repoview: fix memory leak of filtered repo classes

The leak occurs in long-running server processes with
extensions, and is measured at 110kB per request.

Before this change, the contents of the `_filteredrepotypes`
cache are not properly garbage collected, despite it begin
a `WeakKeyDictionary`.

Extensions have a tendency to generate a new repository class
for each `localrepo` instantiation. Server processes based
on `hgwebdir_mod` will instantiate a new `localrepo` for each
HTTP request that involves a repository.

As a result, with a testing process that repeatedly opens a
repository with several extensions activated
(`topic` notably among them), we see a steady increase in
resident memory of 110kB per repository instantiation before this
change. This is also true, if we call `gc.collect()` at each
instantiation, like `hgwebdir_mod` does, or not.

The cause of the leak is that the *values* aren't weak references.
This change uses `weakref.ref` for the values, and this makes
in our measurements the resident size increase drop to 5kB per
repository instantiation, with no explicit call of `gc.collect()`
at all.
There is currently no reason to believe that this remaining leak
of 5kB is related to or even due to Mercurial core.

We've also seen evidence that `ui.ui` instances weren't properly
garbage collected before the change (with the change, they are).
This could explain why the figures are relatively high.

In theory, the collection of weak references could lead to
much more misses in the cache, so we measured the impact on
the original case that was motivation for introducing that cache
in 7e89bd0cfb86 (see also issue5043): `hg convert` of the
mozilla-central repository. The bad news here is that there is a
major memory leak there, both with and without the present changeset.
There were no more cache misses, and we could see no
more memory leak with this change: the resident size after importing
roughly 100000 changesets was at 12.4GB before, and 12.5GB after.
The small increase is mentioned for completeness only, and we
believe that it should be ignored, at least as long as the main
leak isn't fixed. At less than 1% of the main leak, even finding out
whether it is merely noise would be wasteful.

Original context where this was spotted and first mitigated:
  https://foss.heptapod.net/heptapod/heptapod/-/issues/466

The leak reduction was also obtained in Heptapod inner HTTP server,
which amounts to the same as `hgwebdir_mod` for these questions.

The measurements done with Python 3.9, similar figures seen with 3.8.
More work on our side would be needed to give measurements with 2.7,
because of testing server process does not support it.

mercurial/repoview.py

@@ -1,483 +1,486 b''
 # repoview.py - Filtered view of a localrepo object
 #
 # Copyright 2012 Pierre-Yves David <[e-mail]>
 #                Logilab SA        <[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 copy
 import weakref
 
 from .i18n import _
 from .node import (
     hex,
     nullrev,
 )
 from .pycompat import (
     delattr,
     getattr,
     setattr,
 )
 from . import (
     error,
     obsolete,
     phases,
     pycompat,
     tags as tagsmod,
     util,
 )
 from .utils import repoviewutil
 
 
 def hideablerevs(repo):
     """Revision candidates to be hidden
 
     This is a standalone function to allow extensions to wrap it.
 
     Because we use the set of immutable changesets as a fallback subset in
     branchmap (see mercurial.utils.repoviewutils.subsettable), you cannot set
     "public" changesets as "hideable". Doing so would break multiple code
     assertions and lead to crashes."""
     obsoletes = obsolete.getrevs(repo, b'obsolete')
     internals = repo._phasecache.getrevset(repo, phases.localhiddenphases)
     internals = frozenset(internals)
     return obsoletes | internals
 
 
 def pinnedrevs(repo):
     """revisions blocking hidden changesets from being filtered"""
 
     cl = repo.changelog
     pinned = set()
     pinned.update([par.rev() for par in repo[None].parents()])
     pinned.update([cl.rev(bm) for bm in repo._bookmarks.values()])
 
     tags = {}
     tagsmod.readlocaltags(repo.ui, repo, tags, {})
     if tags:
         rev = cl.index.get_rev
         pinned.update(rev(t[0]) for t in tags.values())
         pinned.discard(None)
 
     # Avoid cycle: mercurial.filemerge -> mercurial.templater ->
     # mercurial.templatefuncs -> mercurial.revset -> mercurial.repoview ->
     # mercurial.mergestate -> mercurial.filemerge
     from . import mergestate
 
     ms = mergestate.mergestate.read(repo)
     if ms.active() and ms.unresolvedcount():
         for node in (ms.local, ms.other):
             rev = cl.index.get_rev(node)
             if rev is not None:
                 pinned.add(rev)
 
     return pinned
 
 
 def _revealancestors(pfunc, hidden, revs):
     """reveals contiguous chains of hidden ancestors of 'revs' by removing them
     from 'hidden'
 
     - pfunc(r): a funtion returning parent of 'r',
     - hidden: the (preliminary) hidden revisions, to be updated
     - revs: iterable of revnum,
 
