rebase: clarify comment about merge ancestor when rebasing merges

The code for picking a merge ancestor when rebasing merges had a long and
incorrect comment.

The comment would perhaps have been fine as commit message but does
not make the code more readable or maintainable and is a bad
substitute for correct and readable code.

The correct essense of the comment is quite trivial: a merge of an ancestor of
the rebase destination and an 'outside' revision can be rebased as if it was a
linear change, using 'destination ancestor parent' as base and pretty much
ignoring the 'outside' revision.

The code path where the comment is placed is however also used for other kinds
of merge rebases. The comment is thus not really correct and not helpful. I
think it would be better to drop the comment and rewrite the code.

hgext/rebase.py

@@ -1,1064 +1,1074
 # rebase.py - rebasing feature for mercurial
 #
 # Copyright 2008 Stefano Tortarolo <stefano.tortarolo at gmail dot com>
 #
 # This software may be used and distributed according to the terms of the
 # GNU General Public License version 2 or any later version.
 
 '''command to move sets of revisions to a different ancestor
 
 This extension lets you rebase changesets in an existing Mercurial
 repository.
 
 For more information:
 http://mercurial.selenic.com/wiki/RebaseExtension
 '''
 
 from mercurial import hg, util, repair, merge, cmdutil, commands, bookmarks
 from mercurial import extensions, patch, scmutil, phases, obsolete, error
 from mercurial import copies
 from mercurial.commands import templateopts
 from mercurial.node import nullrev, nullid, hex, short
 from mercurial.lock import release
 from mercurial.i18n import _
 import os, errno
 
 revtodo = -1
 nullmerge = -2
 revignored = -3
 
 cmdtable = {}
 command = cmdutil.command(cmdtable)
 testedwith = 'internal'
 
 def _savegraft(ctx, extra):
     s = ctx.extra().get('source', None)
     if s is not None:
         extra['source'] = s
 
 def _savebranch(ctx, extra):
     extra['branch'] = ctx.branch()
 
 def _makeextrafn(copiers):
     """make an extrafn out of the given copy-functions.
 
     A copy function takes a context and an extra dict, and mutates the
     extra dict as needed based on the given context.
     """
     def extrafn(ctx, extra):
         for c in copiers:
             c(ctx, extra)
     return extrafn
 
 @command('rebase',
     [('s', 'source', '',
      _('rebase the specified changeset and descendants'), _('REV')),
     ('b', 'base', '',
      _('rebase everything from branching point of specified changeset'),
      _('REV')),
     ('r', 'rev', [],
      _('rebase these revisions'),
      _('REV')),
     ('d', 'dest', '',
      _('rebase onto the specified changeset'), _('REV')),
     ('', 'collapse', False, _('collapse the rebased changesets')),
     ('m', 'message', '',
      _('use text as collapse commit message'), _('TEXT')),
     ('e', 'edit', False, _('invoke editor on commit messages')),
     ('l', 'logfile', '',
      _('read collapse commit message from file'), _('FILE')),
     ('', 'keep', False, _('keep original changesets')),
     ('', 'keepbranches', False, _('keep original branch names')),
     ('D', 'detach', False, _('(DEPRECATED)')),
     ('i', 'interactive', False, _('(DEPRECATED)')),
     ('t', 'tool', '', _('specify merge tool')),
     ('c', 'continue', False, _('continue an interrupted rebase')),
     ('a', 'abort', False, _('abort an interrupted rebase'))] +
      templateopts,
     _('[-s REV | -b REV] [-d REV] [OPTION]'))
 def rebase(ui, repo, **opts):
     """move changeset (and descendants) to a different branch
 
     Rebase uses repeated merging to graft changesets from one part of
     history (the source) onto another (the destination). This can be
     useful for linearizing *local* changes relative to a master
     development tree.
 
     You should not rebase changesets that have already been shared
     with others. Doing so will force everybody else to perform the
     same rebase or they will end up with duplicated changesets after
     pulling in your rebased changesets.
 
     In its default configuration, Mercurial will prevent you from
     rebasing published changes. See :hg:`help phases` for details.
 
     If you don't specify a destination changeset (``-d/--dest``),
     rebase uses the current branch tip as the destination. (The
     destination changeset is not modified by rebasing, but new
     changesets are added as its descendants.)
 
     You can specify which changesets to rebase in two ways: as a
     "source" changeset or as a "base" changeset. Both are shorthand
     for a topologically related set of changesets (the "source
     branch"). If you specify source (``-s/--source``), rebase will
     rebase that changeset and all of its descendants onto dest. If you
     specify base (``-b/--base``), rebase will select ancestors of base
     back to but not including the common ancestor with dest. Thus,
     ``-b`` is less precise but more convenient than ``-s``: you can
     specify any changeset in the source branch, and rebase will select
     the whole branch. If you specify neither ``-s`` nor ``-b``, rebase
     uses the parent of the working directory as the base.
 
