branchcache: do not use `__getitem__` in updatecache

The `update_disk` method uses `updatecache` and the point of `update_disk` is to be able to do alternative processing to the one we do in `__getitem__`. So we calling `__getitem__` in `updatecache` defeat this purpose.

Instead we do the equivalent explicitly to preserve the spirit of `update_disk` (that we will actually put to use soon, I promise)

mercurial/branchmap.py

@@ -1,1072 +1,1073 b''
 # branchmap.py - logic to computes, maintain and stores branchmap for local repo
 #
 # Copyright 2005-2007 Olivia 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.
 
 
 import struct
 
 from .node import (
     bin,
     hex,
     nullrev,
 )
 
 from typing import (
     Any,
     Callable,
     Dict,
     Iterable,
     List,
     Optional,
     Set,
     TYPE_CHECKING,
     Tuple,
     Union,
 )
 
 from . import (
     encoding,
     error,
     obsolete,
     scmutil,
     util,
 )
 
 from .utils import (
     repoviewutil,
     stringutil,
 )
 
 if TYPE_CHECKING:
     from . import localrepo
 
     assert [localrepo]
 
 subsettable = repoviewutil.subsettable
 
 calcsize = struct.calcsize
 pack_into = struct.pack_into
 unpack_from = struct.unpack_from
 
 
 class BranchMapCache:
     """mapping of filtered views of repo with their branchcache"""
 
     def __init__(self):
         self._per_filter = {}
 
     def __getitem__(self, repo):
         self.updatecache(repo)
         bcache = self._per_filter[repo.filtername]
         assert bcache._filtername == repo.filtername, (
             bcache._filtername,
             repo.filtername,
         )
         return bcache
 
     def update_disk(self, repo):
         """ensure and up-to-date cache is (or will be) written on disk
 
         The cache for this repository view is updated  if needed and written on
         disk.
 
         If a transaction is in progress, the writing is schedule to transaction
         close. See the `BranchMapCache.write_dirty` method.
 
         This method exist independently of __getitem__ as it is sometime useful
         to signal that we have no intend to use the data in memory yet.
         """
         self.updatecache(repo)
         bcache = self._per_filter[repo.filtername]
         assert bcache._filtername == repo.filtername, (
             bcache._filtername,
             repo.filtername,
         )
         bcache.sync_disk(repo)
 
     def updatecache(self, repo):
         """Update the cache for the given filtered view on a repository"""
         # This can trigger updates for the caches for subsets of the filtered
         # view, e.g. when there is no cache for this filtered view or the cache
         # is stale.
 
         cl = repo.changelog
         filtername = repo.filtername
         bcache = self._per_filter.get(filtername)
         if bcache is None or not bcache.validfor(repo):
             # cache object missing or cache object stale? Read from disk
             bcache = branchcache.fromfile(repo)
 
         revs = []
         if bcache is None:
             # no (fresh) cache available anymore, perhaps we can re-use
             # the cache for a subset, then extend that to add info on missing
             # revisions.
             subsetname = subsettable.get(filtername)
             if subsetname is not None:
                 subset = repo.filtered(subsetname)
-                bcache = self[subset].inherit_for(repo)
+                self.updatecache(subset)
+                bcache = self._per_filter[subset.filtername].inherit_for(repo)
                 extrarevs = subset.changelog.filteredrevs - cl.filteredrevs
                 revs.extend(r for r in extrarevs if r <= bcache.tiprev)
             else:
                 # nothing to fall back on, start empty.
                 bcache = branchcache(repo)
 
         revs.extend(cl.revs(start=bcache.tiprev + 1))
         if revs:
             bcache.update(repo, revs)
 
         assert bcache.validfor(repo), filtername
         self._per_filter[repo.filtername] = bcache
 
     def replace(self, repo, remotebranchmap):
         """Replace the branchmap cache for a repo with a branch mapping.
 
         This is likely only called during clone with a branch map from a
         remote.
 
