fsmonitor: acquire localrepo.wlock prior to emitting hg.update state

we see some weird things in the watchman logs where the mercurial
process is seemingly confused about which hg.update state it is publishing
through watchman.

On closer examination, we're seeing conflicting pids for the clients involved
and this implies a race.

To resolve this, we extend the wlock around the state-enter/state-leave
events that are emitted to watchman.

Test Plan:
Some manual testing:

In one window, run this, and then checkout a different rev:

```
$ watchman -p -j <<<'["subscribe", "/data/users/wez/fbsource", "wez", {"expression": ["name", ".hg/updatestate"]}]'
{
    "version": "4.9.0",
    "subscribe": "wez",
    "clock": "c:1495034090:814028:1:312576"
}
{
    "state-enter": "hg.update",
    "version": "4.9.0",
    "clock": "c:1495034090:814028:1:312596",
    "unilateral": true,
    "subscription": "wez",
    "metadata": {
        "status": "ok",
        "distance": 125,
        "rev": "a1275d79ffa6c58b53116c8ec401c275ca6c1e2a",
        "partial": false
    },
    "root": "/data/users/wez/fbsource"
}
{
    "root": "/data/users/wez/fbsource",
    "metadata": {
        "status": "ok",
        "distance": 125,
        "rev": "a1275d79ffa6c58b53116c8ec401c275ca6c1e2a",
        "partial": false
    },
    "subscription": "wez",
    "unilateral": true,
    "version": "4.9.0",
    "clock": "c:1495034090:814028:1:312627",
    "state-leave": "hg.update"
}
```

Tailed the watchman log file and looked for invalid state assertion errors,
then ran my `rebase-all` script to update/rebase all of my heads.

Didn't trigger the error condition (but couldn't reliably trigger it previously
anyway), and the output captured above shows that the states are being emitted
correctly.

hgext/fsmonitor/__init__.py

@@ -1,726 +1,737 b''
 # __init__.py - fsmonitor initialization and overrides
 #
 # Copyright 2013-2016 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.
 
 '''Faster status operations with the Watchman file monitor (EXPERIMENTAL)
 
 Integrates the file-watching program Watchman with Mercurial to produce faster
 status results.
 
 On a particular Linux system, for a real-world repository with over 400,000
 files hosted on ext4, vanilla `hg status` takes 1.3 seconds. On the same
 system, with fsmonitor it takes about 0.3 seconds.
 
 fsmonitor requires no configuration -- it will tell Watchman about your
 repository as necessary. You'll need to install Watchman from
 https://facebook.github.io/watchman/ and make sure it is in your PATH.
 
 The following configuration options exist:
 
 ::
 
     [fsmonitor]
     mode = {off, on, paranoid}
 
 When `mode = off`, fsmonitor will disable itself (similar to not loading the
 extension at all). When `mode = on`, fsmonitor will be enabled (the default).
 When `mode = paranoid`, fsmonitor will query both Watchman and the filesystem,
 and ensure that the results are consistent.
 
 ::
 
     [fsmonitor]
     timeout = (float)
 
 A value, in seconds, that determines how long fsmonitor will wait for Watchman
 to return results. Defaults to `2.0`.
 
 ::
 
     [fsmonitor]
     blacklistusers = (list of userids)
 
 A list of usernames for which fsmonitor will disable itself altogether.
 
 ::
 
     [fsmonitor]
     walk_on_invalidate = (boolean)
 
 Whether or not to walk the whole repo ourselves when our cached state has been
 invalidated, for example when Watchman has been restarted or .hgignore rules
 have been changed. Walking the repo in that case can result in competing for
 I/O with Watchman. For large repos it is recommended to set this value to
 false. You may wish to set this to true if you have a very fast filesystem
 that can outpace the IPC overhead of getting the result data for the full repo
 from Watchman. Defaults to false.
 
 fsmonitor is incompatible with the largefiles and eol extensions, and
 will disable itself if any of those are active.
 