     For advanced usage, a third way is available through the ``--rev``
     option. It allows you to specify an arbitrary set of changesets to
     rebase. Descendants of revs you specify with this option are not
     automatically included in the rebase.
 
     By default, rebase recreates the changesets in the source branch
     as descendants of dest and then destroys the originals. Use
     ``--keep`` to preserve the original source changesets. Some
     changesets in the source branch (e.g. merges from the destination
     branch) may be dropped if they no longer contribute any change.
 
     One result of the rules for selecting the destination changeset
     and source branch is that, unlike ``merge``, rebase will do
     nothing if you are at the branch tip of a named branch
     with two heads. You need to explicitly specify source and/or
     destination (or ``update`` to the other head, if it's the head of
     the intended source branch).
 
     If a rebase is interrupted to manually resolve a merge, it can be
     continued with --continue/-c or aborted with --abort/-a.
 
     .. container:: verbose
 
       Examples:
 
       - move "local changes" (current commit back to branching point)
         to the current branch tip after a pull::
 
           hg rebase
 
       - move a single changeset to the stable branch::
 
           hg rebase -r 5f493448 -d stable
 
       - splice a commit and all its descendants onto another part of history::
 
           hg rebase --source c0c3 --dest 4cf9
 
       - rebase everything on a branch marked by a bookmark onto the
         default branch::
 
           hg rebase --base myfeature --dest default
 
       - collapse a sequence of changes into a single commit::
 
           hg rebase --collapse -r 1520:1525 -d .
 
       - move a named branch while preserving its name::
 
           hg rebase -r "branch(featureX)" -d 1.3 --keepbranches
 
     Returns 0 on success, 1 if nothing to rebase or there are
     unresolved conflicts.
 
     """
     originalwd = target = None
     activebookmark = None
     external = nullrev
     state = {}
     skipped = set()
     targetancestors = set()
 
 
     lock = wlock = None
     try:
         wlock = repo.wlock()
         lock = repo.lock()
 
         # Validate input and define rebasing points
         destf = opts.get('dest', None)
         srcf = opts.get('source', None)
         basef = opts.get('base', None)
         revf = opts.get('rev', [])
         contf = opts.get('continue')
         abortf = opts.get('abort')
         collapsef = opts.get('collapse', False)
         collapsemsg = cmdutil.logmessage(ui, opts)
         e = opts.get('extrafn') # internal, used by e.g. hgsubversion
         extrafns = [_savegraft]
         if e:
             extrafns = [e]
         keepf = opts.get('keep', False)
         keepbranchesf = opts.get('keepbranches', False)
         # keepopen is not meant for use on the command line, but by
         # other extensions
         keepopen = opts.get('keepopen', False)
 
         if opts.get('interactive'):
             msg = _("interactive history editing is supported by the "
                     "'histedit' extension (see 'hg help histedit')")
             raise util.Abort(msg)
 
         if collapsemsg and not collapsef:
             raise util.Abort(
                 _('message can only be specified with collapse'))
 
         if contf or abortf:
             if contf and abortf:
                 raise util.Abort(_('cannot use both abort and continue'))
             if collapsef:
                 raise util.Abort(
                     _('cannot use collapse with continue or abort'))
             if srcf or basef or destf:
                 raise util.Abort(
                     _('abort and continue do not allow specifying revisions'))
             if opts.get('tool', False):
                 ui.warn(_('tool option will be ignored\n'))
 
             try:
                 (originalwd, target, state, skipped, collapsef, keepf,
                  keepbranchesf, external, activebookmark) = restorestatus(repo)
             except error.RepoLookupError:
                 if abortf:
                     clearstatus(repo)
                     repo.ui.warn(_('rebase aborted (no revision is removed,'
                                    ' only broken state is cleared)\n'))
                     return 0
                 else:
                     msg = _('cannot continue inconsistent rebase')
                     hint = _('use "hg rebase --abort" to clear broken state')
                     raise util.Abort(msg, hint=hint)
             if abortf:
                 return abort(repo, originalwd, target, state)
         else:
             if srcf and basef:
                 raise util.Abort(_('cannot specify both a '
                                    'source and a base'))
             if revf and basef:
                 raise util.Abort(_('cannot specify both a '
                                    'revision and a base'))
             if revf and srcf:
                 raise util.Abort(_('cannot specify both a '
                                    'revision and a source'))
 
             cmdutil.checkunfinished(repo)
             cmdutil.bailifchanged(repo)
 
             if not destf:
                 # Destination defaults to the latest revision in the
                 # current branch
                 branch = repo[None].branch()
                 dest = repo[branch]
             else:
                 dest = scmutil.revsingle(repo, destf)
 
