#28676: Using select_for_update with next save() in multiple threads stucks
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Reporter: M1ha Shvn | Owner: nobody
Type: Bug | Status: new
Component: Database | Version: 1.10
layer (models, ORM) | Keywords: django psycopg
Severity: Normal | PostgreSQL select_for_update
Triage Stage: | Has patch: 0
Unreviewed |
Needs documentation: 0 | Needs tests: 0
Patch needs improvement: 0 | Easy pickings: 0
UI/UX: 0 |
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I use django 1.10.7, PostgreSQL 9.4, psycopg 2.7.3.1 with gunicorn server.
One of celery tasks can be executed simmultiniously by multiple workers.
Here is the code:
{{{
with transaction.atomic(using=settings.DB_SHARD):
# if I remove select_for_update here, everything works fine
# Second thread will wait here with "SELECT ... FOR UPDATE" query
in database with "active" status
conv = Conversation.objects.select_for_update().get(pk=116774)
# Some code, changing conv object without sending
conv.user_unread_count = 0
# First thread is stuck here
# I see "UPDATE ..." query in database with "idle in transaction"
status
conv.save(update_fields=['user_unread_count'])
# This doesn't work as well
# conv.save(update_fields=['user_unread_count'],
force_update=True)
}}}
When 2 workers start on the same Conversation object, second thread is
waiting on select_for_update (which is well), and the first is stuck in
save() operation.
I've ran this code with django_debug toolbar and got stange select, done
by save() method:
{{{
SELECT ••• FROM "messages_conversation" WHERE "messages_conversation"."id"
= 116774 FOR UPDATE
-- ??? What is this select here for
SELECT ••• FROM "messages_conversation" WHERE "messages_conversation"."id"
= 116774
UPDATE "messages_conversation" SET "user_unread_count" = 0 WHERE
"messages_conversation"."id" = 116774
}}}
Moreover, I've fixed the bug by changing save() with update(), it doesn't
do strange select:
{{{
with transaction.atomic(using=settings.DB_SHARD):
conv = Conversation.objects.select_for_update().get(pk=116774)
# Some code here
Conversation.objects.filter(id=conv.id).update(user_unread_count=0)
}}}
--
Ticket URL: <https://code.djangoproject.com/ticket/28676>
Django <https://code.djangoproject.com/>
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