chgserver: auto exit after being idle for too long or lose the socket file

This is a part of the one server per config series. In multiple-server setup,
new server may be started for a temporary config change like in command line,
--config extensions.foo=bar.py. This may end up with a lot of not so useful
server processes. Other questions are about socket file and process management,
How to stop these processes? What if a new server wants to listen on a same
address, replacing the old one?

This patch introduces AutoExitMixIn, which will:

1. Exit after being idle for too long. So useless servers won't run forever.
2. Periodically check the ownership of socket file, exit if it is no longer
   owned. This brings strong consistency between the filesystem and the
   process, and handles some race conditions neatly.
   Since rename is atomic, a new server can just have a same server address
   with an old one and won't worry about how to make sure the old server is
   killed, address conflict, service downtime issues.
   The user can safely stop all servers by simply removing the socket files,
   without worrying about outdated or accidentally removed pidfiles.

hgext/chgserver.py

@@ -1,399 +1,474 b''
 # chgserver.py - command server extension for cHg
 #
 # Copyright 2011 Yuya Nishihara <[e-mail]>
 #
 # This software may be used and distributed according to the terms of the
 # GNU General Public License version 2 or any later version.
 
 """command server extension for cHg (EXPERIMENTAL)
 
 'S' channel (read/write)
     propagate ui.system() request to client
 
 'attachio' command
     attach client's stdio passed by sendmsg()
 
 'chdir' command
     change current directory
 
 'getpager' command
     checks if pager is enabled and which pager should be executed
 
 'setenv' command
     replace os.environ completely
 
 'SIGHUP' signal
     reload configuration files
 """
 
 from __future__ import absolute_import
 
 import SocketServer
 import errno
 import os
 import re
 import signal
 import struct
+import threading
+import time
 import traceback
 
 from mercurial.i18n import _
 
 from mercurial import (
     cmdutil,
     commands,
     commandserver,
     dispatch,
     error,
     osutil,
     util,
 )
 
 # 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'
 
 _log = commandserver.log
 
 # copied from hgext/pager.py:uisetup()
 def _setuppagercmd(ui, options, cmd):
     if not ui.formatted():
         return
 
     p = ui.config("pager", "pager", os.environ.get("PAGER"))
     usepager = False
     always = util.parsebool(options['pager'])
     auto = options['pager'] == 'auto'
 
     if not p:
         pass
     elif always:
         usepager = True
     elif not auto:
         usepager = False
     else:
         attended = ['annotate', 'cat', 'diff', 'export', 'glog', 'log', 'qdiff']
         attend = ui.configlist('pager', 'attend', attended)
         ignore = ui.configlist('pager', 'ignore')
         cmds, _ = cmdutil.findcmd(cmd, commands.table)
 
         for cmd in cmds:
             var = 'attend-%s' % cmd
             if ui.config('pager', var):
                 usepager = ui.configbool('pager', var)
                 break
             if (cmd in attend or
                 (cmd not in ignore and not attend)):
                 usepager = True
                 break
 
     if usepager:
         ui.setconfig('ui', 'formatted', ui.formatted(), 'pager')
         ui.setconfig('ui', 'interactive', False, 'pager')
         return p
 
 _envvarre = re.compile(r'\$[a-zA-Z_]+')
 
 def _clearenvaliases(cmdtable):
     """Remove stale command aliases referencing env vars; variable expansion
     is done at dispatch.addaliases()"""
     for name, tab in cmdtable.items():
         cmddef = tab[0]
         if (isinstance(cmddef, dispatch.cmdalias) and
             not cmddef.definition.startswith('!') and  # shell alias
             _envvarre.search(cmddef.definition)):
             del cmdtable[name]
 
 def _newchgui(srcui, csystem):
     class chgui(srcui.__class__):
         def __init__(self, src=None):
             super(chgui, self).__init__(src)
             if src:
                 self._csystem = getattr(src, '_csystem', csystem)
             else:
                 self._csystem = csystem
 
