core: fix read replica routing during transactions

authentik/tenants/db.py

@@ -1,6 +1,7 @@
 from random import choice
 
 from django.conf import settings
+from django.db import DEFAULT_DB_ALIAS, connections
 
 
 class FailoverRouter:
@@ -10,16 +11,22 @@ class FailoverRouter:
     def __init__(self) -> None:
         super().__init__()
         self.database_aliases = set(settings.DATABASES.keys())
-        self.read_replica_aliases = list(self.database_aliases - {"default"})
+        self.read_replica_aliases = list(self.database_aliases - {DEFAULT_DB_ALIAS})
         self.replica_enabled = len(self.read_replica_aliases) > 0
 
     def db_for_read(self, model, **hints):
         if not self.replica_enabled:
-            return "default"
+            return DEFAULT_DB_ALIAS
+        # Stay on primary for the entire transaction to maintain consistency.
+        # Reading from a replica mid-transaction would give a different snapshot,
+        # breaking transactional semantics (not just read-your-writes, but the
+        # entire consistent point-in-time view that a transaction provides).
+        if connections[DEFAULT_DB_ALIAS].in_atomic_block:
+            return DEFAULT_DB_ALIAS
         return choice(self.read_replica_aliases)  # nosec
 
     def db_for_write(self, model, **hints):
-        return "default"
+        return DEFAULT_DB_ALIAS
 
     def allow_relation(self, obj1, obj2, **hints):
         """Relations between objects are allowed if both objects are