view mercurial/worker.py @ 37051:40206e227412

wireproto: define and implement protocol for issuing requests

The existing HTTP and SSH wire protocols suffer from a host of flaws
and shortcomings. I've been wanting to rewrite the protocol for a while
now. Supporting partial clone - which will require new wire protocol
commands and capabilities - and other advanced server functionality
will be much easier if we start from a clean slate and don't have
to be constrained by limitations of the existing wire protocol.

This commit starts to introduce a new data exchange format for
use over the wire protocol.

The new protocol is built on top of "frames," which are atomic
units of metadata + data. Frames will make it easier to implement
proxies and other mechanisms that want to inspect data without
having to maintain state. The existing frame metadata is very
minimal and it will evolve heavily. (We will eventually support
things like concurrent requests, out-of-order responses,
compression, side-channels for status updates, etc. Some of
these will require additions to the frame header.)

Another benefit of frames is that all reads are of a fixed size.
A reader works by consuming a frame header, extracting the payload
length, then reading that many bytes. No lookahead, buffering, or
memory reallocations are needed.

The new protocol attempts to be transport agnostic. I want all that's
required to use the new protocol to be a pair of unidirectional,
half-duplex pipes. (Yes, we will eventually make use of full-duplex
pipes, but that's for another commit.) Notably, when the SSH
transport switches to this new protocol, stderr will be unused.
This is by design: the lack of stderr on HTTP harms protocol
behavior there. By shoehorning everything into a pair of pipes,
we can have more consistent behavior across transports.

We currently only define the client side parts of the new protocol,
specifically the bits for requesting that a command run. This keeps
the new code and feature small and somewhat easy to review.

We add support to `hg debugwireproto` for writing frames into
HTTP request bodies. Our tests that issue commands to the new
HTTP endpoint have been updated to transmit frames. The server
bits haven't been touched to consume the frames yet. This will
occur in the next commit...

Astute readers may notice that the command name is transmitted in
both the HTTP request URL and the command request frame. This is
partially a kludge from me initially implementing the frame-based
protocol for SSH first. But it is also a feature: I intend to
eventually support issuing multiple commands per HTTP request. This
will allow us to replace the abomination that is the "batch" wire
protocol command with a protocol-level mechanism for performing
multi-dispatch. Because I want the frame-based protocol to be
as similar as possible across transports, I'd rather we (redundantly)
include the command name in the frame than differ behavior between
transports that have out-of-band routing information (like HTTP)
readily available.

Differential Revision: https://phab.mercurial-scm.org/D2851

# worker.py - master-slave parallelism support
#
# Copyright 2013 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.

from __future__ import absolute_import

import errno
import os
import signal
import sys
import threading
import time

from .i18n import _
from . import (
    encoding,
    error,
    pycompat,
    scmutil,
    util,
)

def countcpus():
    '''try to count the number of CPUs on the system'''

    # posix
    try:
        n = int(os.sysconf(r'SC_NPROCESSORS_ONLN'))
        if n > 0:
            return n
    except (AttributeError, ValueError):
        pass

    # windows
    try:
        n = int(encoding.environ['NUMBER_OF_PROCESSORS'])
        if n > 0:
            return n
    except (KeyError, ValueError):
        pass

    return 1

def _numworkers(ui):
    s = ui.config('worker', 'numcpus')
    if s:
        try:
            n = int(s)
            if n >= 1:
                return n
        except ValueError:
            raise error.Abort(_('number of cpus must be an integer'))
    return min(max(countcpus(), 4), 32)

if pycompat.isposix or pycompat.iswindows:
    _startupcost = 0.01
else:
    _startupcost = 1e30

def worthwhile(ui, costperop, nops):
    '''try to determine whether the benefit of multiple processes can
    outweigh the cost of starting them'''
    linear = costperop * nops
    workers = _numworkers(ui)
    benefit = linear - (_startupcost * workers + linear / workers)
    return benefit >= 0.15

def worker(ui, costperarg, func, staticargs, args):
    '''run a function, possibly in parallel in multiple worker
    processes.

