23173095)

* Update src/pretix/helpers/database.py

Co-authored-by: Richard Schreiber <schreiber@rami.io>

* Rename parameter, add test

* Do not run during tests

---------

Co-authored-by: Richard Schreiber <schreiber@rami.io>

src/pretix/helpers/database.py

@@ -21,7 +21,8 @@
 #
 import contextlib
 
-from django.core.exceptions import FieldDoesNotExist
+from django.conf import settings
+from django.core.exceptions import FieldDoesNotExist, ImproperlyConfigured
 from django.db import connection, transaction
 from django.db.models import (
     Aggregate, Expression, F, Field, Lookup, OrderBy, Value,
@@ -62,6 +63,43 @@ def casual_reads():
     yield
 
 
+@contextlib.contextmanager
+def repeatable_reads_transaction():
+    """
+    pretix, and Django, operate in the transaction isolation level READ COMMITTED by default. This is not a strong level
+    of isolation, but we NEED to use it: Otherwise e.g. our quota logic breaks, because we need to be able to get the
+    *current* number of tickets sold at any time in a transaction, not the number of tickets sold *before* our transaction
+    started.
+
+    However, this isolation mode has drawbacks, for example during reporting. When a user retrieves a report from the
+    system, it should return numbers that are consistent with each other. However, if the report makes multiple SQL
+    queries in READ COMMITTED mode, the results might be different for each query, causing numbers to be inconsistent
+    with each other.
+
+    This context manager creates a transaction that is running in REPEATABLE READ mode to avoid this problem.
+
+    **You should only make read-only queries during this transaction and not rely on quota calculations.**
+    """
+    is_under_test = 'tests.testdummy' in settings.INSTALLED_APPS
+    try:
+        with transaction.atomic(durable=not is_under_test):
+            if not is_under_test:
+                # We're not running this in tests, where we can basically not use this since the test runner does its
+                # own transaction logic for efficiency
+                with connection.cursor() as cursor:
+                    if 'postgresql' in settings.DATABASES['default']['ENGINE']:
+                        cursor.execute('SET TRANSACTION ISOLATION LEVEL REPEATABLE READ;')
+                    elif 'sqlite' in settings.DATABASES['default']['ENGINE']:
+                        pass  # noop
+                    else:
+                        raise ImproperlyConfigured("Cannot set transaction isolation mode on this database backend")
+
+            connection.tx_in_repeatable_read = True
+            yield
+    finally:
+        connection.tx_in_repeatable_read = False
+
+
 class GroupConcat(Aggregate):
     function = 'group_concat'
     template = '%(function)s(%(distinct)s%(field)s, "%(separator)s")'