annotate mercurial/branchmap.py @ 46254:c4b792fa109e

branchmap: avoid ancestor computations in absence of non-continous branches

The branchhead computation is one of the more heavy operations for
bigger repositories as it has to scan all changesets and potentially
involves the expensive computation of the ancestor sets. Redo the
computation to handle the common cases directly and use tighter
conditions for when the ancestor scan is necessary. Most importantly,
avoid it completely if the non-continous branches are processed in one
update as seen in the initial computation after a clone.

For the Mercurial repository, it gives a small 2-3% performance boost.
For the NetBSD test repository, it cuts the time in half.

Differential Revision: https://phab.mercurial-scm.org/D9631

# branchmap.py - logic to computes, maintain and stores branchmap for local repo
#
# Copyright 2005-2007 Matt 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.

from __future__ import absolute_import

import struct

from .node import (
    bin,
    hex,
    nullid,
    nullrev,
)
from . import (
    encoding,
    error,
    pycompat,
    scmutil,
    util,
)
from .utils import (
    repoviewutil,
    stringutil,
)

if pycompat.TYPE_CHECKING:
    from typing import (
        Any,
        Callable,
        Dict,
        Iterable,
        List,
        Optional,
        Set,
        Tuple,
        Union,
    )

    assert any(
        (
            Any,
            Callable,
            Dict,
            Iterable,
            List,
            Optional,
            Set,
            Tuple,
            Union,
        )
    )

subsettable = repoviewutil.subsettable

calcsize = struct.calcsize
pack_into = struct.pack_into
unpack_from = struct.unpack_from


class BranchMapCache(object):
    """mapping of filtered views of repo with their branchcache"""

    def __init__(self):
        self._per_filter = {}

    def __getitem__(self, repo):
        self.updatecache(repo)
        return self._per_filter[repo.filtername]

    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()
                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()

        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 pycompat.itervalues(remotebranchmap):
            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(
                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):
                    self._per_filter[candidate] = cache
                    cache.write(rview)
                    return

    def clear(self):
        self._per_filter.clear()


def _unknownnode(node):
    """raises ValueError when branchcache found a node which does not exists"""
    raise ValueError('node %s does not exist' % pycompat.sysstr(hex(node)))


def _branchcachedesc(repo):
    if repo.filtername is not None:
        return b'branch cache (%s)' % repo.filtername
    else:
        return b'branch cache'


class branchcache(object):
    """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 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,
        entries=(),
        tipnode=nullid,
        tiprev=nullrev,
        filteredhash=None,
        closednodes=None,
        hasnode=None,
    ):
        # type: (Union[Dict[bytes, List[bytes]], Iterable[Tuple[bytes, List[bytes]]]], bytes,  int, Optional[bytes], Optional[Set[bytes]], Optional[Callable[[bytes], bool]]) -> 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.tipnode = tipnode
        self.tiprev = tiprev
        self.filteredhash = filteredhash
        # 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 closednodes is None:
            self._closednodes = set()
        else:
            self._closednodes = closednodes
        self._entries = dict(entries)
        # whether closed nodes are verified or not
        self._closedverified = False
        # branches for which nodes are verified
        self._verifiedbranches = set()
        self._hasnode = hasnode
        if self._hasnode is None:
            self._hasnode = lambda x: True

    def _verifyclosed(self):
        """ verify the closed nodes we have """
        if self._closedverified:
            return
        for node in self._closednodes:
            if not self._hasnode(node):
                _unknownnode(node)

        self._closedverified = True

    def _verifybranch(self, branch):
        """ verify head nodes for the given branch. """
        if branch not in self._entries or branch in self._verifiedbranches:
            return
        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 """
        needverification = set(self._entries.keys()) - self._verifiedbranches
        for b in needverification:
            self._verifybranch(b)

    def __iter__(self):
        return iter(self._entries)

    def __setitem__(self, key, value):
        self._entries[key] = value

    def __getitem__(self, key):
        self._verifybranch(key)
        return self._entries[key]

    def __contains__(self, key):
        self._verifybranch(key)
        return key in self._entries

    def iteritems(self):
        for k, v in pycompat.iteritems(self._entries):
            self._verifybranch(k)
            yield k, v

    items = iteritems

    def hasbranch(self, label):
        """ checks whether a branch of this name exists or not """
        self._verifybranch(label)
        return label in self._entries

    @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
            hasnode = repo.changelog.hasnode
            if len(cachekey) > 2:
                filteredhash = bin(cachekey[2])
            bcache = cls(
                tipnode=last,
                tiprev=lrev,
                filteredhash=filteredhash,
                hasnode=hasnode,
            )
            if not bcache.validfor(repo):
                # invalidate the cache
                raise ValueError('tip differs')
            bcache.load(repo, lineiter)
        except (IOError, OSError):
            return None

        except Exception as inst:
            if repo.ui.debugflag:
                msg = b'invalid %s: %s\n'
                repo.ui.debug(
                    msg
                    % (
                        _branchcachedesc(repo),
                        pycompat.bytestr(
                            inst
                        ),  # pytype: disable=wrong-arg-types
                    )
                )
            bcache = None

        finally:
            if f:
                f.close()

        return bcache

    def load(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)

