commandserver: update comment about setpgid

Now setpgid has 2 main purposes: better handling for terminal-generated
SIGTSTP, SIGINT, and process-exit-generated SIGHUP. Update the comment to
explain things more clearly.

mercurial/commandserver.py

@@ -1,536 +1,544 b''
 # commandserver.py - communicate with Mercurial's API over a pipe
 #
 #  Copyright Matt Mackall <[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.
 
 from __future__ import absolute_import
 
 import errno
 import gc
 import os
 import random
 import select
 import signal
 import socket
 import struct
 import sys
 import traceback
 
 from .i18n import _
 from . import (
     encoding,
     error,
     util,
 )
 
 logfile = None
 
 def log(*args):
     if not logfile:
         return
 
     for a in args:
         logfile.write(str(a))
 
     logfile.flush()
 
 class channeledoutput(object):
     """
     Write data to out in the following format:
 
     data length (unsigned int),
     data
     """
     def __init__(self, out, channel):
         self.out = out
         self.channel = channel
 
     @property
     def name(self):
         return '<%c-channel>' % self.channel
 
     def write(self, data):
         if not data:
             return
         self.out.write(struct.pack('>cI', self.channel, len(data)))
         self.out.write(data)
         self.out.flush()
 
     def __getattr__(self, attr):
         if attr in ('isatty', 'fileno', 'tell', 'seek'):
             raise AttributeError(attr)
         return getattr(self.out, attr)
 
 class channeledinput(object):
     """
     Read data from in_.
 
     Requests for input are written to out in the following format:
     channel identifier - 'I' for plain input, 'L' line based (1 byte)
     how many bytes to send at most (unsigned int),
 
     The client replies with:
     data length (unsigned int), 0 meaning EOF
     data
     """
 
     maxchunksize = 4 * 1024
 
     def __init__(self, in_, out, channel):
         self.in_ = in_
         self.out = out
         self.channel = channel
 
     @property
     def name(self):
         return '<%c-channel>' % self.channel
 
     def read(self, size=-1):
         if size < 0:
             # if we need to consume all the clients input, ask for 4k chunks
             # so the pipe doesn't fill up risking a deadlock
             size = self.maxchunksize
             s = self._read(size, self.channel)
             buf = s
             while s:
                 s = self._read(size, self.channel)
                 buf += s
 
             return buf
         else:
             return self._read(size, self.channel)
 
     def _read(self, size, channel):
         if not size:
             return ''
         assert size > 0
 
         # tell the client we need at most size bytes
         self.out.write(struct.pack('>cI', channel, size))
         self.out.flush()
 
         length = self.in_.read(4)
         length = struct.unpack('>I', length)[0]
         if not length:
             return ''
         else:
             return self.in_.read(length)
 
     def readline(self, size=-1):
         if size < 0:
             size = self.maxchunksize
             s = self._read(size, 'L')
             buf = s
             # keep asking for more until there's either no more or
             # we got a full line
             while s and s[-1] != '\n':
                 s = self._read(size, 'L')
                 buf += s
 
             return buf
         else:
             return self._read(size, 'L')
 
     def __iter__(self):
         return self
 
     def next(self):
         l = self.readline()
         if not l:
             raise StopIteration
         return l
 
     def __getattr__(self, attr):
         if attr in ('isatty', 'fileno', 'tell', 'seek'):
             raise AttributeError(attr)
         return getattr(self.in_, attr)
 
 class server(object):
     """
     Listens for commands on fin, runs them and writes the output on a channel
     based stream to fout.
     """
     def __init__(self, ui, repo, fin, fout):
         self.cwd = os.getcwd()
 
         # developer config: cmdserver.log
         logpath = ui.config("cmdserver", "log", None)
         if logpath:
             global logfile
             if logpath == '-':
                 # write log on a special 'd' (debug) channel
                 logfile = channeledoutput(fout, 'd')
             else:
                 logfile = open(logpath, 'a')
 
         if repo:
             # the ui here is really the repo ui so take its baseui so we don't
             # end up with its local configuration
             self.ui = repo.baseui
             self.repo = repo
             self.repoui = repo.ui
         else:
             self.ui = ui
             self.repo = self.repoui = None
 
         self.cerr = channeledoutput(fout, 'e')
         self.cout = channeledoutput(fout, 'o')
         self.cin = channeledinput(fin, fout, 'I')
         self.cresult = channeledoutput(fout, 'r')
 
         self.client = fin
 
     def cleanup(self):
         """release and restore resources taken during server session"""
         pass
 
     def _read(self, size):
         if not size:
             return ''
 
         data = self.client.read(size)
 
         # is the other end closed?
         if not data:
             raise EOFError
 
         return data
 
     def _readstr(self):
         """read a string from the channel
 
         format:
         data length (uint32), data
         """
         length = struct.unpack('>I', self._read(4))[0]
         if not length:
             return ''
         return self._read(length)
 
     def _readlist(self):
         """read a list of NULL separated strings from the channel"""
         s = self._readstr()
         if s:
             return s.split('\0')
         else:
             return []
 
     def runcommand(self):
         """ reads a list of \0 terminated arguments, executes
         and writes the return code to the result channel """
         from . import dispatch  # avoid cycle
 
         args = self._readlist()
 
