tags: avoid double-reversing a list

Differential Revision: https://phab.mercurial-scm.org/D6337

mercurial/tags.py

@@ -1,815 +1,816 b''
 # tags.py - read tag info from local repository
 #
 # Copyright 2009 Matt Mackall <[e-mail]>
 # Copyright 2009 Greg Ward <[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.
 
 # Currently this module only deals with reading and caching tags.
 # Eventually, it could take care of updating (adding/removing/moving)
 # tags too.
 
 from __future__ import absolute_import
 
 import errno
 
 from .node import (
     bin,
     hex,
     nullid,
     nullrev,
     short,
 )
 from .i18n import _
 from . import (
     encoding,
     error,
     match as matchmod,
     scmutil,
     util,
 )
 from .utils import (
     stringutil,
 )
 
 # Tags computation can be expensive and caches exist to make it fast in
 # the common case.
 #
 # The "hgtagsfnodes1" cache file caches the .hgtags filenode values for
 # each revision in the repository. The file is effectively an array of
 # fixed length records. Read the docs for "hgtagsfnodescache" for technical
 # details.
 #
 # The .hgtags filenode cache grows in proportion to the length of the
 # changelog. The file is truncated when the # changelog is stripped.
 #
 # The purpose of the filenode cache is to avoid the most expensive part
 # of finding global tags, which is looking up the .hgtags filenode in the
 # manifest for each head. This can take dozens or over 100ms for
 # repositories with very large manifests. Multiplied by dozens or even
 # hundreds of heads and there is a significant performance concern.
 #
 # There also exist a separate cache file for each repository filter.
 # These "tags-*" files store information about the history of tags.
 #
 # The tags cache files consists of a cache validation line followed by
 # a history of tags.
 #
 # The cache validation line has the format:
 #
 #   <tiprev> <tipnode> [<filteredhash>]
 #
 # <tiprev> is an integer revision and <tipnode> is a 40 character hex
 # node for that changeset. These redundantly identify the repository
 # tip from the time the cache was written. In addition, <filteredhash>,
 # if present, is a 40 character hex hash of the contents of the filtered
 # revisions for this filter. If the set of filtered revs changes, the
 # hash will change and invalidate the cache.
 #
 # The history part of the tags cache consists of lines of the form:
 #
 #   <node> <tag>
 #
 # (This format is identical to that of .hgtags files.)
 #
 # <tag> is the tag name and <node> is the 40 character hex changeset
 # the tag is associated with.
 #
 # Tags are written sorted by tag name.
 #
 # Tags associated with multiple changesets have an entry for each changeset.
 # The most recent changeset (in terms of revlog ordering for the head
 # setting it) for each tag is last.
 
 def fnoderevs(ui, repo, revs):
     """return the list of '.hgtags' fnodes used in a set revisions
 
     This is returned as list of unique fnodes. We use a list instead of a set
     because order matters when it comes to tags."""
     unfi = repo.unfiltered()
     tonode = unfi.changelog.node
     nodes = [tonode(r) for r in revs]
-    fnodes = _getfnodes(ui, repo, nodes[::-1]) # reversed help the cache
+    fnodes = _getfnodes(ui, repo, nodes)
     fnodes = _filterfnodes(fnodes, nodes)
     return fnodes
 
 def _nulltonone(value):
     """convert nullid to None
 
     For tag value, nullid means "deleted". This small utility function helps
     translating that to None."""
     if value == nullid:
         return None
     return value
 
 def difftags(ui, repo, oldfnodes, newfnodes):
     """list differences between tags expressed in two set of file-nodes
 
     The list contains entries in the form: (tagname, oldvalue, new value).
     None is used to expressed missing value:
         ('foo', None, 'abcd') is a new tag,
         ('bar', 'ef01', None) is a deletion,
         ('baz', 'abcd', 'ef01') is a tag movement.
     """
     if oldfnodes == newfnodes:
         return []
     oldtags = _tagsfromfnodes(ui, repo, oldfnodes)
     newtags = _tagsfromfnodes(ui, repo, newfnodes)
 
