locking: take the `wlock` for the full `hg remove` duration
Otherwise, there is a race condition window between the time we resolve the
file to remove with the matcher and the time we lock the repo and modify the
dirstate.
For example, the working copy might have been updated away, or purged, and the
matched files would no longer be correct.
# branchmap.py - logic to computes, maintain and stores branchmap for local repo
#
# Copyright 2005-2007 Olivia Mackall <olivia@selenic.com>
#
# 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 . import (
encoding,
error,
obsolete,
pycompat,
scmutil,
util,
)
from .utils import (
repoviewutil,
stringutil,
)
if pycompat.TYPE_CHECKING:
from typing import (
Any,
Callable,
Dict,
Iterable,
List,
Optional,
Set,
Tuple,
Union,
)
from . import localrepo
assert any(
(
Any,
Callable,
Dict,
Iterable,
List,
Optional,
Set,
Tuple,
Union,
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)
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(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):
self._per_filter[candidate] = cache
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 branchcache:
"""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,
entries=(),
tipnode=None,
tiprev=nullrev,
filteredhash=None,
closednodes=None,
hasnode=None,
):
# type: (localrepo.localrepository, 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._repo = repo
self._delayed = False
if tipnode is None:
self.tipnode = repo.nullid
else:
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 self._entries.items():
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(
repo,
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),
stringutil.forcebytestr(inst),
)
)
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):
"""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
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 self.items():
yield (bn, heads) + self._branchtip(heads)
def iterheads(self):
"""returns all the heads"""
self._verifyall()
return self._entries.values()
def copy(self):
"""return an deep copy of the branchcache object"""
return type(self)(
self._repo,
self._entries,
self.tipnode,
self.tiprev,
self.filteredhash,
self._closednodes,
)
def write(self, repo):
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 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
# 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)]
tiprev = max(newheadrevs)
if tiprev > ntiprev:
ntiprev = tiprev
if ntiprev > self.tiprev:
self.tiprev = ntiprev
self.tipnode = cl.node(ntiprev)
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
)
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)
_rbcmininc = 64 * _rbcrecsize
_rbcnodelen = 4
_rbcbranchidxmask = 0x7FFFFFFF
_rbccloseflag = 0x80000000
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 = 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, 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
requiredsize = rbcrevidx + _rbcrecsize
rbccur = len(self._rbcrevs)
if rbccur < requiredsize:
# 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
# use the classic doubling technique for dynamic arrays instead.
# If there was a gap in the map before, less space will be reserved.
self._rbcrevs.extend(b'\0' * max(_rbcmininc, requiredsize))
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