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).

mercurial/dispatch.py

@@ -59,6 +59,23 b' class request(object):'
         self.fout = fout
         self.ferr = ferr
 
+    def _runexithandlers(self):
+        exc = None
+        handlers = self.ui._exithandlers
+        try:
+            while handlers:
+                func, args, kwargs = handlers.pop()
+                try:
+                    func(*args, **kwargs)
+                except: # re-raises below
+                    if exc is None:
+                        exc = sys.exc_info()[1]
+                    self.ui.warn(('error in exit handlers:\n'))
+                    self.ui.traceback(force=True)
+        finally:
+            if exc is not None:
+                raise exc
+
 def run():
     "run the command in sys.argv"
     sys.exit((dispatch(request(pycompat.sysargv[1:])) or 0) & 255)
@@ -146,6 +163,10 b' def dispatch(req):'
             req.ui.log('uiblocked', 'ui blocked ms', **req.ui._blockedtimes)
         req.ui.log("commandfinish", "%s exited %s after %0.2f seconds\n",
                    msg, ret or 0, duration)
+        try:
+            req._runexithandlers()
+        except: # exiting, so no re-raises
+            ret = ret or -1
     return ret
 
 def _runcatch(req):

mercurial/ui.py

@@ -139,6 +139,8 b' class ui(object):'
         """
         # _buffers: used for temporary capture of output
         self._buffers = []
+        # _exithandlers: callbacks run at the end of a request
+        self._exithandlers = []
         # 3-tuple describing how each buffer in the stack behaves.
         # Values are (capture stderr, capture subprocesses, apply labels).
         self._bufferstates = []
@@ -163,6 +165,7 b' class ui(object):'
         self._styles = {}
 
         if src:
+            self._exithandlers = src._exithandlers
             self.fout = src.fout
             self.ferr = src.ferr
             self.fin = src.fin
@@ -946,6 +949,13 b' class ui(object):'
 
         return True
 
+    def atexit(self, func, *args, **kwargs):
+        '''register a function to run after dispatching a request
+
+        Handlers do not stay registered across request boundaries.'''
+        self._exithandlers.append((func, args, kwargs))
+        return func
+
     def interface(self, feature):
         """what interface to use for interactive console features?