 '''
 
 # Platforms Supported
 # ===================
 #
 # **Linux:** *Stable*. Watchman and fsmonitor are both known to work reliably,
 #   even under severe loads.
 #
 # **Mac OS X:** *Stable*. The Mercurial test suite passes with fsmonitor
 #   turned on, on case-insensitive HFS+. There has been a reasonable amount of
 #   user testing under normal loads.
 #
 # **Solaris, BSD:** *Alpha*. watchman and fsmonitor are believed to work, but
 #   very little testing has been done.
 #
 # **Windows:** *Alpha*. Not in a release version of watchman or fsmonitor yet.
 #
 # Known Issues
 # ============
 #
 # * fsmonitor will disable itself if any of the following extensions are
 #   enabled: largefiles, inotify, eol; or if the repository has subrepos.
 # * fsmonitor will produce incorrect results if nested repos that are not
 #   subrepos exist. *Workaround*: add nested repo paths to your `.hgignore`.
 #
 # The issues related to nested repos and subrepos are probably not fundamental
 # ones. Patches to fix them are welcome.
 
 from __future__ import absolute_import
 
 import codecs
 import hashlib
 import os
 import stat
 import sys
 
 from mercurial.i18n import _
 from mercurial import (
     context,
     encoding,
     error,
     extensions,
     localrepo,
     merge,
     pathutil,
     pycompat,
     scmutil,
     util,
 )
 from mercurial import match as matchmod
 
 from . import (
     pywatchman,
     state,
     watchmanclient,
 )
 
 # Note for extension authors: ONLY specify testedwith = 'ships-with-hg-core' 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 = 'ships-with-hg-core'
 
 # This extension is incompatible with the following blacklisted extensions
 # and will disable itself when encountering one of these:
 _blacklist = ['largefiles', 'eol']
 
 def _handleunavailable(ui, state, ex):
     """Exception handler for Watchman interaction exceptions"""
     if isinstance(ex, watchmanclient.Unavailable):
         if ex.warn:
             ui.warn(str(ex) + '\n')
         if ex.invalidate:
             state.invalidate()
         ui.log('fsmonitor', 'Watchman unavailable: %s\n', ex.msg)
     else:
         ui.log('fsmonitor', 'Watchman exception: %s\n', ex)
 
 def _hashignore(ignore):
     """Calculate hash for ignore patterns and filenames
 
     If this information changes between Mercurial invocations, we can't
     rely on Watchman information anymore and have to re-scan the working
     copy.
 
     """
     sha1 = hashlib.sha1()
     if util.safehasattr(ignore, 'includepat'):
         sha1.update(ignore.includepat)
     sha1.update('\0\0')
     if util.safehasattr(ignore, 'excludepat'):
         sha1.update(ignore.excludepat)
     sha1.update('\0\0')
     if util.safehasattr(ignore, 'patternspat'):
         sha1.update(ignore.patternspat)
     sha1.update('\0\0')
     if util.safehasattr(ignore, '_files'):
         for f in ignore._files:
             sha1.update(f)
     sha1.update('\0')
     return sha1.hexdigest()
 
 _watchmanencoding = pywatchman.encoding.get_local_encoding()
 _fsencoding = sys.getfilesystemencoding() or sys.getdefaultencoding()
 _fixencoding = codecs.lookup(_watchmanencoding) != codecs.lookup(_fsencoding)
 
 def _watchmantofsencoding(path):
     """Fix path to match watchman and local filesystem encoding
 
     watchman's paths encoding can differ from filesystem encoding. For example,
     on Windows, it's always utf-8.
     """
     try:
         decoded = path.decode(_watchmanencoding)
     except UnicodeDecodeError as e:
         raise error.Abort(str(e), hint='watchman encoding error')
 
     try:
         encoded = decoded.encode(_fsencoding, 'strict')
     except UnicodeEncodeError as e:
         raise error.Abort(str(e))
 
     return encoded
 
 def overridewalk(orig, self, match, subrepos, unknown, ignored, full=True):
     '''Replacement for dirstate.walk, hooking into Watchman.
 