    @staticmethod
    def _filename(repo):
        """name of a branchcache file for a given repo or repoview"""
        filename = b"branch2"
        if repo.filtername:
            filename = b'%s-%s' % (filename, repo.filtername)
        return filename

    def validfor(self, repo):
        """Is the cache content valid regarding a repo

        - False when cached tipnode is unknown or if we detect a strip.
        - True when cache is up to date or a subset of current repo."""
        try:
            return (self.tipnode == repo.changelog.node(self.tiprev)) and (
                self.filteredhash == scmutil.filteredhash(repo, self.tiprev)
            )
        except IndexError:
            return False

    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):
        self._verifybranch(branch)
        heads = self._entries[branch]
        if not closed:
            heads = list(self.iteropen(heads))
        return heads

    def iterbranches(self):
        for bn, heads in pycompat.iteritems(self):
            yield (bn, heads) + self._branchtip(heads)

    def iterheads(self):
        """ returns all the heads """
        self._verifyall()
        return pycompat.itervalues(self._entries)

    def copy(self):
        """return an deep copy of the branchcache object"""
        return type(self)(
            self._entries,
            self.tipnode,
            self.tiprev,
            self.filteredhash,
            self._closednodes,
        )

    def write(self, repo):
        try:
            f = repo.cachevfs(self._filename(repo), b"w", atomictemp=True)
            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(pycompat.iteritems(self._entries)):
                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))
            f.close()
            repo.ui.log(
                b'branchcache',
                b'wrote %s with %d labels and %d nodes\n',
                _branchcachedesc(repo),
                len(self._entries),
                nodecount,
            )
        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 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
        for r in revgen:
            branch, closesbranch = getbranchinfo(r)
            newbranches.setdefault(branch, []).append(r)
            if closesbranch:
                self._closednodes.add(cl.node(r))

        # new tip revision which we found after iterating items from new
        # branches
        ntiprev = self.tiprev

        # 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

        for branch, newheadrevs in pycompat.iteritems(newbranches):
            # 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.setdefault(branch, [])
            bheadset = {cl.rev(node) for node in bheads}
            uncertain = set()
            for newrev in sorted(newheadrevs):
                if not bheadset:
                    bheadset.add(newrev)
                    continue

                parents = [p for p in parentrevs(newrev) if p != nullrev]
                samebranch = set()
                otherbranch = set()
                for p in parents:
                    if p in bheadset or getbranchinfo(p)[0] == branch:
                        samebranch.add(p)
                    else:
                        otherbranch.add(p)
                if otherbranch and not (len(bheadset) == len(samebranch) == 1):
                    uncertain.update(otherbranch)
                bheadset.difference_update(samebranch)
                bheadset.add(newrev)

            if uncertain:
                if topoheads is None:
                    topoheads = set(cl.headrevs())
                if bheadset - topoheads:
                    floorrev = min(bheadset)
                    ancestors = set(cl.ancestors(newheadrevs, floorrev))
                    bheadset -= ancestors
            bheadrevs = sorted(bheadset)
            self[branch] = [cl.node(rev) for rev in bheadrevs]
            tiprev = bheadrevs[-1]
            if tiprev > ntiprev:
                ntiprev = tiprev

        if ntiprev > self.tiprev:
            self.tiprev = ntiprev
            self.tipnode = cl.node(ntiprev)

        if not self.validfor(repo):
            # cache key are not valid anymore
            self.tipnode = nullid
            self.tiprev = nullrev
            for heads in self.iterheads():
                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)

        duration = util.timer() - starttime
        repo.ui.log(
            b'branchcache',
            b'updated %s in %.4f seconds\n',
            _branchcachedesc(repo),
            duration,
        )

        self.write(repo)


class remotebranchcache(branchcache):
    """Branchmap info for a remote connection, should not write locally"""

    def write(self, repo):
        pass


# 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)
_rbcnodelen = 4
_rbcbranchidxmask = 0x7FFFFFFF
_rbccloseflag = 0x80000000


class revbranchcache(object):
    """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 = 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:
                data = repo.cachevfs.read(_rbcrevs)
                self._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 = 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 = unpack_from(
            _rbcrecfmt, util.buffer(self._rbcrevs), 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
            del self._rbcrevs[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, branch, rev, node, close):
        """add new data information to the cache"""
        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, node, 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
        if len(self._rbcrevs) < rbcrevidx + _rbcrecsize:
            self._rbcrevs.extend(
                b'\0'
                * (len(self._repo.changelog) * _rbcrecsize - len(self._rbcrevs))
            )
        pack_into(_rbcrecfmt, self._rbcrevs, 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[start:end])
        self._rbcrevslen = revs