         """
         cl = repo.changelog
         clrev = cl.rev
         clbranchinfo = cl.branchinfo
         rbheads = []
         closed = set()
         for bheads in remotebranchmap.values():
             rbheads += bheads
             for h in bheads:
                 r = clrev(h)
                 b, c = clbranchinfo(r)
                 if c:
                     closed.add(h)
 
         if rbheads:
             rtiprev = max((int(clrev(node)) for node in rbheads))
             cache = branchcache(
                 repo,
                 remotebranchmap,
                 repo[rtiprev].node(),
                 rtiprev,
                 closednodes=closed,
             )
 
             # Try to stick it as low as possible
             # filter above served are unlikely to be fetch from a clone
             for candidate in (b'base', b'immutable', b'served'):
                 rview = repo.filtered(candidate)
                 if cache.validfor(rview):
                     cache._filtername = candidate
                     self._per_filter[candidate] = cache
                     cache._state = STATE_DIRTY
                     cache.write(rview)
                     return
 
     def clear(self):
         self._per_filter.clear()
 
     def write_dirty(self, repo):
         unfi = repo.unfiltered()
         for filtername in repoviewutil.get_ordered_subset():
             cache = self._per_filter.get(filtername)
             if cache is None:
                 continue
             if filtername is None:
                 repo = unfi
             else:
                 repo = unfi.filtered(filtername)
             cache.sync_disk(repo)
 
 
 def _unknownnode(node):
     """raises ValueError when branchcache found a node which does not exists"""
     raise ValueError('node %s does not exist' % node.hex())
 
 
 def _branchcachedesc(repo):
     if repo.filtername is not None:
         return b'branch cache (%s)' % repo.filtername
     else:
         return b'branch cache'
 
 
 class _BaseBranchCache:
     """A dict like object that hold branches heads cache.
 
     This cache is used to avoid costly computations to determine all the
     branch heads of a repo.
 
     The cache is serialized on disk in the following format:
 
     <tip hex node> <tip rev number> [optional filtered repo hex hash]
     <branch head hex node> <open/closed state> <branch name>
     <branch head hex node> <open/closed state> <branch name>
     ...
 
     The first line is used to check if the cache is still valid. If the
     branch cache is for a filtered repo view, an optional third hash is
     included that hashes the hashes of all filtered and obsolete revisions.
 
     The open/closed state is represented by a single letter 'o' or 'c'.
     This field can be used to avoid changelog reads when determining if a
     branch head closes a branch or not.
     """
 
     def __init__(
         self,
         repo: "localrepo.localrepository",
         entries: Union[
             Dict[bytes, List[bytes]], Iterable[Tuple[bytes, List[bytes]]]
         ] = (),
         closed_nodes: Optional[Set[bytes]] = None,
     ) -> None:
         """hasnode is a function which can be used to verify whether changelog
         has a given node or not. If it's not provided, we assume that every node
         we have exists in changelog"""
         # closednodes is a set of nodes that close their branch. If the branch
         # cache has been updated, it may contain nodes that are no longer
         # heads.
         if closed_nodes is None:
             closed_nodes = set()
         self._closednodes = set(closed_nodes)
         self._entries = dict(entries)
 
     def __iter__(self):
         return iter(self._entries)
 
     def __setitem__(self, key, value):
         self._entries[key] = value
 
     def __getitem__(self, key):
         return self._entries[key]
 
     def __contains__(self, key):
         return key in self._entries
 
     def iteritems(self):
         return self._entries.items()
 
     items = iteritems
 
     def hasbranch(self, label):
         """checks whether a branch of this name exists or not"""
         return label in self._entries
 
     def _branchtip(self, heads):
         """Return tuple with last open head in heads and false,
         otherwise return last closed head and true."""
         tip = heads[-1]
         closed = True
         for h in reversed(heads):
             if h not in self._closednodes:
                 tip = h
                 closed = False
                 break
         return tip, closed
 
     def branchtip(self, branch):
         """Return the tipmost open head on branch head, otherwise return the
         tipmost closed head on branch.
         Raise KeyError for unknown branch."""
         return self._branchtip(self[branch])[0]
 
     def iteropen(self, nodes):
         return (n for n in nodes if n not in self._closednodes)
 
     def branchheads(self, branch, closed=False):
         heads = self._entries[branch]
         if not closed:
             heads = list(self.iteropen(heads))
         return heads
 
     def iterbranches(self):
         for bn, heads in self.items():
             yield (bn, heads) + self._branchtip(heads)
 
     def iterheads(self):
         """returns all the heads"""
         return self._entries.values()
 
     def update(self, repo, revgen):
         """Given a branchhead cache, self, that may have extra nodes or be
         missing heads, and a generator of nodes that are strictly a superset of
         heads missing, this function updates self to be correct.
         """
         starttime = util.timer()
         cl = repo.changelog
         # collect new branch entries
         newbranches = {}
         getbranchinfo = repo.revbranchcache().branchinfo
         max_rev = -1
         for r in revgen:
             branch, closesbranch = getbranchinfo(r)
             newbranches.setdefault(branch, []).append(r)
             if closesbranch:
                 self._closednodes.add(cl.node(r))
             max_rev = max(max_rev, r)
         if max_rev < 0:
             msg = "running branchcache.update without revision to update"
             raise error.ProgrammingError(msg)
 
