branchcache: introduce revbranchcache for caching of revision branch names

It is expensive to retrieve the branch name of a revision. Very expensive when
creating a changectx and calling .branch() every time - slightly less when
using changelog.branchinfo().

Now, to speed things up, provide a way to cache the results on disk in an
efficient format. Each branchname is assigned a number, and for each revision
we store the number of the corresponding branch name. The branch names are
stored in a dedicated file which is strictly append only.

Branch names are usually reused across several revisions, and the total list of
branch names will thus be so small that it is feasible to read the whole set of
names before using the cache. It will however do that it might be more
efficient to use the changelog for retrieving the branch info for a single
revision.

The revision entries are stored in another file. This file is usually append
only, but if the repository has been modified, the file will be truncated and
the relevant parts rewritten on demand.

The entries for each revision are 8 bytes each, and the whole revision file
will thus be 1/8 of 00changelog.i.

Each revision entry contains the first 4 bytes of the corresponding node hash.
This is used as a check sum that always is verified before the entry is used.
That check is relatively expensive but it makes sure history modification is
detected and handled correctly. It will also detect and handle most revision
file corruptions.

This is just a cache. A new format can always be introduced if other
requirements or ideas make that seem like a good idea. Rebuilding the cache is
not really more expensive than it was to run for example 'hg log -b branchname'
before this cache was introduced.

This new method is still unused but promise to make some operations several
times faster once it actually is used.

Abandoning Python 2.4 would make it possible to implement this more efficiently
by using struct classes and pack_into. The Python code could probably also be
micro optimized or it could be implemented very efficiently in C where it would
be easy to control the data access.

mercurial/branchmap.py

@@ -1,287 +1,436 b''
 # 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 node import bin, hex, nullid, nullrev
 import encoding
 import util
 import time
+from array import array
+from struct import calcsize, pack, unpack
 
 def _filename(repo):
     """name of a branchcache file for a given repo or repoview"""
     filename = "cache/branch2"
     if repo.filtername:
         filename = '%s-%s' % (filename, repo.filtername)
     return filename
 
 def read(repo):
     try:
         f = repo.opener(_filename(repo))
         lines = f.read().split('\n')
         f.close()
     except (IOError, OSError):
         return None
 
     try:
         cachekey = lines.pop(0).split(" ", 2)
         last, lrev = cachekey[:2]
         last, lrev = bin(last), int(lrev)
         filteredhash = None
         if len(cachekey) > 2:
             filteredhash = bin(cachekey[2])
         partial = branchcache(tipnode=last, tiprev=lrev,
                               filteredhash=filteredhash)
         if not partial.validfor(repo):
             # invalidate the cache
             raise ValueError('tip differs')
         for l in lines:
             if not l:
                 continue
             node, state, label = l.split(" ", 2)
             if state not in 'oc':
                 raise ValueError('invalid branch state')
             label = encoding.tolocal(label.strip())
             if not node in repo:
                 raise ValueError('node %s does not exist' % node)
             node = bin(node)
             partial.setdefault(label, []).append(node)
             if state == 'c':
                 partial._closednodes.add(node)
     except KeyboardInterrupt:
         raise
     except Exception, inst:
         if repo.ui.debugflag:
             msg = 'invalid branchheads cache'
             if repo.filtername is not None:
                 msg += ' (%s)' % repo.filtername
             msg += ': %s\n'
             repo.ui.debug(msg % inst)
         partial = None
     return partial
 
 ### Nearest subset relation
 # Nearest subset of filter X is a filter Y so that:
 # * Y is included in X,
 # * X - Y is as small as possible.
 # This create and ordering used for branchmap purpose.
 # the ordering may be partial
 subsettable = {None: 'visible',
                'visible': 'served',
                'served': 'immutable',
                'immutable': 'base'}
 
 def updatecache(repo):
     cl = repo.changelog
     filtername = repo.filtername
     partial = repo._branchcaches.get(filtername)
 
     revs = []
     if partial is None or not partial.validfor(repo):
         partial = read(repo)
         if partial is None:
             subsetname = subsettable.get(filtername)
             if subsetname is None:
                 partial = branchcache()
             else:
                 subset = repo.filtered(subsetname)
                 partial = subset.branchmap().copy()
                 extrarevs = subset.changelog.filteredrevs - cl.filteredrevs
                 revs.extend(r for  r in extrarevs if r <= partial.tiprev)
     revs.extend(cl.revs(start=partial.tiprev + 1))
     if revs:
         partial.update(repo, revs)
         partial.write(repo)
     assert partial.validfor(repo), filtername
     repo._branchcaches[repo.filtername] = partial
 
 class branchcache(dict):
     """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):
         super(branchcache, self).__init__(entries)
         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
 
     def _hashfiltered(self, repo):
         """build hash of revision filtered in the current cache
 
         Tracking tipnode and tiprev is not enough to ensure validity of the
         cache as they do not help to distinct cache that ignored various
         revision bellow tiprev.
 