         # copy the uis so changes (e.g. --config or --verbose) don't
         # persist between requests
         copiedui = self.ui.copy()
         uis = [copiedui]
         if self.repo:
             self.repo.baseui = copiedui
             # clone ui without using ui.copy because this is protected
             repoui = self.repoui.__class__(self.repoui)
             repoui.copy = copiedui.copy # redo copy protection
             uis.append(repoui)
             self.repo.ui = self.repo.dirstate._ui = repoui
             self.repo.invalidateall()
 
         for ui in uis:
             ui.resetstate()
             # any kind of interaction must use server channels, but chg may
             # replace channels by fully functional tty files. so nontty is
             # enforced only if cin is a channel.
             if not util.safehasattr(self.cin, 'fileno'):
                 ui.setconfig('ui', 'nontty', 'true', 'commandserver')
 
         req = dispatch.request(args[:], copiedui, self.repo, self.cin,
                                self.cout, self.cerr)
 
         ret = (dispatch.dispatch(req) or 0) & 255 # might return None
 
         # restore old cwd
         if '--cwd' in args:
             os.chdir(self.cwd)
 
         self.cresult.write(struct.pack('>i', int(ret)))
 
     def getencoding(self):
         """ writes the current encoding to the result channel """
         self.cresult.write(encoding.encoding)
 
     def serveone(self):
         cmd = self.client.readline()[:-1]
         if cmd:
             handler = self.capabilities.get(cmd)
             if handler:
                 handler(self)
             else:
                 # clients are expected to check what commands are supported by
                 # looking at the servers capabilities
                 raise error.Abort(_('unknown command %s') % cmd)
 
         return cmd != ''
 
     capabilities = {'runcommand'  : runcommand,
                     'getencoding' : getencoding}
 
     def serve(self):
         hellomsg = 'capabilities: ' + ' '.join(sorted(self.capabilities))
         hellomsg += '\n'
         hellomsg += 'encoding: ' + encoding.encoding
         hellomsg += '\n'
         hellomsg += 'pid: %d' % util.getpid()
         if util.safehasattr(os, 'getpgid'):
             hellomsg += '\n'
             hellomsg += 'pgid: %d' % os.getpgid(0)
 
         # write the hello msg in -one- chunk
         self.cout.write(hellomsg)
 
         try:
             while self.serveone():
                 pass
         except EOFError:
             # we'll get here if the client disconnected while we were reading
             # its request
             return 1
 
         return 0
 
 def _protectio(ui):
     """ duplicates streams and redirect original to null if ui uses stdio """
     ui.flush()
     newfiles = []
     nullfd = os.open(os.devnull, os.O_RDWR)
     for f, sysf, mode in [(ui.fin, sys.stdin, 'rb'),
                           (ui.fout, sys.stdout, 'wb')]:
         if f is sysf:
             newfd = os.dup(f.fileno())
             os.dup2(nullfd, f.fileno())
             f = os.fdopen(newfd, mode)
         newfiles.append(f)
     os.close(nullfd)
     return tuple(newfiles)
 
 def _restoreio(ui, fin, fout):
     """ restores streams from duplicated ones """
     ui.flush()
     for f, uif in [(fin, ui.fin), (fout, ui.fout)]:
         if f is not uif:
             os.dup2(f.fileno(), uif.fileno())
             f.close()
 
 class pipeservice(object):
     def __init__(self, ui, repo, opts):
         self.ui = ui
         self.repo = repo
 
     def init(self):
         pass
 
     def run(self):
         ui = self.ui
         # redirect stdio to null device so that broken extensions or in-process
         # hooks will never cause corruption of channel protocol.
         fin, fout = _protectio(ui)
         try:
             sv = server(ui, self.repo, fin, fout)
             return sv.serve()
         finally:
             sv.cleanup()
             _restoreio(ui, fin, fout)
 
 def _initworkerprocess():
-    # 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.
+    # use a different process group from the master process, in order to:
+    # 1. make the current process group no longer "orphaned" (because the
+    #    parent of this process is in a different process group while
+    #    remains in a same session)
+    #    according to POSIX 2.2.2.52, orphaned process group will ignore
+    #    terminal-generated stop signals like SIGTSTP (Ctrl+Z), which will
+    #    cause trouble for things like ncurses.
+    # 2. the client can use kill(-pgid, sig) to simulate terminal-generated
+    #    SIGINT (Ctrl+C) and process-exit-generated SIGHUP. our child
+    #    processes like ssh will be killed properly, without affecting
+    #    unrelated processes.
     os.setpgid(0, 0)
     # change random state otherwise forked request handlers would have a
     # same state inherited from parent.
     random.seed()
 
 def _serverequest(ui, repo, conn, createcmdserver):
     fin = conn.makefile('rb')
     fout = conn.makefile('wb')
     sv = None
     try:
         sv = createcmdserver(repo, conn, fin, fout)
         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.
         if sv:
             cerr = sv.cerr
         else:
             cerr = channeledoutput(fout, 'e')
         traceback.print_exc(file=cerr)
         raise
     finally:
         fin.close()
         try:
             fout.close()  # implicit flush() may cause another EPIPE
         except IOError as inst:
             if inst.errno != errno.EPIPE:
                 raise
 
 class unixservicehandler(object):
     """Set of pluggable operations for unix-mode services
 
