Refs #19544 -- Ignored auto-created through additions conflicts if supported.

This prevents IntegrityError caused by race conditions between missing ids
retrieval and bulk insertions.

django/db/models/fields/related_descriptors.py

@@ -1110,14 +1110,20 @@ def create_forward_many_to_many_manager(superclass, rel, reverse):
                             model=self.model, pk_set=missing_target_ids, using=db,
                         )
 
-                    # Add the ones that aren't there already
+                    # Add the ones that aren't there already. Conflicts can be
+                    # ignored when the intermediary model is auto-created as
+                    # the only possible collision is on the (sid_id, tid_id)
+                    # tuple. The same assertion doesn't hold for user-defined
+                    # intermediary models as they could have other fields
+                    # causing conflicts which must be surfaced.
+                    ignore_conflicts = self.through._meta.auto_created is not False
                     self.through._default_manager.using(db).bulk_create([
                         self.through(**through_defaults, **{
                             '%s_id' % source_field_name: self.related_val[0],
                             '%s_id' % target_field_name: target_id,
                         })
                         for target_id in missing_target_ids
-                    ])
+                    ], ignore_conflicts=ignore_conflicts)
 
                     if self.reverse or source_field_name == self.source_field_name:
                         # Don't send the signal when we are inserting the

tests/many_to_many/tests.py

@@ -117,6 +117,16 @@ class ManyToManyTests(TestCase):
             ]
         )
 
+    @skipUnlessDBFeature('supports_ignore_conflicts')
+    def test_add_ignore_conflicts(self):
+        manager_cls = self.a1.publications.__class__
+        # Simulate a race condition between the missing ids retrieval and
+        # the bulk insertion attempt.
+        missing_target_ids = {self.p1.id}
+        with mock.patch.object(manager_cls, '_get_missing_target_ids', return_value=missing_target_ids) as mocked:
+            self.a1.publications.add(self.p1)
+        mocked.assert_called_once()
+
     def test_related_sets(self):
         # Article objects have access to their related Publication objects.
         self.assertQuerysetEqual(self.a1.publications.all(), ['<Publication: The Python Journal>'])