     # list of (tag, old, new): None means missing
     entries = []
     for tag, (new, __) in newtags.items():
         new = _nulltonone(new)
         old, __ = oldtags.pop(tag, (None, None))
         old = _nulltonone(old)
         if old != new:
             entries.append((tag, old, new))
     # handle deleted tags
     for tag, (old, __) in oldtags.items():
         old = _nulltonone(old)
         if old is not None:
             entries.append((tag, old, None))
     entries.sort()
     return entries
 
 def writediff(fp, difflist):
     """write tags diff information to a file.
 
     Data are stored with a line based format:
 
         <action> <hex-node> <tag-name>\n
 
     Action are defined as follow:
        -R tag is removed,
        +A tag is added,
        -M tag is moved (old value),
        +M tag is moved (new value),
 
     Example:
 
          +A 875517b4806a848f942811a315a5bce30804ae85 t5
 
     See documentation of difftags output for details about the input.
     """
     add = '+A %s %s\n'
     remove = '-R %s %s\n'
     updateold = '-M %s %s\n'
     updatenew = '+M %s %s\n'
     for tag, old, new in difflist:
         # translate to hex
         if old is not None:
             old = hex(old)
         if new is not None:
             new = hex(new)
         # write to file
         if old is None:
             fp.write(add % (new, tag))
         elif new is None:
             fp.write(remove % (old, tag))
         else:
             fp.write(updateold % (old, tag))
             fp.write(updatenew % (new, tag))
 
 def findglobaltags(ui, repo):
     '''Find global tags in a repo: return a tagsmap
 
     tagsmap: tag name to (node, hist) 2-tuples.
 
     The tags cache is read and updated as a side-effect of calling.
     '''
     (heads, tagfnode, valid, cachetags, shouldwrite) = _readtagcache(ui, repo)
     if cachetags is not None:
         assert not shouldwrite
         # XXX is this really 100% correct?  are there oddball special
         # cases where a global tag should outrank a local tag but won't,
         # because cachetags does not contain rank info?
         alltags = {}
         _updatetags(cachetags, alltags)
         return alltags
 
     for head in reversed(heads):  # oldest to newest
         assert head in repo.changelog.nodemap, (
                "tag cache returned bogus head %s" % short(head))
     fnodes = _filterfnodes(tagfnode, reversed(heads))
     alltags = _tagsfromfnodes(ui, repo, fnodes)
 
     # and update the cache (if necessary)
     if shouldwrite:
         _writetagcache(ui, repo, valid, alltags)
     return alltags
 
 def _filterfnodes(tagfnode, nodes):
     """return a list of unique fnodes
 
     The order of this list matches the order of "nodes". Preserving this order
     is important as reading tags in different order provides different
     results."""
     seen = set()  # set of fnode
     fnodes = []
     for no in nodes:  # oldest to newest
         fnode = tagfnode.get(no)
         if fnode and fnode not in seen:
             seen.add(fnode)
             fnodes.append(fnode)
     return fnodes
 
 def _tagsfromfnodes(ui, repo, fnodes):
     """return a tagsmap from a list of file-node
 
     tagsmap: tag name to (node, hist) 2-tuples.
 
     The order of the list matters."""
     alltags = {}
     fctx = None
     for fnode in fnodes:
         if fctx is None:
             fctx = repo.filectx('.hgtags', fileid=fnode)
         else:
             fctx = fctx.filectx(fnode)
         filetags = _readtags(ui, repo, fctx.data().splitlines(), fctx)
         _updatetags(filetags, alltags)
     return alltags
 
 def readlocaltags(ui, repo, alltags, tagtypes):
     '''Read local tags in repo. Update alltags and tagtypes.'''
     try:
         data = repo.vfs.read("localtags")
     except IOError as inst:
         if inst.errno != errno.ENOENT:
             raise
         return
 