         To detect such difference, we build a cache of all ignored revisions.
         """
         cl = repo.changelog
         if not cl.filteredrevs:
             return None
         key = None
         revs = sorted(r for r in cl.filteredrevs if r <= self.tiprev)
         if revs:
             s = util.sha1()
             for rev in revs:
                 s.update('%s;' % rev)
             key = s.digest()
         return key
 
     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 == self._hashfiltered(repo)))
         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 branchheads(self, branch, closed=False):
         heads = self[branch]
         if not closed:
             heads = [h for h in heads if h not in self._closednodes]
         return heads
 
     def iterbranches(self):
         for bn, heads in self.iteritems():
             yield (bn, heads) + self._branchtip(heads)
 
     def copy(self):
         """return an deep copy of the branchcache object"""
         return branchcache(self, self.tipnode, self.tiprev, self.filteredhash,
                            self._closednodes)
 
     def write(self, repo):
         try:
             f = repo.opener(_filename(repo), "w", atomictemp=True)
             cachekey = [hex(self.tipnode), str(self.tiprev)]
             if self.filteredhash is not None:
                 cachekey.append(hex(self.filteredhash))
             f.write(" ".join(cachekey) + '\n')
             nodecount = 0
             for label, nodes in sorted(self.iteritems()):
                 for node in nodes:
                     nodecount += 1
                     if node in self._closednodes:
                         state = 'c'
                     else:
                         state = 'o'
                     f.write("%s %s %s\n" % (hex(node), state,
                                             encoding.fromlocal(label)))
             f.close()
             repo.ui.log('branchcache',
                         'wrote %s branch cache with %d labels and %d nodes\n',
                         repo.filtername, len(self), nodecount)
         except (IOError, OSError, util.Abort), inst:
             repo.ui.debug("couldn't write branch cache: %s\n" % inst)
             # Abort may be raise by read only opener
             pass
 
     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 = time.time()
         cl = repo.changelog
         # collect new branch entries
         newbranches = {}
         getbranchinfo = cl.branchinfo
         for r in revgen:
             branch, closesbranch = getbranchinfo(r)
             newbranches.setdefault(branch, []).append(r)
             if closesbranch:
                 self._closednodes.add(cl.node(r))
 
         # fetch current topological heads to speed up filtering
         topoheads = set(cl.headrevs())
 
         # if older branchheads are reachable from new ones, they aren't
         # really branchheads. Note checking parents is insufficient:
         # 1 (branch a) -> 2 (branch b) -> 3 (branch a)
         for branch, newheadrevs in newbranches.iteritems():
             bheads = self.setdefault(branch, [])
             bheadset = set(cl.rev(node) for node in bheads)
 
             # This have been tested True on all internal usage of this function.
             # run it again in case of doubt
             # assert not (set(bheadrevs) & set(newheadrevs))
             newheadrevs.sort()
             bheadset.update(newheadrevs)
 
             # This prunes out two kinds of heads - heads that are superseded by
             # a head in newheadrevs, and newheadrevs that are not heads because
             # an existing head is their descendant.
             uncertain = bheadset - topoheads
             if uncertain:
                 floorrev = min(uncertain)
                 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 > self.tiprev:
                 self.tipnode = cl.node(tiprev)
                 self.tiprev = tiprev
 
         if not self.validfor(repo):
             # cache key are not valid anymore
             self.tipnode = nullid
             self.tiprev = nullrev
             for heads in self.values():
                 tiprev = max(cl.rev(node) for node in heads)
                 if tiprev > self.tiprev:
                     self.tipnode = cl.node(tiprev)
                     self.tiprev = tiprev
         self.filteredhash = self._hashfiltered(repo)
 