     Whenever full is False, ignored is False, and the Watchman client is
     available, use Watchman combined with saved state to possibly return only a
     subset of files.'''
     def bail():
         return orig(match, subrepos, unknown, ignored, full=True)
 
     if full or ignored or not self._watchmanclient.available():
         return bail()
     state = self._fsmonitorstate
     clock, ignorehash, notefiles = state.get()
     if not clock:
         if state.walk_on_invalidate:
             return bail()
         # Initial NULL clock value, see
         # https://facebook.github.io/watchman/docs/clockspec.html
         clock = 'c:0:0'
         notefiles = []
 
     def fwarn(f, msg):
         self._ui.warn('%s: %s\n' % (self.pathto(f), msg))
         return False
 
     def badtype(mode):
         kind = _('unknown')
         if stat.S_ISCHR(mode):
             kind = _('character device')
         elif stat.S_ISBLK(mode):
             kind = _('block device')
         elif stat.S_ISFIFO(mode):
             kind = _('fifo')
         elif stat.S_ISSOCK(mode):
             kind = _('socket')
         elif stat.S_ISDIR(mode):
             kind = _('directory')
         return _('unsupported file type (type is %s)') % kind
 
     ignore = self._ignore
     dirignore = self._dirignore
     if unknown:
         if _hashignore(ignore) != ignorehash and clock != 'c:0:0':
             # ignore list changed -- can't rely on Watchman state any more
             if state.walk_on_invalidate:
                 return bail()
             notefiles = []
             clock = 'c:0:0'
     else:
         # always ignore
         ignore = util.always
         dirignore = util.always
 
     matchfn = match.matchfn
     matchalways = match.always()
     dmap = self._map
     nonnormalset = getattr(self, '_nonnormalset', None)
 
     copymap = self._copymap
     getkind = stat.S_IFMT
     dirkind = stat.S_IFDIR
     regkind = stat.S_IFREG
     lnkkind = stat.S_IFLNK
     join = self._join
     normcase = util.normcase
     fresh_instance = False
 
     exact = skipstep3 = False
     if match.isexact():  # match.exact
         exact = True
         dirignore = util.always  # skip step 2
     elif match.prefix():  # match.match, no patterns
         skipstep3 = True
 
     if not exact and self._checkcase:
         # note that even though we could receive directory entries, we're only
         # interested in checking if a file with the same name exists. So only
         # normalize files if possible.
         normalize = self._normalizefile
         skipstep3 = False
     else:
         normalize = None
 
     # step 1: find all explicit files
     results, work, dirsnotfound = self._walkexplicit(match, subrepos)
 
     skipstep3 = skipstep3 and not (work or dirsnotfound)
     work = [d for d in work if not dirignore(d[0])]
 
     if not work and (exact or skipstep3):
         for s in subrepos:
             del results[s]
         del results['.hg']
         return results
 
     # step 2: query Watchman
     try:
         # Use the user-configured timeout for the query.
         # Add a little slack over the top of the user query to allow for
         # overheads while transferring the data
         self._watchmanclient.settimeout(state.timeout + 0.1)
         result = self._watchmanclient.command('query', {
             'fields': ['mode', 'mtime', 'size', 'exists', 'name'],
             'since': clock,
             'expression': [
                 'not', [
                     'anyof', ['dirname', '.hg'],
                     ['name', '.hg', 'wholename']
                 ]
             ],
             'sync_timeout': int(state.timeout * 1000),
             'empty_on_fresh_instance': state.walk_on_invalidate,
         })
     except Exception as ex:
         _handleunavailable(self._ui, state, ex)
         self._watchmanclient.clearconnection()
         return bail()
     else:
         # We need to propagate the last observed clock up so that we
         # can use it for our next query
         state.setlastclock(result['clock'])
         if result['is_fresh_instance']:
             if state.walk_on_invalidate:
                 state.invalidate()
                 return bail()
             fresh_instance = True
             # Ignore any prior noteable files from the state info
             notefiles = []
 