    returns a progress iterator

    costperarg - cost of a single task

    func - function to run

    staticargs - arguments to pass to every invocation of the function

    args - arguments to split into chunks, to pass to individual
    workers
    '''
    enabled = ui.configbool('worker', 'enabled')
    if enabled and worthwhile(ui, costperarg, len(args)):
        return _platformworker(ui, func, staticargs, args)
    return func(*staticargs + (args,))

def _posixworker(ui, func, staticargs, args):
    rfd, wfd = os.pipe()
    workers = _numworkers(ui)
    oldhandler = signal.getsignal(signal.SIGINT)
    signal.signal(signal.SIGINT, signal.SIG_IGN)
    pids, problem = set(), [0]
    def killworkers():
        # unregister SIGCHLD handler as all children will be killed. This
        # function shouldn't be interrupted by another SIGCHLD; otherwise pids
        # could be updated while iterating, which would cause inconsistency.
        signal.signal(signal.SIGCHLD, oldchldhandler)
        # if one worker bails, there's no good reason to wait for the rest
        for p in pids:
            try:
                os.kill(p, signal.SIGTERM)
            except OSError as err:
                if err.errno != errno.ESRCH:
                    raise
    def waitforworkers(blocking=True):
        for pid in pids.copy():
            p = st = 0
            while True:
                try:
                    p, st = os.waitpid(pid, (0 if blocking else os.WNOHANG))
                    break
                except OSError as e:
                    if e.errno == errno.EINTR:
                        continue
                    elif e.errno == errno.ECHILD:
                        # child would already be reaped, but pids yet been
                        # updated (maybe interrupted just after waitpid)
                        pids.discard(pid)
                        break
                    else:
                        raise
            if not p:
                # skip subsequent steps, because child process should
                # be still running in this case
                continue
            pids.discard(p)
            st = _exitstatus(st)
            if st and not problem[0]:
                problem[0] = st
    def sigchldhandler(signum, frame):
        waitforworkers(blocking=False)
        if problem[0]:
            killworkers()
    oldchldhandler = signal.signal(signal.SIGCHLD, sigchldhandler)
    ui.flush()
    parentpid = os.getpid()
    for pargs in partition(args, workers):
        # make sure we use os._exit in all worker code paths. otherwise the
        # worker may do some clean-ups which could cause surprises like
        # deadlock. see sshpeer.cleanup for example.
        # override error handling *before* fork. this is necessary because
        # exception (signal) may arrive after fork, before "pid =" assignment
        # completes, and other exception handler (dispatch.py) can lead to
        # unexpected code path without os._exit.
        ret = -1
        try:
            pid = os.fork()
            if pid == 0:
                signal.signal(signal.SIGINT, oldhandler)
                signal.signal(signal.SIGCHLD, oldchldhandler)

                def workerfunc():
                    os.close(rfd)
                    for i, item in func(*(staticargs + (pargs,))):
                        os.write(wfd, '%d %s\n' % (i, item))
                    return 0

                ret = scmutil.callcatch(ui, workerfunc)
        except: # parent re-raises, child never returns
            if os.getpid() == parentpid:
                raise
            exctype = sys.exc_info()[0]
            force = not issubclass(exctype, KeyboardInterrupt)
            ui.traceback(force=force)
        finally:
            if os.getpid() != parentpid:
                try:
                    ui.flush()
                except: # never returns, no re-raises
                    pass
                finally:
                    os._exit(ret & 255)
        pids.add(pid)
    os.close(wfd)
    fp = os.fdopen(rfd, r'rb', 0)
    def cleanup():
        signal.signal(signal.SIGINT, oldhandler)
        waitforworkers()
        signal.signal(signal.SIGCHLD, oldchldhandler)
        status = problem[0]
        if status:
            if status < 0:
                os.kill(os.getpid(), -status)
            sys.exit(status)
    try:
        for line in util.iterfile(fp):
            l = line.split(' ', 1)
            yield int(l[0]), l[1][:-1]
    except: # re-raises
        killworkers()
        cleanup()
        raise
    cleanup()

def _posixexitstatus(code):
    '''convert a posix exit status into the same form returned by
    os.spawnv