     # localtags is in the local encoding; re-encode to UTF-8 on
     # input for consistency with the rest of this module.
     filetags = _readtags(
         ui, repo, data.splitlines(), "localtags",
         recode=encoding.fromlocal)
 
     # remove tags pointing to invalid nodes
     cl = repo.changelog
     for t in list(filetags):
         try:
             cl.rev(filetags[t][0])
         except (LookupError, ValueError):
             del filetags[t]
 
     _updatetags(filetags, alltags, 'local', tagtypes)
 
 def _readtaghist(ui, repo, lines, fn, recode=None, calcnodelines=False):
     '''Read tag definitions from a file (or any source of lines).
 
     This function returns two sortdicts with similar information:
 
     - the first dict, bintaghist, contains the tag information as expected by
       the _readtags function, i.e. a mapping from tag name to (node, hist):
         - node is the node id from the last line read for that name,
         - hist is the list of node ids previously associated with it (in file
           order). All node ids are binary, not hex.
 
     - the second dict, hextaglines, is a mapping from tag name to a list of
       [hexnode, line number] pairs, ordered from the oldest to the newest node.
 
     When calcnodelines is False the hextaglines dict is not calculated (an
     empty dict is returned). This is done to improve this function's
     performance in cases where the line numbers are not needed.
     '''
 
     bintaghist = util.sortdict()
     hextaglines = util.sortdict()
     count = 0
 
     def dbg(msg):
         ui.debug("%s, line %d: %s\n" % (fn, count, msg))
 
     for nline, line in enumerate(lines):
         count += 1
         if not line:
             continue
         try:
             (nodehex, name) = line.split(" ", 1)
         except ValueError:
             dbg("cannot parse entry")
             continue
         name = name.strip()
         if recode:
             name = recode(name)
         try:
             nodebin = bin(nodehex)
         except TypeError:
             dbg("node '%s' is not well formed" % nodehex)
             continue
 
         # update filetags
         if calcnodelines:
             # map tag name to a list of line numbers
             if name not in hextaglines:
                 hextaglines[name] = []
             hextaglines[name].append([nodehex, nline])
             continue
         # map tag name to (node, hist)
         if name not in bintaghist:
             bintaghist[name] = []
         bintaghist[name].append(nodebin)
     return bintaghist, hextaglines
 
 def _readtags(ui, repo, lines, fn, recode=None, calcnodelines=False):
     '''Read tag definitions from a file (or any source of lines).
 
     Returns a mapping from tag name to (node, hist).
 
     "node" is the node id from the last line read for that name. "hist"
     is the list of node ids previously associated with it (in file order).
     All node ids are binary, not hex.
     '''
     filetags, nodelines = _readtaghist(ui, repo, lines, fn, recode=recode,
                                        calcnodelines=calcnodelines)
     # util.sortdict().__setitem__ is much slower at replacing then inserting
     # new entries. The difference can matter if there are thousands of tags.
     # Create a new sortdict to avoid the performance penalty.
     newtags = util.sortdict()
     for tag, taghist in filetags.items():
         newtags[tag] = (taghist[-1], taghist[:-1])
     return newtags
 
 def _updatetags(filetags, alltags, tagtype=None, tagtypes=None):
     """Incorporate the tag info read from one file into dictionnaries
 
     The first one, 'alltags', is a "tagmaps" (see 'findglobaltags' for details).
 
