Revert "Drop sync extra checks"

This reverts commit 7673814.

Author: Lode Rosseel

docs/database.rst

@@ -132,6 +132,15 @@ actively restricts where database connections may be opened:
   or by requesting ``transactional_db`` if you must keep sync fixtures.
   See :ref:`async-db-behavior` for more details.
 
+- In sync tests: database access is only allowed from the main thread. Attempting to use the database connection
+  from a different thread will raise::
+
+      RuntimeError: Database access is only allowed in the main thread, modify your
+      test fixtures to be sync or use the transactional_db fixture.
+
+  Fix this by ensuring all database transactions run in the main thread (e.g., avoiding the use of async fixtures),
+  or use ``transactional_db`` to allow mixing.
+
 
 .. _`multi-db`:
 

pytest_django/plugin.py

@@ -858,10 +858,25 @@ def _unblocked_async_only(self, *args, **kwargs):
         elif self._real_ensure_connection is not None:
             self._real_ensure_connection(*args, **kwargs)
 
-    def unblock(self, async_only=False) -> AbstractContextManager[None]:
+    def _unblocked_sync_only(self, *args, **kwargs):
+        __tracebackhide__ = True
+        if threading.current_thread() != threading.main_thread():
+            raise RuntimeError(
+                "Database access is only allowed in the main thread, "
+                "modify your test fixtures to be sync or use the transactional_db fixture."
+                "See https://pytest-django.readthedocs.io/en/latest/database.html#db-thread-safeguards for more information."
+            )
+        elif self._real_ensure_connection is not None:
+            self._real_ensure_connection(*args, **kwargs)
+
+    def unblock(self, sync_only=False, async_only=False) -> AbstractContextManager[None]:
         """Enable access to the Django database."""
+        if sync_only and async_only:
+            raise ValueError("Cannot use both sync_only and async_only. Choose at most one.")
         self._save_active_wrapper()
-        if async_only:
+        if sync_only:
+            self._dj_db_wrapper.ensure_connection = self._unblocked_sync_only
+        elif async_only:
             self._dj_db_wrapper.ensure_connection = self._unblocked_async_only
         else:
             self._dj_db_wrapper.ensure_connection = self._real_ensure_connection