    returns None if the process was stopped instead of exiting'''
    if os.WIFEXITED(code):
        return os.WEXITSTATUS(code)
    elif os.WIFSIGNALED(code):
        return -os.WTERMSIG(code)

def _windowsworker(ui, func, staticargs, args):
    class Worker(threading.Thread):
        def __init__(self, taskqueue, resultqueue, func, staticargs,
                     group=None, target=None, name=None, verbose=None):
            threading.Thread.__init__(self, group=group, target=target,
                                      name=name, verbose=verbose)
            self._taskqueue = taskqueue
            self._resultqueue = resultqueue
            self._func = func
            self._staticargs = staticargs
            self._interrupted = False
            self.daemon = True
            self.exception = None

        def interrupt(self):
            self._interrupted = True

        def run(self):
            try:
                while not self._taskqueue.empty():
                    try:
                        args = self._taskqueue.get_nowait()
                        for res in self._func(*self._staticargs + (args,)):
                            self._resultqueue.put(res)
                            # threading doesn't provide a native way to
                            # interrupt execution. handle it manually at every
                            # iteration.
                            if self._interrupted:
                                return
                    except util.empty:
                        break
            except Exception as e:
                # store the exception such that the main thread can resurface
                # it as if the func was running without workers.
                self.exception = e
                raise

    threads = []
    def trykillworkers():
        # Allow up to 1 second to clean worker threads nicely
        cleanupend = time.time() + 1
        for t in threads:
            t.interrupt()
        for t in threads:
            remainingtime = cleanupend - time.time()
            t.join(remainingtime)
            if t.is_alive():
                # pass over the workers joining failure. it is more
                # important to surface the inital exception than the
                # fact that one of workers may be processing a large
                # task and does not get to handle the interruption.
                ui.warn(_("failed to kill worker threads while "
                          "handling an exception\n"))
                return

    workers = _numworkers(ui)
    resultqueue = util.queue()
    taskqueue = util.queue()
    # partition work to more pieces than workers to minimize the chance
    # of uneven distribution of large tasks between the workers
    for pargs in partition(args, workers * 20):
        taskqueue.put(pargs)
    for _i in range(workers):
        t = Worker(taskqueue, resultqueue, func, staticargs)
        threads.append(t)
        t.start()
    try:
        while len(threads) > 0:
            while not resultqueue.empty():
                yield resultqueue.get()
            threads[0].join(0.05)
            finishedthreads = [_t for _t in threads if not _t.is_alive()]
            for t in finishedthreads:
                if t.exception is not None:
                    raise t.exception
                threads.remove(t)
    except (Exception, KeyboardInterrupt): # re-raises
        trykillworkers()
        raise
    while not resultqueue.empty():
        yield resultqueue.get()

if pycompat.iswindows:
    _platformworker = _windowsworker
else:
    _platformworker = _posixworker
    _exitstatus = _posixexitstatus

def partition(lst, nslices):
    '''partition a list into N slices of roughly equal size

    The current strategy takes every Nth element from the input. If
    we ever write workers that need to preserve grouping in input
    we should consider allowing callers to specify a partition strategy.

    mpm is not a fan of this partitioning strategy when files are involved.
    In his words:

        Single-threaded Mercurial makes a point of creating and visiting
        files in a fixed order (alphabetical). When creating files in order,
        a typical filesystem is likely to allocate them on nearby regions on
        disk. Thus, when revisiting in the same order, locality is maximized
        and various forms of OS and disk-level caching and read-ahead get a
        chance to work.

        This effect can be quite significant on spinning disks. I discovered it
        circa Mercurial v0.4 when revlogs were named by hashes of filenames.
        Tarring a repo and copying it to another disk effectively randomized
        the revlog ordering on disk by sorting the revlogs by hash and suddenly
        performance of my kernel checkout benchmark dropped by ~10x because the
        "working set" of sectors visited no longer fit in the drive's cache and
        the workload switched from streaming to random I/O.

        What we should really be doing is have workers read filenames from a
        ordered queue. This preserves locality and also keeps any worker from
        getting more than one file out of balance.
    '''
    for i in range(nslices):
        yield lst[i::nslices]