         # Delay fetching the topological heads until they are needed.
         # A repository without non-continous branches can skip this part.
         topoheads = None
 
         # If a changeset is visible, its parents must be visible too, so
         # use the faster unfiltered parent accessor.
         parentrevs = repo.unfiltered().changelog.parentrevs
 
         # Faster than using ctx.obsolete()
         obsrevs = obsolete.getrevs(repo, b'obsolete')
 
         for branch, newheadrevs in newbranches.items():
             # For every branch, compute the new branchheads.
             # A branchhead is a revision such that no descendant is on
             # the same branch.
             #
             # The branchheads are computed iteratively in revision order.
             # This ensures topological order, i.e. parents are processed
             # before their children. Ancestors are inclusive here, i.e.
             # any revision is an ancestor of itself.
             #
             # Core observations:
             # - The current revision is always a branchhead for the
             #   repository up to that point.
             # - It is the first revision of the branch if and only if
             #   there was no branchhead before. In that case, it is the
             #   only branchhead as there are no possible ancestors on
             #   the same branch.
             # - If a parent is on the same branch, a branchhead can
             #   only be an ancestor of that parent, if it is parent
             #   itself. Otherwise it would have been removed as ancestor
             #   of that parent before.
             # - Therefore, if all parents are on the same branch, they
             #   can just be removed from the branchhead set.
             # - If one parent is on the same branch and the other is not
             #   and there was exactly one branchhead known, the existing
             #   branchhead can only be an ancestor if it is the parent.
             #   Otherwise it would have been removed as ancestor of
             #   the parent before. The other parent therefore can't have
             #   a branchhead as ancestor.
             # - In all other cases, the parents on different branches
             #   could have a branchhead as ancestor. Those parents are
             #   kept in the "uncertain" set. If all branchheads are also
             #   topological heads, they can't have descendants and further
             #   checks can be skipped. Otherwise, the ancestors of the
             #   "uncertain" set are removed from branchheads.
             #   This computation is heavy and avoided if at all possible.
             bheads = self._entries.get(branch, [])
             bheadset = {cl.rev(node) for node in bheads}
             uncertain = set()
             for newrev in sorted(newheadrevs):
                 if newrev in obsrevs:
                     # We ignore obsolete changesets as they shouldn't be
                     # considered heads.
                     continue
 
                 if not bheadset:
                     bheadset.add(newrev)
                     continue
 
                 parents = [p for p in parentrevs(newrev) if p != nullrev]
                 samebranch = set()
                 otherbranch = set()
                 obsparents = set()
                 for p in parents:
                     if p in obsrevs:
                         # We ignored this obsolete changeset earlier, but now
                         # that it has non-ignored children, we need to make
                         # sure their ancestors are not considered heads. To
                         # achieve that, we will simply treat this obsolete
                         # changeset as a parent from other branch.
                         obsparents.add(p)
                     elif p in bheadset or getbranchinfo(p)[0] == branch:
                         samebranch.add(p)
                     else:
                         otherbranch.add(p)
                 if not (len(bheadset) == len(samebranch) == 1):
                     uncertain.update(otherbranch)
                     uncertain.update(obsparents)
                 bheadset.difference_update(samebranch)
                 bheadset.add(newrev)
 
             if uncertain:
                 if topoheads is None:
                     topoheads = set(cl.headrevs())
                 if bheadset - topoheads:
                     floorrev = min(bheadset)
                     if floorrev <= max(uncertain):
                         ancestors = set(cl.ancestors(uncertain, floorrev))
                         bheadset -= ancestors
             if bheadset:
                 self[branch] = [cl.node(rev) for rev in sorted(bheadset)]
 
         duration = util.timer() - starttime
         repo.ui.log(
             b'branchcache',
             b'updated %s in %.4f seconds\n',
             _branchcachedesc(repo),
             duration,
         )
         return max_rev
 
 
 STATE_CLEAN = 1
 STATE_INHERITED = 2
 STATE_DIRTY = 3
 
 
 class branchcache(_BaseBranchCache):
     """Branchmap info for a local repo or repoview"""
 
