strip: make query to get new bookmark target cheaper

The current query to get the new bookmark target for stripped revisions
involves multiple walks up the DAG, and is really expensive, taking over 2.5
seconds on a repository with over 400,000 changesets even if just one
changeset is being stripped.

A slightly simplified version of the current query is

max(heads(::<tostrip> - <tostrip>))

We make two observations here.

1. For any set s, max(heads(s)) == max(s). That is because revision numbers
   define a topological order, so that the element with the highest revision
   number in s will not have any children in s.

2. For any set s, max(::s - s) == max(parents(s) - s). In other words, the
   ancestor of s with the highest revision number not in s is a parent of one
   of the revs in s. Why? Because if it were an ancestor but not a parent of s,
   it would have a descendant that would be a parent of s. This descendant
   would have a higher revision number, leading to a contradiction.

Combining these two observations, we rewrite the revset query as

max(parents(<tostrip>) - <tostrip>)

The time complexity is now linear in the number of changesets being stripped.
For the above repository, the query now takes 0.1 seconds when one changeset
is stripped. This speeds up operations that use repair.strip, like the rebase
and strip commands.

mercurial/repair.py

@@ -1,201 +1,203 b''
 # repair.py - functions for repository repair for mercurial
 #
 # Copyright 2005, 2006 Chris Mason <[e-mail]>
 # Copyright 2007 Matt Mackall
 #
 # This software may be used and distributed according to the terms of the
 # GNU General Public License version 2 or any later version.
 
 from mercurial import changegroup
 from mercurial.node import short
 from mercurial.i18n import _
 import os
 import errno
 
 def _bundle(repo, bases, heads, node, suffix, compress=True):
     """create a bundle with the specified revisions as a backup"""
     cg = repo.changegroupsubset(bases, heads, 'strip')
     backupdir = repo.join("strip-backup")
     if not os.path.isdir(backupdir):
         os.mkdir(backupdir)
     name = os.path.join(backupdir, "%s-%s.hg" % (short(node), suffix))
     if compress:
         bundletype = "HG10BZ"
     else:
         bundletype = "HG10UN"
     return changegroup.writebundle(cg, name, bundletype)
 
 def _collectfiles(repo, striprev):
     """find out the filelogs affected by the strip"""
     files = set()
 
     for x in xrange(striprev, len(repo)):
         files.update(repo[x].files())
 
     return sorted(files)
 
 def _collectbrokencsets(repo, files, striprev):
     """return the changesets which will be broken by the truncation"""
     s = set()
     def collectone(revlog):
         linkgen = (revlog.linkrev(i) for i in revlog)
         # find the truncation point of the revlog
         for lrev in linkgen:
             if lrev >= striprev:
                 break
         # see if any revision after this point has a linkrev
         # less than striprev (those will be broken by strip)
         for lrev in linkgen:
             if lrev < striprev:
                 s.add(lrev)
 
     collectone(repo.manifest)
     for fname in files:
         collectone(repo.file(fname))
 
     return s
 
 def strip(ui, repo, nodelist, backup="all", topic='backup'):
     repo = repo.unfiltered()
     # It simplifies the logic around updating the branchheads cache if we only
     # have to consider the effect of the stripped revisions and not revisions
     # missing because the cache is out-of-date.
     repo.updatebranchcache()
 
     cl = repo.changelog
     # TODO handle undo of merge sets
     if isinstance(nodelist, str):
         nodelist = [nodelist]
     striplist = [cl.rev(node) for node in nodelist]
     striprev = min(striplist)
 
     # Generate set of branches who will have nodes stripped.
     striprevs = repo.revs("%ld::", striplist)
     stripbranches = set([repo[rev].branch() for rev in striprevs])
 
     # Set of potential new heads resulting from the strip.  The parents of any
     # node removed could be a new head because the node to be removed could have
     # been the only child of the parent.
     newheadrevs = repo.revs("parents(%ld::) - %ld::", striprevs, striprevs)
     newheadnodes = set([cl.node(rev) for rev in newheadrevs])
     newheadbranches = set([repo[rev].branch() for rev in newheadrevs])
 
     keeppartialbundle = backup == 'strip'
 
