view mercurial/worker.py @ 48687:f8f2ecdde4b5

branchmap: skip obsolete revisions while computing heads

It's time to make this part of core Mercurial obsolescence-aware.

Not considering obsolete revisions when computing heads is clearly what
Mercurial should do. But there are a couple of small issues:

- Let's say tip of the repo is obsolete. There are two ways of finding tiprev
  for branchcache (both are in use): looking at input data for update() and
  looking at computed heads after update(). Previously, repo tip would be
  tiprev of the branchcache. With this patch, an obsolete revision can no
  longer be tiprev. And depending on what way we use for finding tiprev (input
  data vs computed heads) we'll get a different result. This is relevant when
  recomputing cache key from cache contents, and may lead to updating cache for
  obsolete revisions multiple times (not from scratch, because it still would
  be considered valid for a subset of revisions in the repo).

- If all commits on a branch are obsolete, the branchcache will include that
  branch, but the list of heads will be empty (that's why there's now `if not
  heads` when recomputing tiprev/tipnode from cache contents). Having an entry
  for every branch is currently required for notify extension (and
  test-notify.t to pass), because notify doesn't handle revsets in its
  subscription config very well and will throw an error if e.g. a branch
  doesn't exist.

- Cloning static HTTP repos may try to stat() a non-existent obsstore file. The
  issue is that we now care about obsolescence during clone, but statichttpvfs
  doesn't implement a stat method, so a regular vfs.stat() is used, and it
  assumes that file is local and calls os.stat().  During a clone, we're trying
  to stat() .hg/store/obsstore, but in static HTTP case we provide a literal
  URL to the obsstore file on the remote as if it were a local file path. On
  windows it actually results in a failure in test-static-http.t.

The first issue is going to be addressed in a series dedicated to making sure
branchcache is properly and timely written on disk (it wasn't perfect even
before this patch, but there aren't enough tests to demonstrate that). The
second issue will be addressed in a future patch for notify extension that will
make it not raise an exception if a branch doesn't exist. And the third one was
partially addressed in the previous patch in this series and will be properly
fixed in a future patch when this series is accepted.

filteredhash() grows a keyword argument to make sure that branchcache is also
invalidated when there are new obsolete revisions in its repo view. This way
the on-disk cache format is unchanged and compatible between versions (although
it will obviously be recomputed when switching versions before/after this patch
and the repo has obsolete revisions).

There's one test that uses plain `hg up` without arguments while updated to a
pruned commit. To make this test pass, simply return current working directory
parent. Later in this series this code will be replaced by what prune command
does: updating to the closest non-obsolete ancestor.

Test changes:

test-branch-change.t: update branch head and cache update message. The head of
default listed in hg heads is changed because revision 2 was rewritten as 7,
and 1 is the closest ancestor on the same branch, so it's the head of default
now.

The cache invalidation message appears now because of the cache hash change,
since we're now accounting for obsolete revisions. Here's some context:

"served.hidden" repo filter means everything is visible (no filtered
revisions), so before this series branch2-served.hidden file would not contain
any cache hash, only revnum and node. Now it also has a hash when there are
obsolete changesets in the repo. The command that the message appears for is
changing branch of 5 and 6, which are now obsolete, so the cache hash changes.

In general, when cache is simply out-of-date, it can be updated using the old
version as a base. But if cache hash differs, then the cache for that
particular repo filter is recomputed (at least with the current
implementation). This is what happens here.

test-obsmarker-template.t: the pull reports 2 heads changed, but after that the
repo correctly sees only 1. The new message could be better, but it's still an
improvement over the previous one where hg pull suggested merging with an
obsolete revision.

test-obsolete.t: we can see these revisions in hg log --hidden, but they
shouldn't be considered heads even with --hidden.

test-rebase-obsolete{,2}.t: there were new heads created previously after
making new orphan changesets, but they weren't detected. Now we are properly
detecting and reporting them.

test-rebase-obsolete4.t: there's only one head now because the other head is
pruned and was falsely reported before.

test-static-http.t: add obsstore to the list of requested files. This file
doesn't exist on the remotes, but clients want it anyway (they get 404). This
is fine, because there are other nonexistent files that clients request, like
.hg/bookmarks or .hg/cache/tags2-served.

Differential Revision: https://phab.mercurial-scm.org/D12097

# 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

try:
    import selectors

    selectors.BaseSelector
except ImportError:
    from .thirdparty import selectors2 as selectors

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('SC_NPROCESSORS_ONLN'))
        if n > 0:
            return n
    except (AttributeError, ValueError):
        pass

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

    return 1


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


if pycompat.ispy3:

    def ismainthread():
        return threading.current_thread() == threading.main_thread()

    class _blockingreader(object):
        def __init__(self, wrapped):
            self._wrapped = wrapped

        # Do NOT implement readinto() by making it delegate to
        # _wrapped.readinto(), since that is unbuffered. The unpickler is fine
        # with just read() and readline(), so we don't need to implement it.

        def readline(self):
            return self._wrapped.readline()