     _base_filename = b"branch2"
 
     def __init__(
         self,
         repo: "localrepo.localrepository",
         entries: Union[
             Dict[bytes, List[bytes]], Iterable[Tuple[bytes, List[bytes]]]
         ] = (),
         tipnode: Optional[bytes] = None,
         tiprev: Optional[int] = nullrev,
         filteredhash: Optional[bytes] = None,
         closednodes: Optional[Set[bytes]] = None,
         hasnode: Optional[Callable[[bytes], bool]] = None,
         verify_node: bool = False,
         inherited: bool = False,
     ) -> None:
         """hasnode is a function which can be used to verify whether changelog
         has a given node or not. If it's not provided, we assume that every node
         we have exists in changelog"""
         self._filtername = repo.filtername
         if tipnode is None:
             self.tipnode = repo.nullid
         else:
             self.tipnode = tipnode
         self.tiprev = tiprev
         self.filteredhash = filteredhash
         self._state = STATE_CLEAN
         if inherited:
             self._state = STATE_INHERITED
 
         super().__init__(repo=repo, entries=entries, closed_nodes=closednodes)
         # closednodes is a set of nodes that close their branch. If the branch
         # cache has been updated, it may contain nodes that are no longer
         # heads.
 
         # Do we need to verify branch at all ?
         self._verify_node = verify_node
         # branches for which nodes are verified
         self._verifiedbranches = set()
         self._hasnode = None
         if self._verify_node:
             self._hasnode = repo.changelog.hasnode
 
     def validfor(self, repo):
         """check that cache contents are valid for (a subset of) this repo
 
         - False when the order of changesets changed or if we detect a strip.
         - True when cache is up-to-date for the current repo or its subset."""
         try:
             node = repo.changelog.node(self.tiprev)
         except IndexError:
             # changesets were stripped and now we don't even have enough to
             # find tiprev
             return False
         if self.tipnode != node:
             # tiprev doesn't correspond to tipnode: repo was stripped, or this
             # repo has a different order of changesets
             return False
         tiphash = scmutil.filteredhash(repo, self.tiprev, needobsolete=True)
         # hashes don't match if this repo view has a different set of filtered
         # revisions (e.g. due to phase changes) or obsolete revisions (e.g.
         # history was rewritten)
         return self.filteredhash == tiphash
 
     @classmethod
     def fromfile(cls, repo):
         f = None
         try:
             f = repo.cachevfs(cls._filename(repo))
             lineiter = iter(f)
             init_kwargs = cls._load_header(repo, lineiter)
             bcache = cls(
                 repo,
                 verify_node=True,
                 **init_kwargs,
             )
             if not bcache.validfor(repo):
                 # invalidate the cache
                 raise ValueError('tip differs')
             bcache._load_heads(repo, lineiter)
         except (IOError, OSError):
             return None
 
         except Exception as inst:
             if repo.ui.debugflag:
                 msg = b'invalid %s: %s\n'
                 msg %= (
                     _branchcachedesc(repo),
                     stringutil.forcebytestr(inst),
                 )
                 repo.ui.debug(msg)
             bcache = None
 
         finally:
             if f:
                 f.close()
 
         return bcache
 
     @classmethod
     def _load_header(cls, repo, lineiter) -> "dict[str, Any]":
         """parse the head of a branchmap file
 
         return parameters to pass to a newly created class instance.
         """
         cachekey = next(lineiter).rstrip(b'\n').split(b" ", 2)
         last, lrev = cachekey[:2]
         last, lrev = bin(last), int(lrev)
         filteredhash = None
         if len(cachekey) > 2:
             filteredhash = bin(cachekey[2])
         return {
             "tipnode": last,
             "tiprev": lrev,
             "filteredhash": filteredhash,
         }
 
     def _load_heads(self, repo, lineiter):
         """fully loads the branchcache by reading from the file using the line
         iterator passed"""
         for line in lineiter:
             line = line.rstrip(b'\n')
             if not line:
                 continue
             node, state, label = line.split(b" ", 2)
             if state not in b'oc':
                 raise ValueError('invalid branch state')
             label = encoding.tolocal(label.strip())
             node = bin(node)
             self._entries.setdefault(label, []).append(node)
             if state == b'c':
                 self._closednodes.add(node)
 