     Almost all methods except for createcmdserver() are called in the main
     process. You can't pass mutable resource back from createcmdserver().
     """
 
     pollinterval = None
 
     def __init__(self, ui):
         self.ui = ui
 
     def bindsocket(self, sock, address):
         util.bindunixsocket(sock, address)
 
     def unlinksocket(self, address):
         os.unlink(address)
 
     def printbanner(self, address):
         self.ui.status(_('listening at %s\n') % address)
         self.ui.flush()  # avoid buffering of status message
 
     def shouldexit(self):
         """True if server should shut down; checked per pollinterval"""
         return False
 
     def newconnection(self):
         """Called when main process notices new connection"""
         pass
 
     def createcmdserver(self, repo, conn, fin, fout):
         """Create new command server instance; called in the process that
         serves for the current connection"""
         return server(self.ui, repo, fin, fout)
 
 class unixforkingservice(object):
     """
     Listens on unix domain socket and forks server per connection
     """
 
     def __init__(self, ui, repo, opts, handler=None):
         self.ui = ui
         self.repo = repo
         self.address = opts['address']
         if not util.safehasattr(socket, 'AF_UNIX'):
             raise error.Abort(_('unsupported platform'))
         if not self.address:
             raise error.Abort(_('no socket path specified with --address'))
         self._servicehandler = handler or unixservicehandler(ui)
         self._sock = None
         self._oldsigchldhandler = None
         self._workerpids = set()  # updated by signal handler; do not iterate
 
     def init(self):
         self._sock = socket.socket(socket.AF_UNIX)
         self._servicehandler.bindsocket(self._sock, self.address)
         self._sock.listen(socket.SOMAXCONN)
         o = signal.signal(signal.SIGCHLD, self._sigchldhandler)
         self._oldsigchldhandler = o
         self._servicehandler.printbanner(self.address)
 
     def _cleanup(self):
         signal.signal(signal.SIGCHLD, self._oldsigchldhandler)
         self._sock.close()
         self._servicehandler.unlinksocket(self.address)
         # don't kill child processes as they have active clients, just wait
         self._reapworkers(0)
 
     def run(self):
         try:
             self._mainloop()
         finally:
             self._cleanup()
 
     def _mainloop(self):
         h = self._servicehandler
         while not h.shouldexit():
             try:
                 ready = select.select([self._sock], [], [], h.pollinterval)[0]
                 if not ready:
                     continue
                 conn, _addr = self._sock.accept()
             except (select.error, socket.error) as inst:
                 if inst.args[0] == errno.EINTR:
                     continue
                 raise
 
             pid = os.fork()
             if pid:
                 try:
                     self.ui.debug('forked worker process (pid=%d)\n' % pid)
                     self._workerpids.add(pid)
                     h.newconnection()
                 finally:
                     conn.close()  # release handle in parent process
             else:
                 try:
                     self._runworker(conn)
                     conn.close()
                     os._exit(0)
                 except:  # never return, hence no re-raises
                     try:
                         self.ui.traceback(force=True)
                     finally:
                         os._exit(255)
 
     def _sigchldhandler(self, signal, frame):
         self._reapworkers(os.WNOHANG)
 
     def _reapworkers(self, options):
         while self._workerpids:
             try:
                 pid, _status = os.waitpid(-1, options)
             except OSError as inst:
                 if inst.errno == errno.EINTR:
                     continue
                 if inst.errno != errno.ECHILD:
                     raise
                 # no child processes at all (reaped by other waitpid()?)
                 self._workerpids.clear()
                 return
             if pid == 0:
                 # no waitable child processes
                 return
             self.ui.debug('worker process exited (pid=%d)\n' % pid)
             self._workerpids.discard(pid)
 
     def _runworker(self, conn):
         signal.signal(signal.SIGCHLD, self._oldsigchldhandler)
         _initworkerprocess()
         h = self._servicehandler
         try:
             _serverequest(self.ui, self.repo, conn, h.createcmdserver)
         finally:
             gc.collect()  # trigger __del__ since worker process uses os._exit
 
 _servicemap = {
     'pipe': pipeservice,
     'unix': unixforkingservice,
     }
 
 def createservice(ui, repo, opts):
     mode = opts['cmdserver']
     try:
         return _servicemap[mode](ui, repo, opts)
     except KeyError:
         raise error.Abort(_('unknown mode %s') % mode)