mercurial/commandserver.py

commandserver: add new forking server implemented without using SocketServer

SocketServer.ForkingMixIn of Python 2.x has a couple of issues, such as:

 - race condition that leads to 100% CPU usage (Python 2.6)
   https://bugs.python.org/issue21491
 - can't wait for children belonging to different process groups (Python 2.6)
 - leaves at least one zombie process (Python 2.6, 2.7)
   https://bugs.python.org/issue11109

The first two are critical because we do setpgid(0, 0) in child process to
isolate terminal signals. The last one isn't, but ForkingMixIn seems to be
doing silly. So there are two choices:

 a) backport and maintain SocketServer until we can drop support for Python 2.x
 b) replace SocketServer by simpler one and eliminate glue codes

I chose (b) because it's great time for getting rid of utterly complicated
SocketServer stuff, and preparing for future move towards prefork service.

New unixforkingservice is implemented loosely based on chg 531f8ef64be6. It
is monolithic but much simpler than SocketServer. unixservicehandler provides
customizing points for chg, and it will be shared with future prefork service.

Old unixservice class is still used by chgserver. It will be removed later.

Thanks to Jun Wu for investigating these issues.

# 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,
)

socketserver = util.socketserver

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()

        # 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 _serverequest(ui, repo, conn, createcmdserver):
    if True:  # TODO: unindent
        # 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)
        # change random state otherwise forked request handlers would have a
        # same state inherited from parent.
        random.seed()

        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
            # trigger __del__ since ForkingMixIn uses os._exit
            gc.collect()

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 _requesthandler(socketserver.BaseRequestHandler):
    def handle(self):
        _serverequest(self.server.ui, self.server.repo, self.request,
                      self._createcmdserver)

    def _createcmdserver(self, repo, conn, fin, fout):
        ui = self.server.ui
        return server(ui, repo, fin, fout)

class unixservice(object):
    """
    Listens on unix domain socket and forks server per connection
    """
    def __init__(self, ui, repo, opts):
        self.ui = ui
        self.repo = repo
        self.address = opts['address']
        if not util.safehasattr(socketserver, 'UnixStreamServer'):
            raise error.Abort(_('unsupported platform'))
        if not self.address:
            raise error.Abort(_('no socket path specified with --address'))

    def init(self):
        class cls(socketserver.ForkingMixIn, socketserver.UnixStreamServer):
            ui = self.ui
            repo = self.repo
        self.server = cls(self.address, _requesthandler)
        self.ui.status(_('listening at %s\n') % self.address)
        self.ui.flush()  # avoid buffering of status message

    def _cleanup(self):
        os.unlink(self.address)

    def run(self):
        try:
            self.server.serve_forever()
        finally:
            self._cleanup()

class unixforkingservice(unixservice):
    def __init__(self, ui, repo, opts, handler=None):
        super(unixforkingservice, self).__init__(ui, repo, opts)
        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(5)
        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._serveworker(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 _serveworker(self, conn):
        signal.signal(signal.SIGCHLD, self._oldsigchldhandler)
        h = self._servicehandler
        _serverequest(self.ui, self.repo, conn, h.createcmdserver)

_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)