     @classmethod
     def _filename(cls, repo):
         """name of a branchcache file for a given repo or repoview"""
         filename = cls._base_filename
         if repo.filtername:
             filename = b'%s-%s' % (filename, repo.filtername)
         return filename
 
     def inherit_for(self, repo):
         """return a deep copy of the branchcache object"""
         assert repo.filtername != self._filtername
         other = type(self)(
             repo=repo,
             # we always do a shally copy of self._entries, and the values is
             # always replaced, so no need to deepcopy until the above remains
             # true.
             entries=self._entries,
             tipnode=self.tipnode,
             tiprev=self.tiprev,
             filteredhash=self.filteredhash,
             closednodes=set(self._closednodes),
             verify_node=self._verify_node,
             inherited=True,
         )
         # also copy information about the current verification state
         other._verifiedbranches = set(self._verifiedbranches)
         return other
 
     def sync_disk(self, repo):
         """synchronise the on disk file with the cache state
 
         If new value specific to this filter level need to be written, the file
         will be updated, if the state of the branchcache is inherited from a
         subset, any stalled on disk file will be deleted.
 
         That method does nothing if there is nothing to do.
         """
         if self._state == STATE_DIRTY:
             self.write(repo)
         elif self._state == STATE_INHERITED:
             filename = self._filename(repo)
             repo.cachevfs.tryunlink(filename)
 
     def write(self, repo):
         assert self._filtername == repo.filtername, (
             self._filtername,
             repo.filtername,
         )
         assert self._state == STATE_DIRTY, self._state
         tr = repo.currenttransaction()
         if not getattr(tr, 'finalized', True):
             # Avoid premature writing.
             #
             # (The cache warming setup by localrepo will update the file later.)
             return
         try:
             filename = self._filename(repo)
             with repo.cachevfs(filename, b"w", atomictemp=True) as f:
                 self._write_header(f)
                 nodecount = self._write_heads(f)
             repo.ui.log(
                 b'branchcache',
                 b'wrote %s with %d labels and %d nodes\n',
                 _branchcachedesc(repo),
                 len(self._entries),
                 nodecount,
             )
             self._state = STATE_CLEAN
         except (IOError, OSError, error.Abort) as inst:
             # Abort may be raised by read only opener, so log and continue
             repo.ui.debug(
                 b"couldn't write branch cache: %s\n"
                 % stringutil.forcebytestr(inst)
             )
 
     def _write_header(self, fp) -> None:
         """write the branch cache header to a file"""
         cachekey = [hex(self.tipnode), b'%d' % self.tiprev]
         if self.filteredhash is not None:
             cachekey.append(hex(self.filteredhash))
         fp.write(b" ".join(cachekey) + b'\n')
 
     def _write_heads(self, fp) -> int:
         """write list of heads to a file
 
         Return the number of heads written."""
         nodecount = 0
         for label, nodes in sorted(self._entries.items()):
             label = encoding.fromlocal(label)
             for node in nodes:
                 nodecount += 1
                 if node in self._closednodes:
                     state = b'c'
                 else:
                     state = b'o'
                 fp.write(b"%s %s %s\n" % (hex(node), state, label))
         return nodecount
 
     def _verifybranch(self, branch):
         """verify head nodes for the given branch."""
         if not self._verify_node:
             return
         if branch not in self._entries or branch in self._verifiedbranches:
             return
         assert self._hasnode is not None
         for n in self._entries[branch]:
             if not self._hasnode(n):
                 _unknownnode(n)
 
         self._verifiedbranches.add(branch)
 
     def _verifyall(self):
         """verifies nodes of all the branches"""
         for b in self._entries.keys():
             if b not in self._verifiedbranches:
                 self._verifybranch(b)
 
     def __getitem__(self, key):
         self._verifybranch(key)
         return super().__getitem__(key)
 
     def __contains__(self, key):
         self._verifybranch(key)
         return super().__contains__(key)
 
     def iteritems(self):
         self._verifyall()
         return super().iteritems()
 
     items = iteritems
 
     def iterheads(self):
         """returns all the heads"""
         self._verifyall()
         return super().iterheads()
 
     def hasbranch(self, label):
         """checks whether a branch of this name exists or not"""
         self._verifybranch(label)
         return super().hasbranch(label)
 
     def branchheads(self, branch, closed=False):
         self._verifybranch(branch)
         return super().branchheads(branch, closed=closed)
 
