Fixed #20571 -- Added an API to control connection.needs_rollback.

This is useful:
- to force a rollback on the exit of an atomic block without having to
  raise and catch an exception;
- to prevent a rollback after handling an exception manually.

django/db/backends/__init__.py

@@ -330,6 +330,15 @@ class BaseDatabaseWrapper(object):
         self._set_autocommit(autocommit)
         self.autocommit = autocommit
 
+    def set_rollback(self, rollback):
+        """
+        Set or unset the "needs rollback" flag -- for *advanced use* only.
+        """
+        if not self.in_atomic_block:
+            raise TransactionManagementError(
+                "needs_rollback doesn't work outside of an 'atomic' block.")
+        self.needs_rollback = rollback
+
     def validate_no_atomic_block(self):
         """
         Raise an error if an atomic block is active.

django/db/transaction.py

@@ -171,6 +171,26 @@ def clean_savepoints(using=None):
     """
     get_connection(using).clean_savepoints()
 
+def get_rollback(using=None):
+    """
+    Gets the "needs rollback" flag -- for *advanced use* only.
+    """
+    return get_connection(using).needs_rollback
+
+def set_rollback(rollback, using=None):
+    """
+    Sets or unsets the "needs rollback" flag -- for *advanced use* only.
+
+    When `rollback` is `True`, it triggers a rollback when exiting the
+    innermost enclosing atomic block that has `savepoint=True` (that's the
+    default). Use this to force a rollback without raising an exception.
+
+    When `rollback` is `False`, it prevents such a rollback. Use this only
+    after rolling back to a known-good state! Otherwise, you break the atomic
+    block and data corruption may occur.
+    """
+    return get_connection(using).set_rollback(rollback)
+
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