[PATCH STABLE] extensions: clear aftercallbacks after execution (issue4646)

[Yuya Nishihara]

On Tue, 05 May 2015 21:35:25 -0700, Gregory Szorc wrote:
> # HG changeset patch
> # User Gregory Szorc <gregory.szorc at gmail.com>
> # Date 1430886916 25200
> #      Tue May 05 21:35:16 2015 -0700
> # Branch stable
> # Node ID 4da7e74074010404744171ad68c1a1bd7d100696
> # Parent  41cd8171e58f991373dcd0b4897dc1e5978d42dd
> extensions: clear aftercallbacks after execution (issue4646)
> 
> It was reported that enabling pager without color could cause a hang.
> Inserting print statements revealed that the callbacks in
> extensions._aftercallbacks were being invoked twice.
> 
> I'm not sure exactly how the hang comes to be. But, clearing
> extensions._aftercallbacks makes the problem go away. This presumably
> has something to do with forked processes inheriting
> extensions._aftercallbacks and that somehow results in badness.
> 
> The reproduce steps in the bug seem to only occur when the output of
> a command is less than the size of the current screen. This is not
> something that can easily be tested. I verified the test case works
> with this patch and that pager and color interaction continues to
> work. Since we have no existing automated tests for pager, this sadly
> appears to be the best I can do.
> 
> diff --git a/mercurial/extensions.py b/mercurial/extensions.py
> --- a/mercurial/extensions.py
> +++ b/mercurial/extensions.py
> @@ -133,8 +133,12 @@ def loadall(ui):
>  
>          for fn in _aftercallbacks[shortname]:
>              fn(loaded=False)
>  
> +    # Forked processes appear to inherit _aftercallbacks, which may cause
> +    # deadlock. See issue4646.
> +    _aftercallbacks.clear()

I don't think this is related to fork(). extensions.loadall() is just called
more than once.

% grep extensions.loadall mercurial/*.py
mercurial/dispatch.py:    extensions.loadall(lui)
mercurial/dispatch.py:    # (uisetup and extsetup are handled in extensions.loadall)
mercurial/localrepo.py:            extensions.loadall(self.ui)