     The second one, 'tagtypes', is optional and will be updated to track the
     "tagtype" of entries in the tagmaps. When set, the 'tagtype' argument also
     needs to be set."""
     if tagtype is None:
         assert tagtypes is None
 
     for name, nodehist in filetags.iteritems():
         if name not in alltags:
             alltags[name] = nodehist
             if tagtype is not None:
                 tagtypes[name] = tagtype
             continue
 
         # we prefer alltags[name] if:
         #  it supersedes us OR
         #  mutual supersedes and it has a higher rank
         # otherwise we win because we're tip-most
         anode, ahist = nodehist
         bnode, bhist = alltags[name]
         if (bnode != anode and anode in bhist and
             (bnode not in ahist or len(bhist) > len(ahist))):
             anode = bnode
         elif tagtype is not None:
             tagtypes[name] = tagtype
         ahist.extend([n for n in bhist if n not in ahist])
         alltags[name] = anode, ahist
 
 def _filename(repo):
     """name of a tagcache file for a given repo or repoview"""
     filename = 'tags2'
     if repo.filtername:
         filename = '%s-%s' % (filename, repo.filtername)
     return filename
 
 def _readtagcache(ui, repo):
     '''Read the tag cache.
 
     Returns a tuple (heads, fnodes, validinfo, cachetags, shouldwrite).
 
     If the cache is completely up-to-date, "cachetags" is a dict of the
     form returned by _readtags() and "heads", "fnodes", and "validinfo" are
     None and "shouldwrite" is False.
 
     If the cache is not up to date, "cachetags" is None. "heads" is a list
     of all heads currently in the repository, ordered from tip to oldest.
     "validinfo" is a tuple describing cache validation info. This is used
     when writing the tags cache. "fnodes" is a mapping from head to .hgtags
     filenode. "shouldwrite" is True.
 
     If the cache is not up to date, the caller is responsible for reading tag
     info from each returned head. (See findglobaltags().)
     '''
     try:
         cachefile = repo.cachevfs(_filename(repo), 'r')
         # force reading the file for static-http
         cachelines = iter(cachefile)
     except IOError:
         cachefile = None
 
     cacherev = None
     cachenode = None
     cachehash = None
     if cachefile:
         try:
             validline = next(cachelines)
             validline = validline.split()
             cacherev = int(validline[0])
             cachenode = bin(validline[1])
             if len(validline) > 2:
                 cachehash = bin(validline[2])
         except Exception:
             # corruption of the cache, just recompute it.
             pass
 
     tipnode = repo.changelog.tip()
     tiprev = len(repo.changelog) - 1
 
     # Case 1 (common): tip is the same, so nothing has changed.
     # (Unchanged tip trivially means no changesets have been added.
     # But, thanks to localrepository.destroyed(), it also means none
     # have been destroyed by strip or rollback.)
     if (cacherev == tiprev
             and cachenode == tipnode
             and cachehash == scmutil.filteredhash(repo, tiprev)):
         tags = _readtags(ui, repo, cachelines, cachefile.name)
         cachefile.close()
         return (None, None, None, tags, False)
     if cachefile:
         cachefile.close()               # ignore rest of file
 
     valid = (tiprev, tipnode, scmutil.filteredhash(repo, tiprev))
 
     repoheads = repo.heads()
     # Case 2 (uncommon): empty repo; get out quickly and don't bother
     # writing an empty cache.
     if repoheads == [nullid]:
         return ([], {}, valid, {}, False)
 
     # Case 3 (uncommon): cache file missing or empty.
 
     # Case 4 (uncommon): tip rev decreased.  This should only happen
     # when we're called from localrepository.destroyed().  Refresh the
     # cache so future invocations will not see disappeared heads in the
     # cache.
 
     # Case 5 (common): tip has changed, so we've added/replaced heads.
 
     # As it happens, the code to handle cases 3, 4, 5 is the same.
 
     # N.B. in case 4 (nodes destroyed), "new head" really means "newly
     # exposed".
     if not len(repo.file('.hgtags')):
         # No tags have ever been committed, so we can avoid a
         # potentially expensive search.
         return ([], {}, valid, None, True)
 
 
     # Now we have to lookup the .hgtags filenode for every new head.
     # This is the most expensive part of finding tags, so performance
     # depends primarily on the size of newheads.  Worst case: no cache
     # file, so newheads == repoheads.
-    cachefnode = _getfnodes(ui, repo, repoheads)
+    # Reversed order helps the cache ('repoheads' is in descending order)
+    cachefnode = _getfnodes(ui, repo, reversed(repoheads))
 