     # for file paths which require normalization and we encounter a case
     # collision, we store our own foldmap
     if normalize:
         foldmap = dict((normcase(k), k) for k in results)
 
     switch_slashes = pycompat.ossep == '\\'
     # The order of the results is, strictly speaking, undefined.
     # For case changes on a case insensitive filesystem we may receive
     # two entries, one with exists=True and another with exists=False.
     # The exists=True entries in the same response should be interpreted
     # as being happens-after the exists=False entries due to the way that
     # Watchman tracks files.  We use this property to reconcile deletes
     # for name case changes.
     for entry in result['files']:
         fname = entry['name']
         if _fixencoding:
             fname = _watchmantofsencoding(fname)
         if switch_slashes:
             fname = fname.replace('\\', '/')
         if normalize:
             normed = normcase(fname)
             fname = normalize(fname, True, True)
             foldmap[normed] = fname
         fmode = entry['mode']
         fexists = entry['exists']
         kind = getkind(fmode)
 
         if not fexists:
             # if marked as deleted and we don't already have a change
             # record, mark it as deleted.  If we already have an entry
             # for fname then it was either part of walkexplicit or was
             # an earlier result that was a case change
             if fname not in results and fname in dmap and (
                     matchalways or matchfn(fname)):
                 results[fname] = None
         elif kind == dirkind:
             if fname in dmap and (matchalways or matchfn(fname)):
                 results[fname] = None
         elif kind == regkind or kind == lnkkind:
             if fname in dmap:
                 if matchalways or matchfn(fname):
                     results[fname] = entry
             elif (matchalways or matchfn(fname)) and not ignore(fname):
                 results[fname] = entry
         elif fname in dmap and (matchalways or matchfn(fname)):
             results[fname] = None
 
     # step 3: query notable files we don't already know about
     # XXX try not to iterate over the entire dmap
     if normalize:
         # any notable files that have changed case will already be handled
         # above, so just check membership in the foldmap
         notefiles = set((normalize(f, True, True) for f in notefiles
                          if normcase(f) not in foldmap))
     visit = set((f for f in notefiles if (f not in results and matchfn(f)
                                           and (f in dmap or not ignore(f)))))
 
     if nonnormalset is not None and not fresh_instance:
         if matchalways:
             visit.update(f for f in nonnormalset if f not in results)
             visit.update(f for f in copymap if f not in results)
         else:
             visit.update(f for f in nonnormalset
                          if f not in results and matchfn(f))
             visit.update(f for f in copymap
                          if f not in results and matchfn(f))
     else:
         if matchalways:
             visit.update(f for f, st in dmap.iteritems()
                          if (f not in results and
                              (st[2] < 0 or st[0] != 'n' or fresh_instance)))
             visit.update(f for f in copymap if f not in results)
         else:
             visit.update(f for f, st in dmap.iteritems()
                          if (f not in results and
                              (st[2] < 0 or st[0] != 'n' or fresh_instance)
                              and matchfn(f)))
             visit.update(f for f in copymap
                          if f not in results and matchfn(f))
 
     audit = pathutil.pathauditor(self._root).check
     auditpass = [f for f in visit if audit(f)]
     auditpass.sort()
     auditfail = visit.difference(auditpass)
     for f in auditfail:
         results[f] = None
 
     nf = iter(auditpass).next
     for st in util.statfiles([join(f) for f in auditpass]):
         f = nf()
         if st or f in dmap:
             results[f] = st
 
     for s in subrepos:
         del results[s]
     del results['.hg']
     return results
 
 def overridestatus(
         orig, self, node1='.', node2=None, match=None, ignored=False,
         clean=False, unknown=False, listsubrepos=False):
     listignored = ignored
     listclean = clean
     listunknown = unknown
 