     # Some revisions with rev > striprev may not be descendants of striprev.
     # We have to find these revisions and put them in a bundle, so that
     # we can restore them after the truncations.
     # To create the bundle we use repo.changegroupsubset which requires
     # the list of heads and bases of the set of interesting revisions.
     # (head = revision in the set that has no descendant in the set;
     #  base = revision in the set that has no ancestor in the set)
     tostrip = set(striplist)
     for rev in striplist:
         for desc in cl.descendants([rev]):
             tostrip.add(desc)
 
     files = _collectfiles(repo, striprev)
     saverevs = _collectbrokencsets(repo, files, striprev)
 
     # compute heads
     saveheads = set(saverevs)
     for r in xrange(striprev + 1, len(cl)):
         if r not in tostrip:
             saverevs.add(r)
             saveheads.difference_update(cl.parentrevs(r))
             saveheads.add(r)
     saveheads = [cl.node(r) for r in saveheads]
 
     # compute base nodes
     if saverevs:
         descendants = set(cl.descendants(saverevs))
         saverevs.difference_update(descendants)
     savebases = [cl.node(r) for r in saverevs]
     stripbases = [cl.node(r) for r in tostrip]
-    newbmtarget = repo.revs('sort(heads((::%ld) - (%ld)), -rev)',
-                            tostrip, tostrip)
+
+    # For a set s, max(parents(s) - s) is the same as max(heads(::s - s)), but
+    # is much faster
+    newbmtarget = repo.revs('max(parents(%ld) - (%ld))', tostrip, tostrip)
     if newbmtarget:
         newbmtarget = repo[newbmtarget[0]].node()
     else:
         newbmtarget = '.'
 
     bm = repo._bookmarks
     updatebm = []
     for m in bm:
         rev = repo[bm[m]].rev()
         if rev in tostrip:
             updatebm.append(m)
 
     # create a changegroup for all the branches we need to keep
     backupfile = None
     if backup == "all":
         backupfile = _bundle(repo, stripbases, cl.heads(), node, topic)
         repo.ui.status(_("saved backup bundle to %s\n") % backupfile)
     if saveheads or savebases:
         # do not compress partial bundle if we remove it from disk later
         chgrpfile = _bundle(repo, savebases, saveheads, node, 'temp',
                             compress=keeppartialbundle)
 
     mfst = repo.manifest
 
     tr = repo.transaction("strip")
     offset = len(tr.entries)
 
     try:
         tr.startgroup()
         cl.strip(striprev, tr)
         mfst.strip(striprev, tr)
         for fn in files:
             repo.file(fn).strip(striprev, tr)
         tr.endgroup()
 
         try:
             for i in xrange(offset, len(tr.entries)):
                 file, troffset, ignore = tr.entries[i]
                 repo.sopener(file, 'a').truncate(troffset)
             tr.close()
         except: # re-raises
             tr.abort()
             raise
 
         if saveheads or savebases:
             ui.note(_("adding branch\n"))
             f = open(chgrpfile, "rb")
             gen = changegroup.readbundle(f, chgrpfile)
             if not repo.ui.verbose:
                 # silence internal shuffling chatter
                 repo.ui.pushbuffer()
             repo.addchangegroup(gen, 'strip', 'bundle:' + chgrpfile, True)
             if not repo.ui.verbose:
                 repo.ui.popbuffer()
             f.close()
             if not keeppartialbundle:
                 os.unlink(chgrpfile)
 
         # remove undo files
         for undofile in repo.undofiles():
             try:
                 os.unlink(undofile)
             except OSError, e:
                 if e.errno != errno.ENOENT:
                     ui.warn(_('error removing %s: %s\n') % (undofile, str(e)))
 
         for m in updatebm:
             bm[m] = repo[newbmtarget].node()
         bm.write()
     except: # re-raises
         if backupfile:
             ui.warn(_("strip failed, full bundle stored in '%s'\n")
                     % backupfile)
         elif saveheads:
             ui.warn(_("strip failed, partial bundle stored in '%s'\n")
                     % chgrpfile)
         raise
 
     if len(stripbranches) == 1 and len(newheadbranches) == 1 \
             and stripbranches == newheadbranches:
         repo.destroyed(newheadnodes)
     else:
         # Multiple branches involved in strip. Will allow branchcache to become
         # invalid and later on rebuilt from scratch
         repo.destroyed()