         def system(self, cmd, environ=None, cwd=None, onerr=None,
                    errprefix=None):
             # copied from mercurial/util.py:system()
             self.flush()
             def py2shell(val):
                 if val is None or val is False:
                     return '0'
                 if val is True:
                     return '1'
                 return str(val)
             env = os.environ.copy()
             if environ:
                 env.update((k, py2shell(v)) for k, v in environ.iteritems())
             env['HG'] = util.hgexecutable()
             rc = self._csystem(cmd, env, cwd)
             if rc and onerr:
                 errmsg = '%s %s' % (os.path.basename(cmd.split(None, 1)[0]),
                                     util.explainexit(rc)[0])
                 if errprefix:
                     errmsg = '%s: %s' % (errprefix, errmsg)
                 raise onerr(errmsg)
             return rc
 
     return chgui(srcui)
 
 def _renewui(srcui):
     newui = srcui.__class__()
     for a in ['fin', 'fout', 'ferr', 'environ']:
         setattr(newui, a, getattr(srcui, a))
     if util.safehasattr(srcui, '_csystem'):
         newui._csystem = srcui._csystem
     # stolen from tortoisehg.util.copydynamicconfig()
     for section, name, value in srcui.walkconfig():
         source = srcui.configsource(section, name)
         if ':' in source:
             # path:line
             continue
         if source == 'none':
             # ui.configsource returns 'none' by default
             source = ''
         newui.setconfig(section, name, value, source)
     return newui
 
 class channeledsystem(object):
     """Propagate ui.system() request in the following format:
 
     payload length (unsigned int),
     cmd, '\0',
     cwd, '\0',
     envkey, '=', val, '\0',
     ...
     envkey, '=', val
 
     and waits:
 
     exitcode length (unsigned int),
     exitcode (int)
     """
     def __init__(self, in_, out, channel):
         self.in_ = in_
         self.out = out
         self.channel = channel
 
     def __call__(self, cmd, environ, cwd):
         args = [util.quotecommand(cmd), cwd or '.']
         args.extend('%s=%s' % (k, v) for k, v in environ.iteritems())
         data = '\0'.join(args)
         self.out.write(struct.pack('>cI', self.channel, len(data)))
         self.out.write(data)
         self.out.flush()
 
         length = self.in_.read(4)
         length, = struct.unpack('>I', length)
         if length != 4:
             raise error.Abort(_('invalid response'))
         rc, = struct.unpack('>i', self.in_.read(4))
         return rc
 
 _iochannels = [
     # server.ch, ui.fp, mode
     ('cin', 'fin', 'rb'),
     ('cout', 'fout', 'wb'),
     ('cerr', 'ferr', 'wb'),
 ]
 
 class chgcmdserver(commandserver.server):
     def __init__(self, ui, repo, fin, fout, sock):
         super(chgcmdserver, self).__init__(
             _newchgui(ui, channeledsystem(fin, fout, 'S')), repo, fin, fout)
         self.clientsock = sock
         self._oldios = []  # original (self.ch, ui.fp, fd) before "attachio"
 
     def cleanup(self):
         # dispatch._runcatch() does not flush outputs if exception is not
         # handled by dispatch._dispatch()
         self.ui.flush()
         self._restoreio()
 
     def attachio(self):
         """Attach to client's stdio passed via unix domain socket; all
         channels except cresult will no longer be used
         """
         # tell client to sendmsg() with 1-byte payload, which makes it
         # distinctive from "attachio\n" command consumed by client.read()
         self.clientsock.sendall(struct.pack('>cI', 'I', 1))
         clientfds = osutil.recvfds(self.clientsock.fileno())
         _log('received fds: %r\n' % clientfds)
 