     # Caller has to iterate over all heads, but can use the filenodes in
     # cachefnode to get to each .hgtags revision quickly.
     return (repoheads, cachefnode, valid, None, True)
 
 def _getfnodes(ui, repo, nodes):
     """return .hgtags fnodes for a list of changeset nodes
 
     Return value is a {node: fnode} mapping. There will be no entry for nodes
     without a '.hgtags' file.
     """
     starttime = util.timer()
     fnodescache = hgtagsfnodescache(repo.unfiltered())
     cachefnode = {}
-    for node in reversed(nodes):
+    for node in nodes:
         fnode = fnodescache.getfnode(node)
         if fnode != nullid:
             cachefnode[node] = fnode
 
     fnodescache.write()
 
     duration = util.timer() - starttime
     ui.log('tagscache',
            '%d/%d cache hits/lookups in %0.4f seconds\n',
            fnodescache.hitcount, fnodescache.lookupcount, duration)
     return cachefnode
 
 def _writetagcache(ui, repo, valid, cachetags):
     filename = _filename(repo)
     try:
         cachefile = repo.cachevfs(filename, 'w', atomictemp=True)
     except (OSError, IOError):
         return
 
     ui.log('tagscache', 'writing .hg/cache/%s with %d tags\n',
            filename, len(cachetags))
 
     if valid[2]:
         cachefile.write('%d %s %s\n' % (valid[0], hex(valid[1]), hex(valid[2])))
     else:
         cachefile.write('%d %s\n' % (valid[0], hex(valid[1])))
 
     # Tag names in the cache are in UTF-8 -- which is the whole reason
     # we keep them in UTF-8 throughout this module.  If we converted
     # them local encoding on input, we would lose info writing them to
     # the cache.
     for (name, (node, hist)) in sorted(cachetags.iteritems()):
         for n in hist:
             cachefile.write("%s %s\n" % (hex(n), name))
         cachefile.write("%s %s\n" % (hex(node), name))
 
     try:
         cachefile.close()
     except (OSError, IOError):
         pass
 
 def tag(repo, names, node, message, local, user, date, editor=False):
     '''tag a revision with one or more symbolic names.
 
     names is a list of strings or, when adding a single tag, names may be a
     string.
 
     if local is True, the tags are stored in a per-repository file.
     otherwise, they are stored in the .hgtags file, and a new
     changeset is committed with the change.
 
     keyword arguments:
 
     local: whether to store tags in non-version-controlled file
     (default False)
 
     message: commit message to use if committing
 
     user: name of user to use if committing
 
     date: date tuple to use if committing'''
 
     if not local:
         m = matchmod.exact(['.hgtags'])
         if any(repo.status(match=m, unknown=True, ignored=True)):
             raise error.Abort(_('working copy of .hgtags is changed'),
                              hint=_('please commit .hgtags manually'))
 
     with repo.wlock():
         repo.tags() # instantiate the cache
         _tag(repo, names, node, message, local, user, date,
              editor=editor)
 
 def _tag(repo, names, node, message, local, user, date, extra=None,
          editor=False):
     if isinstance(names, bytes):
         names = (names,)
 
     branches = repo.branchmap()
     for name in names:
         repo.hook('pretag', throw=True, node=hex(node), tag=name,
                   local=local)
         if name in branches:
             repo.ui.warn(_("warning: tag %s conflicts with existing"
             " branch name\n") % name)
 
     def writetags(fp, names, munge, prevtags):
         fp.seek(0, 2)
         if prevtags and not prevtags.endswith('\n'):
             fp.write('\n')
         for name in names:
             if munge:
                 m = munge(name)
             else:
                 m = name
 
             if (repo._tagscache.tagtypes and
                 name in repo._tagscache.tagtypes):
                 old = repo.tags().get(name, nullid)
                 fp.write('%s %s\n' % (hex(old), m))
             fp.write('%s %s\n' % (hex(node), m))
         fp.close()
 
     prevtags = ''
     if local:
         try:
             fp = repo.vfs('localtags', 'r+')
         except IOError:
             fp = repo.vfs('localtags', 'a')
         else:
             prevtags = fp.read()
 
