diff -r b65966f50058 -r a368da441b32 hgext/logtoprocess.py
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/hgext/logtoprocess.py	Tue Apr 05 17:39:59 2016 +0100
@@ -0,0 +1,129 @@
+# logtoprocess.py - send ui.log() data to a subprocess
+#
+# Copyright 2016 Facebook, Inc.
+#
+# This software may be used and distributed according to the terms of the
+# GNU General Public License version 2 or any later version.
+"""Send ui.log() data to a subprocess (EXPERIMENTAL)
+
+This extension lets you specify a shell command per ui.log() event,
+sending all remaining arguments to as environment variables to that command.
+
+Each positional argument to the method results in a `MSG[N]` key in the
+environment, starting at 1 (so `MSG1`, `MSG2`, etc.). Each keyword argument
+is set as a `OPT_UPPERCASE_KEY` variable (so the key is uppercased, and
+prefixed with `OPT_`). The original event name is passed in the `EVENT`
+environment variable, and the process ID of mercurial is given in `HGPID`.
+
+So given a call `ui.log('foo', 'bar', 'baz', spam='eggs'), a script configured
+for the `foo` event can expect an environment with `MSG1=bar`, `MSG2=baz`, and
+`OPT_SPAM=eggs`.
+
+Scripts are configured in the `[logtoprocess]` section, each key an event name.
+For example::
+
+  [logtoprocess]
+  commandexception = echo "$MSG2$MSG3" > /var/log/mercurial_exceptions.log
+
+would log the warning message and traceback of any failed command dispatch.
+
+Scripts are run asychronously as detached daemon processes; mercurial will
+not ensure that they exit cleanly.
+
+"""
+
+from __future__ import absolute_import
+
+import itertools
+import os
+import platform
+import subprocess
+import sys
+
+# Note for extension authors: ONLY specify testedwith = 'internal' for
+# extensions which SHIP WITH MERCURIAL. Non-mainline extensions should
+# be specifying the version(s) of Mercurial they are tested with, or
+# leave the attribute unspecified.
+testedwith = 'internal'
+
+def uisetup(ui):
+    if platform.system() == 'Windows':
+        # no fork on Windows, but we can create a detached process
+        # https://msdn.microsoft.com/en-us/library/windows/desktop/ms684863.aspx
+        # No stdlib constant exists for this value
+        DETACHED_PROCESS = 0x00000008
+        _creationflags = DETACHED_PROCESS | subprocess.CREATE_NEW_PROCESS_GROUP
+
+        def runshellcommand(script, env):
+            # we can't use close_fds *and* redirect stdin. I'm not sure that we
+            # need to because the detached process has no console connection.
+            subprocess.Popen(
+                script, shell=True, env=env, close_fds=True,
+                creationflags=_creationflags)
+    else:
+        def runshellcommand(script, env):
+            # double-fork to completely detach from the parent process
+            # based on http://code.activestate.com/recipes/278731
+            pid = os.fork()
+            if pid:
+                # parent
+                return
+            # subprocess.Popen() forks again, all we need to add is
+            # flag the new process as a new session.
+            if sys.version_info < (3, 2):
+                newsession = {'preexec_fn': os.setsid}
+            else:
+                newsession = {'start_new_session': True}
+            try:
+                # connect stdin to devnull to make sure the subprocess can't
+                # muck up that stream for mercurial.
+                subprocess.Popen(
+                    script, shell=True, stdin=open(os.devnull, 'r'), env=env,
+                    close_fds=True, **newsession)
+            finally:
+                # mission accomplished, this child needs to exit and not
+                # continue the hg process here.
+                os._exit(0)
+
+    class logtoprocessui(ui.__class__):
+        def log(self, event, *msg, **opts):
+            """Map log events to external commands
+
+            Arguments are passed on as environment variables.
+
+            """
+            script = ui.config('logtoprocess', event)
+            if script:
+                if msg:
+                    # try to format the log message given the remaining
+                    # arguments
+                    try:
+                        # Python string formatting with % either uses a
+                        # dictionary *or* tuple, but not both. If we have
+                        # keyword options, assume we need a mapping.
+                        formatted = msg[0] % (opts or msg[1:])
+                    except (TypeError, KeyError):
+                        # Failed to apply the arguments, ignore
+                        formatted = msg[0]
+                    messages = (formatted,) + msg[1:]
+                else:
+                    messages = msg
+                # positional arguments are listed as MSG[N] keys in the
+                # environment
+                msgpairs = (
+                    ('MSG{0:d}'.format(i), str(m))
+                    for i, m in enumerate(messages, 1))
+                # keyword arguments get prefixed with OPT_ and uppercased
+                optpairs = (
+                    ('OPT_{0}'.format(key.upper()), str(value))
+                    for key, value in opts.iteritems())
+                env = dict(itertools.chain(os.environ.items(),
+                                           msgpairs, optpairs),
+                           EVENT=event, HGPID=str(os.getpid()))
+                # Connect stdin to /dev/null to prevent child processes messing
+                # with mercurial's stdin.
+                runshellcommand(script, env)
+            return super(logtoprocessui, self).log(event, *msg, **opts)
+
+    # Replace the class for this instance and all clones created from it:
+    ui.__class__ = logtoprocessui