#34091: Invalid SQL: FROM clauses can be omitted when QuerySet is accessed from
multiple threads
-------------------------------------+-------------------------------------
Reporter: Marti Raudsepp | Owner: nobody
Type: Bug | Status: new
Component: Database layer | Version: 4.1
(models, ORM) |
Severity: Normal | Resolution:
Keywords: race-condition, | Triage Stage:
orm, threading, SQLCompiler | Unreviewed
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
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Comment (by Simon Charette):
> The access from multiple threads was unintentional, but I believe this
deserves to be fixed in Django anyway
Thank you for your report and investigation but I disagree with your
conclusion.
Django doesn't provide any thread safety guarantees for ORM objects (model
instances, querysets) and it's your responsibility to ensure that copies
of these objects are used in threads instead of sharing them. That's the
reason why thread safe built-in abstractions that interact with the ORM
and share references to objects (e.g. generic model views) make sure
[https://github.com/django/django/blob/da2621c3dfef934000c479750e0276d992025542/django/views/generic/list.py#L32
they create copies of the references before manipulating them].
Even if we were to have the compiler make
[https://github.com/django/django/blob/da2621c3dfef934000c479750e0276d992025542/django/db/models/sql/compiler.py#L709-L926
a copy of its query before altering its alias counts] and
[https://github.com/django/django/blob/da2621c3dfef934000c479750e0276d992025542/django/db/models/sql/compiler.py#L924-L926
then setting it back] it's likely only one of the locations that make
query compilation non-thread safe.
We can't accept this ticket without committing to making the ORM entirely
thread-safe with the performance and complexity penalties it incurs. If
you disagree with my conclusion feel free to open a thread on
[https://docs.djangoproject.com/en/4.1/internals/mailing-lists/#django-
developers the developers mailing list] to discuss the subject further.
--
Ticket URL: <https://code.djangoproject.com/ticket/34091#comment:1>
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