         # local tags are stored in the current charset
         writetags(fp, names, None, prevtags)
         for name in names:
             repo.hook('tag', node=hex(node), tag=name, local=local)
         return
 
     try:
         fp = repo.wvfs('.hgtags', 'rb+')
     except IOError as e:
         if e.errno != errno.ENOENT:
             raise
         fp = repo.wvfs('.hgtags', 'ab')
     else:
         prevtags = fp.read()
 
     # committed tags are stored in UTF-8
     writetags(fp, names, encoding.fromlocal, prevtags)
 
     fp.close()
 
     repo.invalidatecaches()
 
     if '.hgtags' not in repo.dirstate:
         repo[None].add(['.hgtags'])
 
     m = matchmod.exact(['.hgtags'])
     tagnode = repo.commit(message, user, date, extra=extra, match=m,
                           editor=editor)
 
     for name in names:
         repo.hook('tag', node=hex(node), tag=name, local=local)
 
     return tagnode
 
 _fnodescachefile = 'hgtagsfnodes1'
 _fnodesrecsize = 4 + 20 # changeset fragment + filenode
 _fnodesmissingrec = '\xff' * 24
 
 class hgtagsfnodescache(object):
     """Persistent cache mapping revisions to .hgtags filenodes.
 
     The cache is an array of records. Each item in the array corresponds to
     a changelog revision. Values in the array contain the first 4 bytes of
     the node hash and the 20 bytes .hgtags filenode for that revision.
 
     The first 4 bytes are present as a form of verification. Repository
     stripping and rewriting may change the node at a numeric revision in the
     changelog. The changeset fragment serves as a verifier to detect
     rewriting. This logic is shared with the rev branch cache (see
     branchmap.py).
 
     The instance holds in memory the full cache content but entries are
     only parsed on read.
 
     Instances behave like lists. ``c[i]`` works where i is a rev or
     changeset node. Missing indexes are populated automatically on access.
     """
     def __init__(self, repo):
         assert repo.filtername is None
 
         self._repo = repo
 
         # Only for reporting purposes.
         self.lookupcount = 0
         self.hitcount = 0
 
 
         try:
             data = repo.cachevfs.read(_fnodescachefile)
         except (OSError, IOError):
             data = ""
         self._raw = bytearray(data)
 
         # The end state of self._raw is an array that is of the exact length
         # required to hold a record for every revision in the repository.
         # We truncate or extend the array as necessary. self._dirtyoffset is
         # defined to be the start offset at which we need to write the output
         # file. This offset is also adjusted when new entries are calculated
         # for array members.
         cllen = len(repo.changelog)
         wantedlen = cllen * _fnodesrecsize
         rawlen = len(self._raw)
 
         self._dirtyoffset = None
 
         if rawlen < wantedlen:
             self._dirtyoffset = rawlen
             self._raw.extend('\xff' * (wantedlen - rawlen))
         elif rawlen > wantedlen:
             # There's no easy way to truncate array instances. This seems
             # slightly less evil than copying a potentially large array slice.
             for i in range(rawlen - wantedlen):
                 self._raw.pop()
             self._dirtyoffset = len(self._raw)
 
     def getfnode(self, node, computemissing=True):
         """Obtain the filenode of the .hgtags file at a specified revision.
 
         If the value is in the cache, the entry will be validated and returned.
         Otherwise, the filenode will be computed and returned unless
         "computemissing" is False, in which case None will be returned without
         any potentially expensive computation being performed.
 
