ui: add special-purpose atexit functionality
In spite of its longstanding use, Python's built-in atexit code is
not suitable for Mercurial's purposes, for several reasons:
* Handlers run after application code has finished.
* Because of this, the code that runs handlers swallows exceptions
(since there's no possible stacktrace to associate errors with).
If we're lucky, we'll get something spat out to stderr (if stderr
still works), which of course isn't any use in a big deployment
where it's important that exceptions get logged and aggregated.
* Mercurial's current atexit handlers make unfortunate assumptions
about process state (specifically stdio) that, coupled with the
above problems, make it impossible to deal with certain categories
of error (try "hg status > /dev/full" on a Linux box).
* In Python 3, the atexit implementation is completely hidden, so
we can't hijack the platform's atexit code to run handlers at a
time of our choosing.
As a result, here's a perfectly cromulent atexit-like implementation
over which we have control. This lets us decide exactly when the
handlers run (after each request has completed), and control what
the process state is when that occurs (and afterwards).
$ cat > abortcommit.py <<EOF
> from mercurial import error
> def hook(**args):
> raise error.Abort("no commits allowed")
> def reposetup(ui, repo):
> repo.ui.setconfig("hooks", "pretxncommit.nocommits", hook)
> EOF
$ abspath=`pwd`/abortcommit.py
$ cat <<EOF >> $HGRCPATH
> [extensions]
> mq =
> abortcommit = $abspath
> EOF
$ hg init foo
$ cd foo
$ echo foo > foo
$ hg add foo
mq may keep a reference to the repository so __del__ will not be
called and .hg/journal.dirstate will not be deleted:
$ hg ci -m foo
error: pretxncommit.nocommits hook failed: no commits allowed
transaction abort!
rollback completed
abort: no commits allowed
[255]
$ hg ci -m foo
error: pretxncommit.nocommits hook failed: no commits allowed
transaction abort!
rollback completed
abort: no commits allowed
[255]
$ cd ..