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
7475	Race Condition in ModelChoiceIterator (affects ModelChoiceField and ModelMultipleChoiceField)	Jason Davies	nobody	"I have discovered a race condition in {{{ModelChoiceIterator}}}: when {{{ModelChoiceField}}} is initialised with a queryset, this same queryset is kept across multiple requests (due to the fact that form fields are generally only initialised once as class members).  As a result, there is a race condition when multiple requests are made concurrently, and more than one request tries to iterate over the same queryset.  This generally appears in the form of a {{{ValueError: generator already executing}}} occurring.

The {{{Select}}} widget also seems to keep the same {{{ModelChoiceIterator}}} instance in its {{{choices}}} attribute across multiple requests.  Under high load, this {{{ModelChoiceIterator}}} may be consumed by multiple threads simultaneously.  However, as this queryset instance is the same for these multiple threads, it causes a {{{ValueError: generator already executing}}} error when the second or third thread attempts to consume it.

The patch is extremely simple for this: just call {{{.all()}}} on the queryset every time the {{{ModelChoiceIterator}}} is consumed.  I've tested this under high load and it works perfectly."		closed	Forms	dev		fixed	race condition threading ModelChoiceIterator generator ValueError		Unreviewed	1	0	0	0	0	0