     (Ancestors are revealed exclusively, i.e. the elements in 'revs' are
     *not* revealed)
     """
     stack = list(revs)
     while stack:
         for p in pfunc(stack.pop()):
             if p != nullrev and p in hidden:
                 hidden.remove(p)
                 stack.append(p)
 
 
 def computehidden(repo, visibilityexceptions=None):
     """compute the set of hidden revision to filter
 
     During most operation hidden should be filtered."""
     assert not repo.changelog.filteredrevs
 
     hidden = hideablerevs(repo)
     if hidden:
         hidden = set(hidden - pinnedrevs(repo))
         if visibilityexceptions:
             hidden -= visibilityexceptions
         pfunc = repo.changelog.parentrevs
         mutable = repo._phasecache.getrevset(repo, phases.mutablephases)
 
         visible = mutable - hidden
         _revealancestors(pfunc, hidden, visible)
     return frozenset(hidden)
 
 
 def computesecret(repo, visibilityexceptions=None):
     """compute the set of revision that can never be exposed through hgweb
 
     Changeset in the secret phase (or above) should stay unaccessible."""
     assert not repo.changelog.filteredrevs
     secrets = repo._phasecache.getrevset(repo, phases.remotehiddenphases)
     return frozenset(secrets)
 
 
 def computeunserved(repo, visibilityexceptions=None):
     """compute the set of revision that should be filtered when used a server
 
     Secret and hidden changeset should not pretend to be here."""
     assert not repo.changelog.filteredrevs
     # fast path in simple case to avoid impact of non optimised code
     hiddens = filterrevs(repo, b'visible')
     secrets = filterrevs(repo, b'served.hidden')
     if secrets:
         return frozenset(hiddens | secrets)
     else:
         return hiddens
 
 
 def computemutable(repo, visibilityexceptions=None):
     assert not repo.changelog.filteredrevs
     # fast check to avoid revset call on huge repo
     if repo._phasecache.hasnonpublicphases(repo):
         return frozenset(repo._phasecache.getrevset(repo, phases.mutablephases))
     return frozenset()
 
 
 def computeimpactable(repo, visibilityexceptions=None):
     """Everything impactable by mutable revision
 
     The immutable filter still have some chance to get invalidated. This will
     happen when:
 
     - you garbage collect hidden changeset,
     - public phase is moved backward,
     - something is changed in the filtering (this could be fixed)
 
     This filter out any mutable changeset and any public changeset that may be
     impacted by something happening to a mutable revision.
 
     This is achieved by filtered everything with a revision number equal or
     higher than the first mutable changeset is filtered."""
     assert not repo.changelog.filteredrevs
     cl = repo.changelog
     firstmutable = len(cl)
     roots = repo._phasecache.nonpublicphaseroots(repo)
     if roots:
         firstmutable = min(firstmutable, min(cl.rev(r) for r in roots))
     # protect from nullrev root
     firstmutable = max(0, firstmutable)
     return frozenset(pycompat.xrange(firstmutable, len(cl)))
 
 
 # function to compute filtered set
 #
 # When adding a new filter you MUST update the table at:
 #     mercurial.utils.repoviewutil.subsettable
 # Otherwise your filter will have to recompute all its branches cache
 # from scratch (very slow).
 filtertable = {
     b'visible': computehidden,
     b'visible-hidden': computehidden,
     b'served.hidden': computesecret,
     b'served': computeunserved,
     b'immutable': computemutable,
     b'base': computeimpactable,
 }
 
 # set of filter level that will include the working copy parent no matter what.
 filter_has_wc = {b'visible', b'visible-hidden'}
 
 _basefiltername = list(filtertable)
 
 
 def extrafilter(ui):
     """initialize extra filter and return its id
 
     If extra filtering is configured, we make sure the associated filtered view
     are declared and return the associated id.
     """
     frevs = ui.config(b'experimental', b'extra-filter-revs')
     if frevs is None:
         return None
 
     fid = pycompat.sysbytes(util.DIGESTS[b'sha1'](frevs).hexdigest())[:12]
 
     combine = lambda fname: fname + b'%' + fid
 
     subsettable = repoviewutil.subsettable
 
     if combine(b'base') not in filtertable:
         for name in _basefiltername:
 
             def extrafilteredrevs(repo, *args, **kwargs):
                 baserevs = filtertable[name](repo, *args, **kwargs)
                 extrarevs = frozenset(repo.revs(frevs))
                 return baserevs | extrarevs
 
             filtertable[combine(name)] = extrafilteredrevs
             if name in subsettable:
                 subsettable[combine(name)] = combine(subsettable[name])
     return fid
 
 
 def filterrevs(repo, filtername, visibilityexceptions=None):
     """returns set of filtered revision for this filter name
 
