bot: fix race-condition in dispatch #1728
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Exirel
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Bugfix
dgw
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Oct 27, 2019
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Fixed!
Yup, and I believe an overhaul of |
dgw
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All squashed into one single commit! |
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@Exirel This is approaching the top of the merge queue, but it has a conflict now. :/ |
The scenario is: * dispatch is called for a pretrigger * it looks into the list of callable by priority * it triggers one of the first callables, which wants to remove a callable of the same priority * because Python must not allow a dict to be modified while it is iterated over, Python raises an exception If we put a lock here, Python won't raise the exception... but we'll get a deadlock. To prevent that, instead, we copy the list of callables, and return the ones that are triggered. If any of them wants to change the callables of the same priority, they won't raise an error, because we are not iterating anymore on the dict. Hoo-ray! (with applied review suggestions from dgw) Co-Authored-By: dgw <dgw@technobabbl.es>
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Conflict fixed. That was just the little |
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Assigning the tuple returned by `_is_pretrigger_blocked()` to both vars instead of unpacking the values makes both of them always evaluate to `True` in Boolean contexts. Which magically blocks everyone from doing anything, unless `get_triggered_callables()` decides they're unblockable by virtue of `trigger.admin` being `True` (or if the function is marked unblockable, of course). Fun times! Introduced by #1728 — `.blame Exirel`! I share no responsibility for missing this in code review. Nope, none at all. :P Let this be a lesson to us all: Test things as a non-admin sometimes. Just in case. (New testing infrastructure should help us catch this sort of thing automatically in a lot of cases, once our unit tests have been updated to use the new mocks.)
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The scenario is:
If we put a lock here, Python won't raise the exception... but we'll get a deadlock. To prevent that, instead, we copy the list of callables, and return the ones that are triggered. If any of them wants to change the callables of the same priority, they won't raise an error, because we are not iterating anymore on the dict.
Hoo-ray!
@dgw have fun with my miserable grammar at 2am.