     def update(self, repo, revgen):
         assert self._filtername == repo.filtername, (
             self._filtername,
             repo.filtername,
         )
         cl = repo.changelog
         max_rev = super().update(repo, revgen)
         # new tip revision which we found after iterating items from new
         # branches
         if max_rev is not None and max_rev > self.tiprev:
             self.tiprev = max_rev
             self.tipnode = cl.node(max_rev)
 
         if not self.validfor(repo):
             # old cache key is now invalid for the repo, but we've just updated
             # the cache and we assume it's valid, so let's make the cache key
             # valid as well by recomputing it from the cached data
             self.tipnode = repo.nullid
             self.tiprev = nullrev
             for heads in self.iterheads():
                 if not heads:
                     # all revisions on a branch are obsolete
                     continue
                 # note: tiprev is not necessarily the tip revision of repo,
                 # because the tip could be obsolete (i.e. not a head)
                 tiprev = max(cl.rev(node) for node in heads)
                 if tiprev > self.tiprev:
                     self.tipnode = cl.node(tiprev)
                     self.tiprev = tiprev
         self.filteredhash = scmutil.filteredhash(
             repo, self.tiprev, needobsolete=True
         )
         self._state = STATE_DIRTY
         self.write(repo)
 
 
 class remotebranchcache(_BaseBranchCache):
     """Branchmap info for a remote connection, should not write locally"""
 
     def __init__(
         self,
         repo: "localrepo.localrepository",
         entries: Union[
             Dict[bytes, List[bytes]], Iterable[Tuple[bytes, List[bytes]]]
         ] = (),
         closednodes: Optional[Set[bytes]] = None,
     ) -> None:
         super().__init__(repo=repo, entries=entries, closed_nodes=closednodes)
 
 
 # Revision branch info cache
 
 _rbcversion = b'-v1'
 _rbcnames = b'rbc-names' + _rbcversion
 _rbcrevs = b'rbc-revs' + _rbcversion
 # [4 byte hash prefix][4 byte branch name number with sign bit indicating open]
 _rbcrecfmt = b'>4sI'
 _rbcrecsize = calcsize(_rbcrecfmt)
 _rbcmininc = 64 * _rbcrecsize
 _rbcnodelen = 4
 _rbcbranchidxmask = 0x7FFFFFFF
 _rbccloseflag = 0x80000000
 
 
 class rbcrevs:
     """a byte string consisting of an immutable prefix followed by a mutable suffix"""
 
     def __init__(self, revs):
         self._prefix = revs
         self._rest = bytearray()
 
     def __len__(self):
         return len(self._prefix) + len(self._rest)
 
     def unpack_record(self, rbcrevidx):
         if rbcrevidx < len(self._prefix):
             return unpack_from(_rbcrecfmt, util.buffer(self._prefix), rbcrevidx)
         else:
             return unpack_from(
                 _rbcrecfmt,
                 util.buffer(self._rest),
                 rbcrevidx - len(self._prefix),
             )
 
     def make_mutable(self):
         if len(self._prefix) > 0:
             entirety = bytearray()
             entirety[:] = self._prefix
             entirety.extend(self._rest)
             self._rest = entirety
             self._prefix = bytearray()
 
     def truncate(self, pos):
         self.make_mutable()
         del self._rest[pos:]
 
     def pack_into(self, rbcrevidx, node, branchidx):
         if rbcrevidx < len(self._prefix):
             self.make_mutable()
         buf = self._rest
         start_offset = rbcrevidx - len(self._prefix)
         end_offset = start_offset + _rbcrecsize
 
         if len(self._rest) < end_offset:
             # bytearray doesn't allocate extra space at least in Python 3.7.
             # When multiple changesets are added in a row, precise resize would
             # result in quadratic complexity. Overallocate to compensate by
             # using the classic doubling technique for dynamic arrays instead.
             # If there was a gap in the map before, less space will be reserved.
             self._rest.extend(b'\0' * end_offset)
         return pack_into(
             _rbcrecfmt,
             buf,
             start_offset,
             node,
             branchidx,
         )
 
     def extend(self, extension):
         return self._rest.extend(extension)
 
     def slice(self, begin, end):
         if begin < len(self._prefix):
             acc = bytearray()
             acc[:] = self._prefix[begin:end]
             acc.extend(
                 self._rest[begin - len(self._prefix) : end - len(self._prefix)]
             )
             return acc
         return self._rest[begin - len(self._prefix) : end - len(self._prefix)]
 
 
 class revbranchcache:
     """Persistent cache, mapping from revision number to branch name and close.
     This is a low level cache, independent of filtering.
 