     def _cmpsets(l1, l2):
         try:
             if 'FSMONITOR_LOG_FILE' in encoding.environ:
                 fn = encoding.environ['FSMONITOR_LOG_FILE']
                 f = open(fn, 'wb')
             else:
                 fn = 'fsmonitorfail.log'
                 f = self.opener(fn, 'wb')
         except (IOError, OSError):
             self.ui.warn(_('warning: unable to write to %s\n') % fn)
             return
 
         try:
             for i, (s1, s2) in enumerate(zip(l1, l2)):
                 if set(s1) != set(s2):
                     f.write('sets at position %d are unequal\n' % i)
                     f.write('watchman returned: %s\n' % s1)
                     f.write('stat returned: %s\n' % s2)
         finally:
             f.close()
 
     if isinstance(node1, context.changectx):
         ctx1 = node1
     else:
         ctx1 = self[node1]
     if isinstance(node2, context.changectx):
         ctx2 = node2
     else:
         ctx2 = self[node2]
 
     working = ctx2.rev() is None
     parentworking = working and ctx1 == self['.']
     match = match or matchmod.always(self.root, self.getcwd())
 
     # Maybe we can use this opportunity to update Watchman's state.
     # Mercurial uses workingcommitctx and/or memctx to represent the part of
     # the workingctx that is to be committed. So don't update the state in
     # that case.
     # HG_PENDING is set in the environment when the dirstate is being updated
     # in the middle of a transaction; we must not update our state in that
     # case, or we risk forgetting about changes in the working copy.
     updatestate = (parentworking and match.always() and
                    not isinstance(ctx2, (context.workingcommitctx,
                                          context.memctx)) and
                    'HG_PENDING' not in encoding.environ)
 
     try:
         if self._fsmonitorstate.walk_on_invalidate:
             # Use a short timeout to query the current clock.  If that
             # takes too long then we assume that the service will be slow
             # to answer our query.
             # walk_on_invalidate indicates that we prefer to walk the
             # tree ourselves because we can ignore portions that Watchman
             # cannot and we tend to be faster in the warmer buffer cache
             # cases.
             self._watchmanclient.settimeout(0.1)
         else:
             # Give Watchman more time to potentially complete its walk
             # and return the initial clock.  In this mode we assume that
             # the filesystem will be slower than parsing a potentially
             # very large Watchman result set.
             self._watchmanclient.settimeout(
                 self._fsmonitorstate.timeout + 0.1)
         startclock = self._watchmanclient.getcurrentclock()
     except Exception as ex:
         self._watchmanclient.clearconnection()
         _handleunavailable(self.ui, self._fsmonitorstate, ex)
         # boo, Watchman failed. bail
         return orig(node1, node2, match, listignored, listclean,
                     listunknown, listsubrepos)
 
     if updatestate:
         # We need info about unknown files. This may make things slower the
         # first time, but whatever.
         stateunknown = True
     else:
         stateunknown = listunknown
 
     r = orig(node1, node2, match, listignored, listclean, stateunknown,
              listsubrepos)
     modified, added, removed, deleted, unknown, ignored, clean = r
 
     if updatestate:
         notefiles = modified + added + removed + deleted + unknown
         self._fsmonitorstate.set(
             self._fsmonitorstate.getlastclock() or startclock,
             _hashignore(self.dirstate._ignore),
             notefiles)
 
     if not listunknown:
         unknown = []
 
     # don't do paranoid checks if we're not going to query Watchman anyway
     full = listclean or match.traversedir is not None
     if self._fsmonitorstate.mode == 'paranoid' and not full:
         # run status again and fall back to the old walk this time
         self.dirstate._fsmonitordisable = True
 
