Add database.disable_server_side_cursors option (#4016)

doc/admin/config.rst

@@ -158,6 +158,7 @@ Example::
     host=localhost
     port=3306
     advisory_lock_index=1
+    disable_server_side_cursors=0
     sslmode=require
     sslrootcert=/etc/pretix/postgresql-ca.crt
     sslcert=/etc/pretix/postgresql-client-crt.crt
@@ -178,6 +179,11 @@ Example::
     and are not scoped to a specific database. If you run multiple pretix applications with the same PostgreSQL server,
     you should set separate values for this setting (integers up to 256).
 
+``disable_server_side_cursors``
+    On PostgreSQL pretix might use server side cursors for certain operations. This is generally fine but will break in
+    specific circumstances, for example when connecting to PostgreSQL through a PGBouncer configured with a transaction
+    pool mode. Off by default (i.e. by default server side cursors will be used).
+
 ``sslmode``, ``sslrootcert``
     Connection TLS details for the PostgreSQL database connection. Possible values of ``sslmode`` are ``disable``, ``allow``, ``prefer``, ``require``, ``verify-ca``, and ``verify-full``. ``sslrootcert`` should be the accessible path of the ca certificate. Both values are empty by default.
 

src/pretix/settings.py

@@ -142,6 +142,8 @@ if USE_DATABASE_TLS or USE_DATABASE_MTLS:
 
     db_options.update(tls_config)
 
+db_disable_server_side_cursors = db_backend == 'postgresql' and config.getboolean('database', 'disable_server_side_cursors', fallback=False)
+
 DATABASES = {
     'default': {
         'ENGINE': 'django.db.backends.' + db_backend,
@@ -152,6 +154,7 @@ DATABASES = {
         'PORT': config.get('database', 'port', fallback=''),
         'CONN_MAX_AGE': 0 if db_backend == 'sqlite3' else 120,
         'CONN_HEALTH_CHECKS': db_backend != 'sqlite3',  # Will only be used from Django 4.1 onwards
+        'DISABLE_SERVER_SIDE_CURSORS': db_disable_server_side_cursors,
         'OPTIONS': db_options,
         'TEST': {}
     }