     Branch names are stored in rbc-names in internal encoding separated by 0.
     rbc-names is append-only, and each branch name is only stored once and will
     thus have a unique index.
 
     The branch info for each revision is stored in rbc-revs as constant size
     records. The whole file is read into memory, but it is only 'parsed' on
     demand. The file is usually append-only but will be truncated if repo
     modification is detected.
     The record for each revision contains the first 4 bytes of the
     corresponding node hash, and the record is only used if it still matches.
     Even a completely trashed rbc-revs fill thus still give the right result
     while converging towards full recovery ... assuming no incorrectly matching
     node hashes.
     The record also contains 4 bytes where 31 bits contains the index of the
     branch and the last bit indicate that it is a branch close commit.
     The usage pattern for rbc-revs is thus somewhat similar to 00changelog.i
     and will grow with it but be 1/8th of its size.
     """
 
     def __init__(self, repo, readonly=True):
         assert repo.filtername is None
         self._repo = repo
         self._names = []  # branch names in local encoding with static index
         self._rbcrevs = rbcrevs(bytearray())
         self._rbcsnameslen = 0  # length of names read at _rbcsnameslen
         try:
             bndata = repo.cachevfs.read(_rbcnames)
             self._rbcsnameslen = len(bndata)  # for verification before writing
             if bndata:
                 self._names = [
                     encoding.tolocal(bn) for bn in bndata.split(b'\0')
                 ]
         except (IOError, OSError):
             if readonly:
                 # don't try to use cache - fall back to the slow path
                 self.branchinfo = self._branchinfo
 
         if self._names:
             try:
                 if repo.ui.configbool(b'format', b'mmap-revbranchcache'):
                     with repo.cachevfs(_rbcrevs) as fp:
                         data = util.buffer(util.mmapread(fp))
                 else:
                     data = repo.cachevfs.read(_rbcrevs)
                 self._rbcrevs = rbcrevs(data)
             except (IOError, OSError) as inst:
                 repo.ui.debug(
                     b"couldn't read revision branch cache: %s\n"
                     % stringutil.forcebytestr(inst)
                 )
         # remember number of good records on disk
         self._rbcrevslen = min(
             len(self._rbcrevs) // _rbcrecsize, len(repo.changelog)
         )
         if self._rbcrevslen == 0:
             self._names = []
         self._rbcnamescount = len(self._names)  # number of names read at
         # _rbcsnameslen
 
     def _clear(self):
         self._rbcsnameslen = 0
         del self._names[:]
         self._rbcnamescount = 0
         self._rbcrevslen = len(self._repo.changelog)
         self._rbcrevs = rbcrevs(bytearray(self._rbcrevslen * _rbcrecsize))
         util.clearcachedproperty(self, b'_namesreverse')
 
     @util.propertycache
     def _namesreverse(self):
         return {b: r for r, b in enumerate(self._names)}
 
     def branchinfo(self, rev):
         """Return branch name and close flag for rev, using and updating
         persistent cache."""
         changelog = self._repo.changelog
         rbcrevidx = rev * _rbcrecsize
 
         # avoid negative index, changelog.read(nullrev) is fast without cache
         if rev == nullrev:
             return changelog.branchinfo(rev)
 
         # if requested rev isn't allocated, grow and cache the rev info
         if len(self._rbcrevs) < rbcrevidx + _rbcrecsize:
             return self._branchinfo(rev)
 
         # fast path: extract data from cache, use it if node is matching
         reponode = changelog.node(rev)[:_rbcnodelen]
         cachenode, branchidx = self._rbcrevs.unpack_record(rbcrevidx)
         close = bool(branchidx & _rbccloseflag)
         if close:
             branchidx &= _rbcbranchidxmask
         if cachenode == b'\0\0\0\0':
             pass
         elif cachenode == reponode:
             try:
                 return self._names[branchidx], close
             except IndexError:
                 # recover from invalid reference to unknown branch
                 self._repo.ui.debug(
                     b"referenced branch names not found"
                     b" - rebuilding revision branch cache from scratch\n"
                 )
                 self._clear()
         else:
             # rev/node map has changed, invalidate the cache from here up
             self._repo.ui.debug(
                 b"history modification detected - truncating "
                 b"revision branch cache to revision %d\n" % rev
             )
             truncate = rbcrevidx + _rbcrecsize
             self._rbcrevs.truncate(truncate)
             self._rbcrevslen = min(self._rbcrevslen, truncate)
 