         ui = self.ui
         ui.flush()
         first = self._saveio()
         for fd, (cn, fn, mode) in zip(clientfds, _iochannels):
             assert fd > 0
             fp = getattr(ui, fn)
             os.dup2(fd, fp.fileno())
             os.close(fd)
             if not first:
                 continue
             # reset buffering mode when client is first attached. as we want
             # to see output immediately on pager, the mode stays unchanged
             # when client re-attached. ferr is unchanged because it should
             # be unbuffered no matter if it is a tty or not.
             if fn == 'ferr':
                 newfp = fp
             else:
                 # make it line buffered explicitly because the default is
                 # decided on first write(), where fout could be a pager.
                 if fp.isatty():
                     bufsize = 1  # line buffered
                 else:
                     bufsize = -1  # system default
                 newfp = os.fdopen(fp.fileno(), mode, bufsize)
                 setattr(ui, fn, newfp)
             setattr(self, cn, newfp)
 
         self.cresult.write(struct.pack('>i', len(clientfds)))
 
     def _saveio(self):
         if self._oldios:
             return False
         ui = self.ui
         for cn, fn, _mode in _iochannels:
             ch = getattr(self, cn)
             fp = getattr(ui, fn)
             fd = os.dup(fp.fileno())
             self._oldios.append((ch, fp, fd))
         return True
 
     def _restoreio(self):
         ui = self.ui
         for (ch, fp, fd), (cn, fn, _mode) in zip(self._oldios, _iochannels):
             newfp = getattr(ui, fn)
             # close newfp while it's associated with client; otherwise it
             # would be closed when newfp is deleted
             if newfp is not fp:
                 newfp.close()
             # restore original fd: fp is open again
             os.dup2(fd, fp.fileno())
             os.close(fd)
             setattr(self, cn, ch)
             setattr(ui, fn, fp)
         del self._oldios[:]
 
     def chdir(self):
         """Change current directory
 
         Note that the behavior of --cwd option is bit different from this.
         It does not affect --config parameter.
         """
         path = self._readstr()
         if not path:
             return
         _log('chdir to %r\n' % path)
         os.chdir(path)
 
     def setumask(self):
         """Change umask"""
         mask = struct.unpack('>I', self._read(4))[0]
         _log('setumask %r\n' % mask)
         os.umask(mask)
 
     def getpager(self):
         """Read cmdargs and write pager command to r-channel if enabled
 
         If pager isn't enabled, this writes '\0' because channeledoutput
         does not allow to write empty data.
         """
         args = self._readlist()
         try:
             cmd, _func, args, options, _cmdoptions = dispatch._parse(self.ui,
                                                                      args)
         except (error.Abort, error.AmbiguousCommand, error.CommandError,
                 error.UnknownCommand):
             cmd = None
             options = {}
         if not cmd or 'pager' not in options:
             self.cresult.write('\0')
             return
 
         pagercmd = _setuppagercmd(self.ui, options, cmd)
         if pagercmd:
             self.cresult.write(pagercmd)
         else:
             self.cresult.write('\0')
 
     def setenv(self):
         """Clear and update os.environ
 
         Note that not all variables can make an effect on the running process.
         """
         l = self._readlist()
         try:
             newenv = dict(s.split('=', 1) for s in l)
         except ValueError:
             raise ValueError('unexpected value in setenv request')
 
         diffkeys = set(k for k in set(os.environ.keys() + newenv.keys())
                        if os.environ.get(k) != newenv.get(k))
         _log('change env: %r\n' % sorted(diffkeys))
 
         os.environ.clear()
         os.environ.update(newenv)
 
         if set(['HGPLAIN', 'HGPLAINEXCEPT']) & diffkeys:
             # reload config so that ui.plain() takes effect
             self.ui = _renewui(self.ui)
 
         _clearenvaliases(commands.table)
 
     capabilities = commandserver.server.capabilities.copy()
     capabilities.update({'attachio': attachio,
                          'chdir': chdir,
                          'getpager': getpager,
                          'setenv': setenv,
                          'setumask': setumask})
 