         # shut the UI up
         quiet = self.ui.quiet
         self.ui.quiet = True
         fout, ferr = self.ui.fout, self.ui.ferr
         self.ui.fout = self.ui.ferr = open(os.devnull, 'wb')
 
         try:
             rv2 = orig(
                 node1, node2, match, listignored, listclean, listunknown,
                 listsubrepos)
         finally:
             self.dirstate._fsmonitordisable = False
             self.ui.quiet = quiet
             self.ui.fout, self.ui.ferr = fout, ferr
 
         # clean isn't tested since it's set to True above
         _cmpsets([modified, added, removed, deleted, unknown, ignored, clean],
                  rv2)
         modified, added, removed, deleted, unknown, ignored, clean = rv2
 
     return scmutil.status(
         modified, added, removed, deleted, unknown, ignored, clean)
 
 def makedirstate(cls):
     class fsmonitordirstate(cls):
         def _fsmonitorinit(self, fsmonitorstate, watchmanclient):
             # _fsmonitordisable is used in paranoid mode
             self._fsmonitordisable = False
             self._fsmonitorstate = fsmonitorstate
             self._watchmanclient = watchmanclient
 
         def walk(self, *args, **kwargs):
             orig = super(fsmonitordirstate, self).walk
             if self._fsmonitordisable:
                 return orig(*args, **kwargs)
             return overridewalk(orig, self, *args, **kwargs)
 
         def rebuild(self, *args, **kwargs):
             self._fsmonitorstate.invalidate()
             return super(fsmonitordirstate, self).rebuild(*args, **kwargs)
 
         def invalidate(self, *args, **kwargs):
             self._fsmonitorstate.invalidate()
             return super(fsmonitordirstate, self).invalidate(*args, **kwargs)
 
     return fsmonitordirstate
 
 def wrapdirstate(orig, self):
     ds = orig(self)
     # only override the dirstate when Watchman is available for the repo
     if util.safehasattr(self, '_fsmonitorstate'):
         ds.__class__ = makedirstate(ds.__class__)
         ds._fsmonitorinit(self._fsmonitorstate, self._watchmanclient)
     return ds
 
 def extsetup(ui):
     wrapfilecache(localrepo.localrepository, 'dirstate', wrapdirstate)
     if pycompat.sysplatform == 'darwin':
         # An assist for avoiding the dangling-symlink fsevents bug
         extensions.wrapfunction(os, 'symlink', wrapsymlink)
 
     extensions.wrapfunction(merge, 'update', wrapupdate)
 
 def wrapsymlink(orig, source, link_name):
     ''' if we create a dangling symlink, also touch the parent dir
     to encourage fsevents notifications to work more correctly '''
     try:
         return orig(source, link_name)
     finally:
         try:
             os.utime(os.path.dirname(link_name), None)
         except OSError:
             pass
 
 class state_update(object):
     ''' This context manager is responsible for dispatching the state-enter
         and state-leave signals to the watchman service '''
 
     def __init__(self, repo, node, distance, partial):
         self.repo = repo
         self.node = node
         self.distance = distance
         self.partial = partial
+        self._lock = None
 
     def __enter__(self):
+        # We explicitly need to take a lock here, before we proceed to update
+        # watchman about the update operation, so that we don't race with
+        # some other actor.  merge.update is going to take the wlock almost
+        # immediately anyway, so this is effectively extending the lock
+        # around a couple of short sanity checks.
+        self._lock = self.repo.wlock()
         self._state('state-enter')
         return self
 
     def __exit__(self, type_, value, tb):
-        status = 'ok' if type_ is None else 'failed'
-        self._state('state-leave', status=status)
+        try:
+            status = 'ok' if type_ is None else 'failed'
+            self._state('state-leave', status=status)
+        finally:
+            if self._lock:
+                self._lock.release()
 
     def _state(self, cmd, status='ok'):
         if not util.safehasattr(self.repo, '_watchmanclient'):
             return
         try:
             commithash = self.repo[self.node].hex()
             self.repo._watchmanclient.command(cmd, {
                 'name': 'hg.update',
                 'metadata': {
                     # the target revision
                     'rev': commithash,
                     # approximate number of commits between current and target
                     'distance': self.distance,
                     # success/failure (only really meaningful for state-leave)
                     'status': status,
                     # whether the working copy parent is changing
                     'partial': self.partial,
             }})
         except Exception as e:
             # Swallow any errors; fire and forget
             self.repo.ui.log(
                 'watchman', 'Exception %s while running %s\n', e, cmd)
 