     visibilityexceptions is a set of revs which must are exceptions for
     hidden-state and must be visible. They are dynamic and hence we should not
     cache it's result"""
     if filtername not in repo.filteredrevcache:
         if repo.ui.configbool(b'devel', b'debug.repo-filters'):
             msg = b'computing revision filter for "%s"'
             msg %= filtername
             if repo.ui.tracebackflag and repo.ui.debugflag:
                 # XXX use ui.write_err
                 util.debugstacktrace(
                     msg,
                     f=repo.ui._fout,
                     otherf=repo.ui._ferr,
                     prefix=b'debug.filters: ',
                 )
             else:
                 repo.ui.debug(b'debug.filters: %s\n' % msg)
         func = filtertable[filtername]
         if visibilityexceptions:
             return func(repo.unfiltered, visibilityexceptions)
         repo.filteredrevcache[filtername] = func(repo.unfiltered())
     return repo.filteredrevcache[filtername]
 
 
 def wrapchangelog(unfichangelog, filteredrevs):
     cl = copy.copy(unfichangelog)
     cl.filteredrevs = filteredrevs
 
     class filteredchangelog(filteredchangelogmixin, cl.__class__):
         pass
 
     cl.__class__ = filteredchangelog
 
     return cl
 
 
 class filteredchangelogmixin(object):
     def tiprev(self):
         """filtered version of revlog.tiprev"""
         for i in pycompat.xrange(len(self) - 1, -2, -1):
             if i not in self.filteredrevs:
                 return i
 
     def __contains__(self, rev):
         """filtered version of revlog.__contains__"""
         return 0 <= rev < len(self) and rev not in self.filteredrevs
 
     def __iter__(self):
         """filtered version of revlog.__iter__"""
 
         def filterediter():
             for i in pycompat.xrange(len(self)):
                 if i not in self.filteredrevs:
                     yield i
 
         return filterediter()
 
     def revs(self, start=0, stop=None):
         """filtered version of revlog.revs"""
         for i in super(filteredchangelogmixin, self).revs(start, stop):
             if i not in self.filteredrevs:
                 yield i
 
     def _checknofilteredinrevs(self, revs):
         """raise the appropriate error if 'revs' contains a filtered revision
 
         This returns a version of 'revs' to be used thereafter by the caller.
         In particular, if revs is an iterator, it is converted into a set.
         """
         safehasattr = util.safehasattr
         if safehasattr(revs, '__next__'):
             # Note that inspect.isgenerator() is not true for iterators,
             revs = set(revs)
 
         filteredrevs = self.filteredrevs
         if safehasattr(revs, 'first'):  # smartset
             offenders = revs & filteredrevs
         else:
             offenders = filteredrevs.intersection(revs)
 
         for rev in offenders:
             raise error.FilteredIndexError(rev)
         return revs
 
     def headrevs(self, revs=None):
         if revs is None:
             try:
                 return self.index.headrevsfiltered(self.filteredrevs)
             # AttributeError covers non-c-extension environments and
             # old c extensions without filter handling.
             except AttributeError:
                 return self._headrevs()
 
         revs = self._checknofilteredinrevs(revs)
         return super(filteredchangelogmixin, self).headrevs(revs)
 
     def strip(self, *args, **kwargs):
         # XXX make something better than assert
         # We can't expect proper strip behavior if we are filtered.
         assert not self.filteredrevs
         super(filteredchangelogmixin, self).strip(*args, **kwargs)
 
     def rev(self, node):
         """filtered version of revlog.rev"""
         r = super(filteredchangelogmixin, self).rev(node)
         if r in self.filteredrevs:
             raise error.FilteredLookupError(
                 hex(node), self.indexfile, _(b'filtered node')
             )
         return r
 
     def node(self, rev):
         """filtered version of revlog.node"""
         if rev in self.filteredrevs:
             raise error.FilteredIndexError(rev)
         return super(filteredchangelogmixin, self).node(rev)
 
     def linkrev(self, rev):
         """filtered version of revlog.linkrev"""
         if rev in self.filteredrevs:
             raise error.FilteredIndexError(rev)
         return super(filteredchangelogmixin, self).linkrev(rev)
 
     def parentrevs(self, rev):
         """filtered version of revlog.parentrevs"""
         if rev in self.filteredrevs:
             raise error.FilteredIndexError(rev)
         return super(filteredchangelogmixin, self).parentrevs(rev)
 
     def flags(self, rev):
         """filtered version of revlog.flags"""
         if rev in self.filteredrevs:
             raise error.FilteredIndexError(rev)
         return super(filteredchangelogmixin, self).flags(rev)
 
 
 class repoview(object):
     """Provide a read/write view of a repo through a filtered changelog
 
     This object is used to access a filtered version of a repository without
     altering the original repository object itself. We can not alter the
     original object for two main reasons:
     - It prevents the use of a repo with multiple filters at the same time. In
       particular when multiple threads are involved.
     - It makes scope of the filtering harder to control.
 
