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).
https://bz.mercurial-scm.org/1502
Initialize repository
$ hg init foo
$ touch foo/a && hg -R foo commit -A -m "added a"
adding a
$ hg clone foo foo1
updating to branch default
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ echo "bar" > foo1/a && hg -R foo1 commit -m "edit a in foo1"
$ echo "hi" > foo/a && hg -R foo commit -m "edited a foo"
$ hg -R foo1 pull
pulling from $TESTTMP/foo (glob)
searching for changes
adding changesets
adding manifests
adding file changes
added 1 changesets with 1 changes to 1 files (+1 heads)
(run 'hg heads' to see heads, 'hg merge' to merge)
$ hg -R foo1 book branchy
$ hg -R foo1 book
* branchy 1:e3e522925eff
Pull. Bookmark should not jump to new head.
$ echo "there" >> foo/a && hg -R foo commit -m "edited a again"
$ hg -R foo1 pull
pulling from $TESTTMP/foo (glob)
searching for changes
adding changesets
adding manifests
adding file changes
added 1 changesets with 1 changes to 1 files
(run 'hg update' to get a working copy)
$ hg -R foo1 book
* branchy 1:e3e522925eff