#17251: select_for_update tests threads should close connections manually
-------------------------------------+-------------------------------------
Reporter: akaariai | Owner: nobody
Type: | Status: new
Cleanup/optimization | Version:
Component: Uncategorized | Resolution:
Severity: Normal | Triage Stage:
Keywords: | Unreviewed
Has patch: 1 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
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Changes (by akaariai):
* needs_better_patch: => 0
* needs_tests: => 0
* needs_docs: => 0
Comment:
The GCed part of the description is somewhat wrong. Threadlocals do not
get GCed when the thread goes away. However it seems they do get GCed if
the thread has gone away and _another_ thread is then accessing the same
thread-local object. Or at least this is my working hypothesis based on
the fact that the connection gets closed on this line in the
transactions.py, managed() [L111]:
{{{
connection.managed(flag)
}}}
And this is ran from main thread! I assume this is the first time the
default_db_alias' DatabaseWrapper object (which is thread-local) is used
after the another thread has gone away. I assume Python GCes the state of
the other thread because it has gone away, and it sees it only when the
object is accessed.
If you want to try tracking this down, I have a branch where you can run:
{{{
python runtests.py --settings=test_pgsql
select_for_update.SelectForUpdateTests.test_nowait_raises_error_on_block
}}}
Then wait until you get into PDB. Begin stepping through the code. Note
when the 'I got deleted' for thread 1 is printed. Note also which thread
you are currently in. It is also good to use postgresql and set
log_connections / log_disconnections to ON, and also log at least the
backend pid in the log_line_prefix (%p).
I think this enforces the point that it would be good to close the
connection in the tester threads. The current behavior seems to be based
on pure luck :) I am also beginning to see why some of the core developers
don't like thread-locals...
The test code for this is at:
https://github.com/akaariai/django/tree/connection_close
--
Ticket URL: <https://code.djangoproject.com/ticket/17251#comment:1>
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