         If an .hgtags does not exist at the specified revision, nullid is
         returned.
         """
         if node == nullid:
             return nullid
 
         ctx = self._repo[node]
         rev = ctx.rev()
 
         self.lookupcount += 1
 
         offset = rev * _fnodesrecsize
         record = '%s' % self._raw[offset:offset + _fnodesrecsize]
         properprefix = node[0:4]
 
         # Validate and return existing entry.
         if record != _fnodesmissingrec:
             fileprefix = record[0:4]
 
             if fileprefix == properprefix:
                 self.hitcount += 1
                 return record[4:]
 
             # Fall through.
 
         # If we get here, the entry is either missing or invalid.
 
         if not computemissing:
             return None
 
         fnode = None
         cl = self._repo.changelog
         p1rev, p2rev = cl._uncheckedparentrevs(rev)
         p1node = cl.node(p1rev)
         p1fnode = self.getfnode(p1node, computemissing=False)
         if p2rev != nullrev:
             # There is some no-merge changeset where p1 is null and p2 is set
             # Processing them as merge is just slower, but still gives a good
             # result.
             p2node = cl.node(p1rev)
             p2fnode = self.getfnode(p2node, computemissing=False)
             if p1fnode != p2fnode:
                 # we cannot rely on readfast because we don't know against what
                 # parent the readfast delta is computed
                 p1fnode = None
         if p1fnode is not None:
             mctx = ctx.manifestctx()
             fnode = mctx.readfast().get('.hgtags')
             if fnode is None:
                 fnode = p1fnode
         if fnode is None:
             # Populate missing entry.
             try:
                 fnode = ctx.filenode('.hgtags')
             except error.LookupError:
                 # No .hgtags file on this revision.
                 fnode = nullid
 
         self._writeentry(offset, properprefix, fnode)
         return fnode
 
     def setfnode(self, node, fnode):
         """Set the .hgtags filenode for a given changeset."""
         assert len(fnode) == 20
         ctx = self._repo[node]
 
         # Do a lookup first to avoid writing if nothing has changed.
         if self.getfnode(ctx.node(), computemissing=False) == fnode:
             return
 
         self._writeentry(ctx.rev() * _fnodesrecsize, node[0:4], fnode)
 
     def _writeentry(self, offset, prefix, fnode):
         # Slices on array instances only accept other array.
         entry = bytearray(prefix + fnode)
         self._raw[offset:offset + _fnodesrecsize] = entry
         # self._dirtyoffset could be None.
         self._dirtyoffset = min(self._dirtyoffset or 0, offset or 0)
 
     def write(self):
         """Perform all necessary writes to cache file.
 
         This may no-op if no writes are needed or if a write lock could
         not be obtained.
         """
         if self._dirtyoffset is None:
             return
 
         data = self._raw[self._dirtyoffset:]
         if not data:
             return
 
         repo = self._repo
 
         try:
             lock = repo.wlock(wait=False)
         except error.LockError:
             repo.ui.log('tagscache', 'not writing .hg/cache/%s because '
                         'lock cannot be acquired\n' % (_fnodescachefile))
             return
 
         try:
             f = repo.cachevfs.open(_fnodescachefile, 'ab')
             try:
                 # if the file has been truncated
                 actualoffset = f.tell()
                 if actualoffset < self._dirtyoffset:
                     self._dirtyoffset = actualoffset
                     data = self._raw[self._dirtyoffset:]
                 f.seek(self._dirtyoffset)
                 f.truncate()
                 repo.ui.log('tagscache',
                             'writing %d bytes to cache/%s\n' % (
                             len(data), _fnodescachefile))
                 f.write(data)
                 self._dirtyoffset = None
             finally:
                 f.close()
         except (IOError, OSError) as inst:
             repo.ui.log('tagscache',
                         "couldn't write cache/%s: %s\n" % (
                             _fnodescachefile, stringutil.forcebytestr(inst)))
         finally:
             lock.release()