 # copied from mercurial/commandserver.py
 class _requesthandler(SocketServer.StreamRequestHandler):
     def handle(self):
         # use a different process group from the master process, making this
         # process pass kernel "is_current_pgrp_orphaned" check so signals like
         # SIGTSTP, SIGTTIN, SIGTTOU are not ignored.
         os.setpgid(0, 0)
         ui = self.server.ui
         repo = self.server.repo
         sv = chgcmdserver(ui, repo, self.rfile, self.wfile, self.connection)
         try:
             try:
                 sv.serve()
             # handle exceptions that may be raised by command server. most of
             # known exceptions are caught by dispatch.
             except error.Abort as inst:
                 ui.warn(_('abort: %s\n') % inst)
             except IOError as inst:
                 if inst.errno != errno.EPIPE:
                     raise
             except KeyboardInterrupt:
                 pass
             finally:
                 sv.cleanup()
         except: # re-raises
             # also write traceback to error channel. otherwise client cannot
             # see it because it is written to server's stderr by default.
             traceback.print_exc(file=sv.cerr)
             raise
 
+def _tempaddress(address):
+    return '%s.%d.tmp' % (address, os.getpid())
+
+class AutoExitMixIn:  # use old-style to comply with SocketServer design
+    lastactive = time.time()
+    idletimeout = 3600  # default 1 hour
+
+    def startautoexitthread(self):
+        # note: the auto-exit check here is cheap enough to not use a thread,
+        # be done in serve_forever. however SocketServer is hook-unfriendly,
+        # you simply cannot hook serve_forever without copying a lot of code.
+        # besides, serve_forever's docstring suggests using thread.
+        thread = threading.Thread(target=self._autoexitloop)
+        thread.daemon = True
+        thread.start()
+
+    def _autoexitloop(self, interval=1):
+        while True:
+            time.sleep(interval)
+            if not self.issocketowner():
+                _log('%s is not owned, exiting.\n' % self.server_address)
+                break
+            if time.time() - self.lastactive > self.idletimeout:
+                _log('being idle too long. exiting.\n')
+                break
+        self.shutdown()
+
+    def process_request(self, request, address):
+        self.lastactive = time.time()
+        return SocketServer.ForkingMixIn.process_request(
+            self, request, address)
+
+    def server_bind(self):
+        # use a unique temp address so we can stat the file and do ownership
+        # check later
+        tempaddress = _tempaddress(self.server_address)
+        self.socket.bind(tempaddress)
+        self._socketstat = os.stat(tempaddress)
+        # rename will replace the old socket file if exists atomically. the
+        # old server will detect ownership change and exit.
+        util.rename(tempaddress, self.server_address)
+
+    def issocketowner(self):
+        try:
+            stat = os.stat(self.server_address)
+            return (stat.st_ino == self._socketstat.st_ino and
+                    stat.st_mtime == self._socketstat.st_mtime)
+        except OSError:
+            return False
+
+    def unlinksocketfile(self):
+        if not self.issocketowner():
+            return
+        # it is possible to have a race condition here that we may
+        # remove another server's socket file. but that's okay
+        # since that server will detect and exit automatically and
+        # the client will start a new server on demand.
+        try:
+            os.unlink(self.server_address)
+        except OSError as exc:
+            if exc.errno != errno.ENOENT:
+                raise
+
 class chgunixservice(commandserver.unixservice):
     def init(self):
         # drop options set for "hg serve --cmdserver" command
         self.ui.setconfig('progress', 'assume-tty', None)
         signal.signal(signal.SIGHUP, self._reloadconfig)
-        class cls(SocketServer.ForkingMixIn, SocketServer.UnixStreamServer):
+        class cls(AutoExitMixIn, SocketServer.ForkingMixIn,
+                  SocketServer.UnixStreamServer):
             ui = self.ui
             repo = self.repo
         self.server = cls(self.address, _requesthandler)
+        self.server.idletimeout = self.ui.configint(
+            'chgserver', 'idletimeout', self.server.idletimeout)
+        self.server.startautoexitthread()
         # avoid writing "listening at" message to stdout before attachio
         # request, which calls setvbuf()
 
     def _reloadconfig(self, signum, frame):
         self.ui = self.server.ui = _renewui(self.ui)
 
+    def run(self):
+        try:
+            self.server.serve_forever()
+        finally:
+            self.server.unlinksocketfile()
+
 def uisetup(ui):
     commandserver._servicemap['chgunix'] = chgunixservice