id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
15507	Savepoint support for MySQL backend	Chris Lamb	nobody	"Currently, it is not possible for the following code to deadlock when using the MySQL backend:

{{{#!python
from django.db import models, transaction

class Parent(models.Model):
    lookup = models.CharField(max_length=20, unique=True)
    num_children = models.IntegerField(default=0)

class Child(models.Model):
    parent = models.ForeignKey(Parent, related_name='children')
    name = models.CharField(max_length=20)

    def save(self, *args, **kwargs):
        created = not self.pk

        super(Child, self).save(*args, **kwargs)

        if created:
            Parent.objects.filter(pk=self.parent_id).update(
                num_children=models.F('num_children') + 1,
            )

@transaction.commit_on_success
def my_view(request):
    parent = Parent.objects.get_or_create(lookup='a')
    parent.children.create(name='foo')
}}}

This is because the `INSERT` in `get_or_create` gains an `IX` lock on that row even though it fails and raises `IntegrityError`. This lock causes the deadlock later on when `UPDATE` is called from `Child.save`.

Adding savepoint support would fix this as the rollback to savepoint made just before the `INSERT` would remove the `IX` lock.

Patch attached. Tested on 5.0.84-1. "	New feature	closed	Database layer (models, ORM)	1.2	Normal	fixed		Chris Lamb tomasz.zielinski@…	Accepted	1	0	0	0	0	0