branchcache: move the header loading in a `_load_header` class method

This will help changing header parsing in format variants.

mercurial/branchmap.py

@@ -1,1019 +1,1031 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_delayed` 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.write(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].copy(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 = self._per_filter[candidate] = cache.copy(rview)
                     cache.write(rview)
                     return
 
     def clear(self):
         self._per_filter.clear()
 
     def write_delayed(self, repo):
         unfi = repo.unfiltered()
         for filtername, cache in self._per_filter.items():
             if cache._delayed:
                 repo = unfi.filtered(filtername)
                 cache.write(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:
             max_rev = None
 
         # 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
 
 
 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,
     ) -> 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
         self._delayed = False
         if tipnode is None:
             self.tipnode = repo.nullid
         else:
             self.tipnode = tipnode
         self.tiprev = tiprev
         self.filteredhash = filteredhash
 
         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)
-            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])
+            init_kwargs = cls._load_header(repo, lineiter)
             bcache = cls(
                 repo,
-                tipnode=last,
-                tiprev=lrev,
-                filteredhash=filteredhash,
                 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 copy(self, repo):
         """return a deep copy of the branchcache object"""
         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,
         )
         # we copy will likely schedule a write anyway, but that does not seems
         # to hurt to overschedule
         other._delayed = self._delayed
         # also copy information about the current verification state
         other._verifiedbranches = set(self._verifiedbranches)
         return other
 
     def write(self, repo):
         assert self._filtername == repo.filtername, (
             self._filtername,
             repo.filtername,
         )
         tr = repo.currenttransaction()
         if not getattr(tr, 'finalized', True):
             # Avoid premature writing.
             #
             # (The cache warming setup by localrepo will update the file later.)
             self._delayed = True
             return
         try:
             filename = self._filename(repo)
             with repo.cachevfs(filename, b"w", atomictemp=True) as f:
                 cachekey = [hex(self.tipnode), b'%d' % self.tiprev]
                 if self.filteredhash is not None:
                     cachekey.append(hex(self.filteredhash))
                 f.write(b" ".join(cachekey) + b'\n')
                 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'
                         f.write(b"%s %s %s\n" % (hex(node), state, label))
             repo.ui.log(
                 b'branchcache',
                 b'wrote %s with %d labels and %d nodes\n',
                 _branchcachedesc(repo),
                 len(self._entries),
                 nodecount,
             )
             self._delayed = False
         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 _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.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