 # Bracket working copy updates with calls to the watchman state-enter
 # and state-leave commands.  This allows clients to perform more intelligent
 # settling during bulk file change scenarios
 # https://facebook.github.io/watchman/docs/cmd/subscribe.html#advanced-settling
 def wrapupdate(orig, repo, node, branchmerge, force, ancestor=None,
                mergeancestor=False, labels=None, matcher=None, **kwargs):
 
     distance = 0
     partial = True
     if matcher is None or matcher.always():
         partial = False
         wc = repo[None]
         parents = wc.parents()
         if len(parents) == 2:
             anc = repo.changelog.ancestor(parents[0].node(), parents[1].node())
             ancrev = repo[anc].rev()
             distance = abs(repo[node].rev() - ancrev)
         elif len(parents) == 1:
             distance = abs(repo[node].rev() - parents[0].rev())
 
     with state_update(repo, node, distance, partial):
         return orig(
             repo, node, branchmerge, force, ancestor, mergeancestor,
             labels, matcher, *kwargs)
 
 def reposetup(ui, repo):
     # We don't work with largefiles or inotify
     exts = extensions.enabled()
     for ext in _blacklist:
         if ext in exts:
             ui.warn(_('The fsmonitor extension is incompatible with the %s '
                       'extension and has been disabled.\n') % ext)
             return
 
     if util.safehasattr(repo, 'dirstate'):
         # We don't work with subrepos either. Note that we can get passed in
         # e.g. a statichttprepo, which throws on trying to access the substate.
         # XXX This sucks.
         try:
             # if repo[None].substate can cause a dirstate parse, which is too
             # slow. Instead, look for a file called hgsubstate,
             if repo.wvfs.exists('.hgsubstate') or repo.wvfs.exists('.hgsub'):
                 return
         except AttributeError:
             return
 
         fsmonitorstate = state.state(repo)
         if fsmonitorstate.mode == 'off':
             return
 
         try:
             client = watchmanclient.client(repo)
         except Exception as ex:
             _handleunavailable(ui, fsmonitorstate, ex)
             return
 
         repo._fsmonitorstate = fsmonitorstate
         repo._watchmanclient = client
 
         # at this point since fsmonitorstate wasn't present, repo.dirstate is
         # not a fsmonitordirstate
         dirstate = repo.dirstate
         dirstate.__class__ = makedirstate(dirstate.__class__)
         dirstate._fsmonitorinit(fsmonitorstate, client)
         # invalidate property cache, but keep filecache which contains the
         # wrapped dirstate object
         del repo.unfiltered().__dict__['dirstate']
         assert dirstate is repo._filecache['dirstate'].obj
 
         class fsmonitorrepo(repo.__class__):
             def status(self, *args, **kwargs):
                 orig = super(fsmonitorrepo, self).status
                 return overridestatus(orig, self, *args, **kwargs)
 
         repo.__class__ = fsmonitorrepo
 
 def wrapfilecache(cls, propname, wrapper):
     """Wraps a filecache property. These can't be wrapped using the normal
     wrapfunction. This should eventually go into upstream Mercurial.
     """
     assert callable(wrapper)
     for currcls in cls.__mro__:
         if propname in currcls.__dict__:
             origfn = currcls.__dict__[propname].func
             assert callable(origfn)
             def wrap(*args, **kwargs):
                 return wrapper(origfn, *args, **kwargs)
             currcls.__dict__[propname].func = wrap
             break
 
     if currcls is object:
         raise AttributeError(
             _("type '%s' has no property '%s'") % (cls, propname))