     This object behaves very closely to the original repository. All attribute
     operations are done on the original repository:
     - An access to `repoview.someattr` actually returns `repo.someattr`,
     - A write to `repoview.someattr` actually sets value of `repo.someattr`,
     - A deletion of `repoview.someattr` actually drops `someattr`
       from `repo.__dict__`.
 
     The only exception is the `changelog` property. It is overridden to return
     a (surface) copy of `repo.changelog` with some revisions filtered. The
     `filtername` attribute of the view control the revisions that need to be
     filtered.  (the fact the changelog is copied is an implementation detail).
 
     Unlike attributes, this object intercepts all method calls. This means that
     all methods are run on the `repoview` object with the filtered `changelog`
     property. For this purpose the simple `repoview` class must be mixed with
     the actual class of the repository. This ensures that the resulting
     `repoview` object have the very same methods than the repo object. This
     leads to the property below.
 
         repoview.method() --> repo.__class__.method(repoview)
 
     The inheritance has to be done dynamically because `repo` can be of any
     subclasses of `localrepo`. Eg: `bundlerepo` or `statichttprepo`.
     """
 
     def __init__(self, repo, filtername, visibilityexceptions=None):
         object.__setattr__(self, '_unfilteredrepo', repo)
         object.__setattr__(self, 'filtername', filtername)
         object.__setattr__(self, '_clcachekey', None)
         object.__setattr__(self, '_clcache', None)
         # revs which are exceptions and must not be hidden
         object.__setattr__(self, '_visibilityexceptions', visibilityexceptions)
 
     # not a propertycache on purpose we shall implement a proper cache later
     @property
     def changelog(self):
         """return a filtered version of the changeset
 
         this changelog must not be used for writing"""
         # some cache may be implemented later
         unfi = self._unfilteredrepo
         unfichangelog = unfi.changelog
         # bypass call to changelog.method
         unfiindex = unfichangelog.index
         unfilen = len(unfiindex)
         unfinode = unfiindex[unfilen - 1][7]
         with util.timedcm('repo filter for %s', self.filtername):
             revs = filterrevs(unfi, self.filtername, self._visibilityexceptions)
         cl = self._clcache
         newkey = (unfilen, unfinode, hash(revs), unfichangelog._delayed)
         # if cl.index is not unfiindex, unfi.changelog would be
         # recreated, and our clcache refers to garbage object
         if cl is not None and (
             cl.index is not unfiindex or newkey != self._clcachekey
         ):
             cl = None
         # could have been made None by the previous if
         if cl is None:
             # Only filter if there's something to filter
             cl = wrapchangelog(unfichangelog, revs) if revs else unfichangelog
             object.__setattr__(self, '_clcache', cl)
             object.__setattr__(self, '_clcachekey', newkey)
         return cl
 
     def unfiltered(self):
         """Return an unfiltered version of a repo"""
         return self._unfilteredrepo
 
     def filtered(self, name, visibilityexceptions=None):
         """Return a filtered version of a repository"""
         if name == self.filtername and not visibilityexceptions:
             return self
         return self.unfiltered().filtered(name, visibilityexceptions)
 
     def __repr__(self):
         return '<%s:%s %r>' % (
             self.__class__.__name__,
             pycompat.sysstr(self.filtername),
             self.unfiltered(),
         )
 
     # everything access are forwarded to the proxied repo
     def __getattr__(self, attr):
         return getattr(self._unfilteredrepo, attr)
 
     def __setattr__(self, attr, value):
         return setattr(self._unfilteredrepo, attr, value)
 
     def __delattr__(self, attr):
         return delattr(self._unfilteredrepo, attr)
 
 
 # Python <3.4 easily leaks types via __mro__. See
 # https://bugs.python.org/issue17950. We cache dynamically created types
 # so they won't be leaked on every invocation of repo.filtered().
 _filteredrepotypes = weakref.WeakKeyDictionary()
 
 
 def newtype(base):
     """Create a new type with the repoview mixin and the given base class"""
-    cls = _filteredrepotypes.get(base)
-    if cls is not None:
-        return cls
+    ref = _filteredrepotypes.get(base)
+    if ref is not None:
+        cls = ref()
+        if cls is not None:
+            return cls
 
     class filteredrepo(repoview, base):
         pass
 
-    _filteredrepotypes[base] = filteredrepo
+    _filteredrepotypes[base] = weakref.ref(filteredrepo)
+    # do not reread from weakref to be 100% sure not to return None
     return filteredrepo