        # issue multiple reads until size is fulfilled
        def read(self, size=-1):
            if size < 0:
                return self._wrapped.readall()

            buf = bytearray(size)
            view = memoryview(buf)
            pos = 0

            while pos < size:
                ret = self._wrapped.readinto(view[pos:])
                if not ret:
                    break
                pos += ret

            del view
            del buf[pos:]
            return bytes(buf)


else:

    def ismainthread():
        # pytype: disable=module-attr
        return isinstance(threading.current_thread(), threading._MainThread)
        # pytype: enable=module-attr

    def _blockingreader(wrapped):
        return wrapped


if pycompat.isposix or pycompat.iswindows:
    _STARTUP_COST = 0.01
    # The Windows worker is thread based. If tasks are CPU bound, threads
    # in the presence of the GIL result in excessive context switching and
    # this overhead can slow down execution.
    _DISALLOW_THREAD_UNSAFE = pycompat.iswindows
else:
    _STARTUP_COST = 1e30
    _DISALLOW_THREAD_UNSAFE = False


def worthwhile(ui, costperop, nops, threadsafe=True):
    """try to determine whether the benefit of multiple processes can
    outweigh the cost of starting them"""

    if not threadsafe and _DISALLOW_THREAD_UNSAFE:
        return False

    linear = costperop * nops
    workers = _numworkers(ui)
    benefit = linear - (_STARTUP_COST * workers + linear / workers)
    return benefit >= 0.15


def worker(
    ui, costperarg, func, staticargs, args, hasretval=False, threadsafe=True
):
    """run a function, possibly in parallel in multiple worker
    processes.

    returns a progress iterator

    costperarg - cost of a single task

    func - function to run. It is expected to return a progress iterator.

    staticargs - arguments to pass to every invocation of the function

    args - arguments to split into chunks, to pass to individual
    workers

    hasretval - when True, func and the current function return an progress
    iterator then a dict (encoded as an iterator that yield many (False, ..)
    then a (True, dict)). The dicts are joined in some arbitrary order, so
    overlapping keys are a bad idea.

    threadsafe - whether work items are thread safe and can be executed using
    a thread-based worker. Should be disabled for CPU heavy tasks that don't
    release the GIL.
    """
    enabled = ui.configbool(b'worker', b'enabled')
    if enabled and _platformworker is _posixworker and not ismainthread():
        # The POSIX worker has to install a handler for SIGCHLD.
        # Python up to 3.9 only allows this in the main thread.
        enabled = False

    if enabled and worthwhile(ui, costperarg, len(args), threadsafe=threadsafe):
        return _platformworker(ui, func, staticargs, args, hasretval)
    return func(*staticargs + (args,))


def _posixworker(ui, func, staticargs, args, hasretval):
    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()
    pipes = []
    retval = {}
    for pargs in partition(args, min(workers, len(args))):
        # Every worker gets its own pipe to send results on, so we don't have to
        # implement atomic writes larger than PIPE_BUF. Each forked process has
        # its own pipe's descriptors in the local variables, and the parent
        # process has the full list of pipe descriptors (and it doesn't really
        # care what order they're in).
        rfd, wfd = os.pipe()
        pipes.append((rfd, wfd))
        # 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():
                    for r, w in pipes[:-1]:
                        os.close(r)
                        os.close(w)
                    os.close(rfd)
                    for result in func(*(staticargs + (pargs,))):
                        os.write(wfd, util.pickle.dumps(result))
                    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)
    selector = selectors.DefaultSelector()
    for rfd, wfd in pipes:
        os.close(wfd)
        selector.register(os.fdopen(rfd, 'rb', 0), selectors.EVENT_READ)

    def cleanup():
        signal.signal(signal.SIGINT, oldhandler)
        waitforworkers()
        signal.signal(signal.SIGCHLD, oldchldhandler)
        selector.close()
        return problem[0]

    try:
        openpipes = len(pipes)
        while openpipes > 0:
            for key, events in selector.select():
                try:
                    res = util.pickle.load(_blockingreader(key.fileobj))
                    if hasretval and res[0]:
                        retval.update(res[1])
                    else:
                        yield res
                except EOFError:
                    selector.unregister(key.fileobj)
                    key.fileobj.close()
                    openpipes -= 1
                except IOError as e:
                    if e.errno == errno.EINTR:
                        continue
                    raise
    except:  # re-raises
        killworkers()
        cleanup()
        raise
    status = cleanup()
    if status:
        if status < 0:
            os.kill(os.getpid(), -status)
        raise error.WorkerError(status)
    if hasretval:
        yield True, retval


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, hasretval):
    class Worker(threading.Thread):
        def __init__(
            self, taskqueue, resultqueue, func, staticargs, *args, **kwargs
        ):
            threading.Thread.__init__(self, *args, **kwargs)
            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 pycompat.queue.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(
                    _(
                        b"failed to kill worker threads while "
                        b"handling an exception\n"
                    )
                )
                return

    workers = _numworkers(ui)
    resultqueue = pycompat.queue.Queue()
    taskqueue = pycompat.queue.Queue()
    retval = {}
    # 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():
                res = resultqueue.get()
                if hasretval and res[0]:
                    retval.update(res[1])
                else:
                    yield res
            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():
        res = resultqueue.get()
        if hasretval and res[0]:
            retval.update(res[1])
        else:
            yield res
    if hasretval:
        yield True, retval


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.

    olivia 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]