         duration = time.time() - starttime
         repo.ui.log('branchcache', 'updated %s branch cache in %.4f seconds\n',
                     repo.filtername, duration)
+
+# Revision branch info cache
+
+_rbcversion = '-v1'
+_rbcnames = 'cache/rbc-names' + _rbcversion
+_rbcrevs = 'cache/rbc-revs' + _rbcversion
+# [4 byte hash prefix][4 byte branch name number with sign bit indicating open]
+_rbcrecfmt = '>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):
+        assert repo.filtername is None
+        self._names = [] # branch names in local encoding with static index
+        self._rbcrevs = array('c') # structs of type _rbcrecfmt
+        self._rbcsnameslen = 0
+        try:
+            bndata = repo.vfs.read(_rbcnames)
+            self._rbcsnameslen = len(bndata) # for verification before writing
+            self._names = [encoding.tolocal(bn) for bn in bndata.split('\0')]
+        except (IOError, OSError), inst:
+            repo.ui.debug("couldn't read revision branch cache names: %s\n" %
+                          inst)
+        if self._names:
+            try:
+                data = repo.vfs.read(_rbcrevs)
+                self._rbcrevs.fromstring(data)
+            except (IOError, OSError), inst:
+                repo.ui.debug("couldn't read revision branch cache: %s\n" %
+                              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 good names on disk
+        self._namesreverse = dict((b, r) for r, b in enumerate(self._names))
+
+    def branchinfo(self, changelog, rev):
+        """Return branch name and close flag for rev, using and updating
+        persistent cache."""
+        rbcrevidx = rev * _rbcrecsize
+
+        # if requested rev is missing, add and populate all missing revs
+        if len(self._rbcrevs) < rbcrevidx + _rbcrecsize:
+            first = len(self._rbcrevs) // _rbcrecsize
+            self._rbcrevs.extend('\0' * (len(changelog) * _rbcrecsize -
+                                         len(self._rbcrevs)))
+            for r in xrange(first, len(changelog)):
+                self._branchinfo(r)
+
+        # fast path: extract data from cache, use it if node is matching
+        reponode = changelog.node(rev)[:_rbcnodelen]
+        cachenode, branchidx = unpack(
+            _rbcrecfmt, buffer(self._rbcrevs, rbcrevidx, _rbcrecsize))
+        close = bool(branchidx & _rbccloseflag)
+        if close:
+            branchidx &= _rbcbranchidxmask
+        if cachenode == reponode:
+            return self._names[branchidx], close
+        # fall back to slow path and make sure it will be written to disk
+        self._rbcrevslen = min(self._rbcrevslen, rev)
+        return self._branchinfo(rev)
+
+    def _branchinfo(self, changelog, rev):
+        """Retrieve branch info from changelog and update _rbcrevs"""
+        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
+        rbcrevidx = rev * _rbcrecsize
+        rec = array('c')
+        rec.fromstring(pack(_rbcrecfmt, reponode, branchidx))
+        self._rbcrevs[rbcrevidx:rbcrevidx + _rbcrecsize] = rec
+        return b, close
+
+    def write(self, repo):
+        """Save branch cache if it is dirty."""
+        if self._rbcnamescount < len(self._names):
+            try:
+                if self._rbcnamescount != 0:
+                    f = repo.vfs.open(_rbcnames, 'ab')
+                    if f.tell() == self._rbcsnameslen:
+                        f.write('\0')
+                    else:
+                        f.close()
+                        self._rbcnamescount = 0
+                        self._rbcrevslen = 0
+                if self._rbcnamescount == 0:
+                    f = repo.vfs.open(_rbcnames, 'wb')
+                f.write('\0'.join(encoding.fromlocal(b)
+                                  for b in self._names[self._rbcnamescount:]))
+                self._rbcsnameslen = f.tell()
+                f.close()
+            except (IOError, OSError, util.Abort), inst:
+                repo.ui.debug("couldn't write revision branch cache names: "
+                              "%s\n" % inst)
+                return
+            self._rbcnamescount = len(self._names)
+
+        start = self._rbcrevslen * _rbcrecsize
+        if start != len(self._rbcrevs):
+            self._rbcrevslen = min(len(repo.changelog),
+                                   len(self._rbcrevs) // _rbcrecsize)
+            try:
+                f = repo.vfs.open(_rbcrevs, 'ab')
+                if f.tell() != start:
+                    f.seek(start)
+                    f.truncate()
+                end = self._rbcrevslen * _rbcrecsize
+                f.write(self._rbcrevs[start:end])
+                f.close()
+            except (IOError, OSError, util.Abort), inst:
+                repo.ui.debug("couldn't write revision branch cache: %s\n" %
+                              inst)
+                return