         # fall back to slow path and make sure it will be written to disk
         return self._branchinfo(rev)
 
     def _branchinfo(self, rev):
         """Retrieve branch info from changelog and update _rbcrevs"""
         changelog = self._repo.changelog
         b, close = changelog.branchinfo(rev)
         if b in self._namesreverse:
             branchidx = self._namesreverse[b]
         else:
             branchidx = len(self._names)
             self._names.append(b)
             self._namesreverse[b] = branchidx
         reponode = changelog.node(rev)
         if close:
             branchidx |= _rbccloseflag
         self._setcachedata(rev, reponode, branchidx)
         return b, close
 
     def setdata(self, rev, changelogrevision):
         """add new data information to the cache"""
         branch, close = changelogrevision.branchinfo
 
         if branch in self._namesreverse:
             branchidx = self._namesreverse[branch]
         else:
             branchidx = len(self._names)
             self._names.append(branch)
             self._namesreverse[branch] = branchidx
         if close:
             branchidx |= _rbccloseflag
         self._setcachedata(rev, self._repo.changelog.node(rev), branchidx)
         # If no cache data were readable (non exists, bad permission, etc)
         # the cache was bypassing itself by setting:
         #
         #   self.branchinfo = self._branchinfo
         #
         # Since we now have data in the cache, we need to drop this bypassing.
         if 'branchinfo' in vars(self):
             del self.branchinfo
 
     def _setcachedata(self, rev, node, branchidx):
         """Writes the node's branch data to the in-memory cache data."""
         if rev == nullrev:
             return
         rbcrevidx = rev * _rbcrecsize
         self._rbcrevs.pack_into(rbcrevidx, node, branchidx)
         self._rbcrevslen = min(self._rbcrevslen, rev)
 
         tr = self._repo.currenttransaction()
         if tr:
             tr.addfinalize(b'write-revbranchcache', self.write)
 
     def write(self, tr=None):
         """Save branch cache if it is dirty."""
         repo = self._repo
         wlock = None
         step = b''
         try:
             # write the new names
             if self._rbcnamescount < len(self._names):
                 wlock = repo.wlock(wait=False)
                 step = b' names'
                 self._writenames(repo)
 
             # write the new revs
             start = self._rbcrevslen * _rbcrecsize
             if start != len(self._rbcrevs):
                 step = b''
                 if wlock is None:
                     wlock = repo.wlock(wait=False)
                 self._writerevs(repo, start)
 
         except (IOError, OSError, error.Abort, error.LockError) as inst:
             repo.ui.debug(
                 b"couldn't write revision branch cache%s: %s\n"
                 % (step, stringutil.forcebytestr(inst))
             )
         finally:
             if wlock is not None:
                 wlock.release()
 
     def _writenames(self, repo):
         """write the new branch names to revbranchcache"""
         if self._rbcnamescount != 0:
             f = repo.cachevfs.open(_rbcnames, b'ab')
             if f.tell() == self._rbcsnameslen:
                 f.write(b'\0')
             else:
                 f.close()
                 repo.ui.debug(b"%s changed - rewriting it\n" % _rbcnames)
                 self._rbcnamescount = 0
                 self._rbcrevslen = 0
         if self._rbcnamescount == 0:
             # before rewriting names, make sure references are removed
             repo.cachevfs.unlinkpath(_rbcrevs, ignoremissing=True)
             f = repo.cachevfs.open(_rbcnames, b'wb')
         f.write(
             b'\0'.join(
                 encoding.fromlocal(b)
                 for b in self._names[self._rbcnamescount :]
             )
         )
         self._rbcsnameslen = f.tell()
         f.close()
         self._rbcnamescount = len(self._names)
 
     def _writerevs(self, repo, start):
         """write the new revs to revbranchcache"""
         revs = min(len(repo.changelog), len(self._rbcrevs) // _rbcrecsize)
         with repo.cachevfs.open(_rbcrevs, b'ab') as f:
             if f.tell() != start:
                 repo.ui.debug(
                     b"truncating cache/%s to %d\n" % (_rbcrevs, start)
                 )
                 f.seek(start)
                 if f.tell() != start:
                     start = 0
                     f.seek(start)
                 f.truncate()
             end = revs * _rbcrecsize
             f.write(self._rbcrevs.slice(start, end))
         self._rbcrevslen = revs