             if revf:
                 rebaseset = scmutil.revrange(repo, revf)
                 if not rebaseset:
                     ui.status(_('empty "rev" revision set - '
                                 'nothing to rebase\n'))
                     return 1
             elif srcf:
                 src = scmutil.revrange(repo, [srcf])
                 if not src:
                     ui.status(_('empty "source" revision set - '
                                 'nothing to rebase\n'))
                     return 1
                 rebaseset = repo.revs('(%ld)::', src)
                 assert rebaseset
             else:
                 base = scmutil.revrange(repo, [basef or '.'])
                 if not base:
                     ui.status(_('empty "base" revision set - '
                                 "can't compute rebase set\n"))
                     return 1
                 commonanc = repo.revs('ancestor(%ld, %d)', base, dest).first()
                 if commonanc is not None:
                     rebaseset = repo.revs('(%d::(%ld) - %d)::',
                                           commonanc, base, commonanc)
                 else:
                     rebaseset = []
 
                 if not rebaseset:
                     # transform to list because smartsets are not comparable to
                     # lists. This should be improved to honor laziness of
                     # smartset.
                     if list(base) == [dest.rev()]:
                         if basef:
                             ui.status(_('nothing to rebase - %s is both "base"'
                                         ' and destination\n') % dest)
                         else:
                             ui.status(_('nothing to rebase - working directory '
                                         'parent is also destination\n'))
                     elif not repo.revs('%ld - ::%d', base, dest):
                         if basef:
                             ui.status(_('nothing to rebase - "base" %s is '
                                         'already an ancestor of destination '
                                         '%s\n') %
                                       ('+'.join(str(repo[r]) for r in base),
                                        dest))
                         else:
                             ui.status(_('nothing to rebase - working '
                                         'directory parent is already an '
                                         'ancestor of destination %s\n') % dest)
                     else: # can it happen?
                         ui.status(_('nothing to rebase from %s to %s\n') %
                                   ('+'.join(str(repo[r]) for r in base), dest))
                     return 1
 
             allowunstable = obsolete.isenabled(repo, obsolete.allowunstableopt)
             if (not (keepf or allowunstable)
                   and repo.revs('first(children(%ld) - %ld)',
                                 rebaseset, rebaseset)):
                 raise util.Abort(
                     _("can't remove original changesets with"
                       " unrebased descendants"),
                     hint=_('use --keep to keep original changesets'))
 
             result = buildstate(repo, dest, rebaseset, collapsef)
             if not result:
                 # Empty state built, nothing to rebase
                 ui.status(_('nothing to rebase\n'))
                 return 1
 
             root = min(rebaseset)
             if not keepf and not repo[root].mutable():
                 raise util.Abort(_("can't rebase immutable changeset %s")
                                  % repo[root],
                                  hint=_('see hg help phases for details'))
 
             originalwd, target, state = result
             if collapsef:
                 targetancestors = repo.changelog.ancestors([target],
                                                            inclusive=True)
                 external = externalparent(repo, state, targetancestors)
 
             if dest.closesbranch() and not keepbranchesf:
                 ui.status(_('reopening closed branch head %s\n') % dest)
 
         if keepbranchesf:
             # insert _savebranch at the start of extrafns so if
             # there's a user-provided extrafn it can clobber branch if
             # desired
             extrafns.insert(0, _savebranch)
             if collapsef:
                 branches = set()
                 for rev in state:
                     branches.add(repo[rev].branch())
                     if len(branches) > 1:
                         raise util.Abort(_('cannot collapse multiple named '
                             'branches'))
 
         # Rebase
         if not targetancestors:
             targetancestors = repo.changelog.ancestors([target], inclusive=True)
 
         # Keep track of the current bookmarks in order to reset them later
         currentbookmarks = repo._bookmarks.copy()
         activebookmark = activebookmark or repo._bookmarkcurrent
         if activebookmark:
             bookmarks.unsetcurrent(repo)
 
         extrafn = _makeextrafn(extrafns)
 
         sortedstate = sorted(state)
         total = len(sortedstate)
         pos = 0
         for rev in sortedstate:
             ctx = repo[rev]
             desc = '%d:%s "%s"' % (ctx.rev(), ctx,
                                    ctx.description().split('\n', 1)[0])
             names = repo.nodetags(ctx.node()) + repo.nodebookmarks(ctx.node())
             if names:
                 desc += ' (%s)' % ' '.join(names)
             pos += 1
             if state[rev] == revtodo:
                 ui.status(_('rebasing %s\n') % desc)
                 ui.progress(_("rebasing"), pos, ("%d:%s" % (rev, ctx)),
                             _('changesets'), total)
                 p1, p2, base = defineparents(repo, rev, target, state,
                                              targetancestors)
                 storestatus(repo, originalwd, target, state, collapsef, keepf,
                             keepbranchesf, external, activebookmark)
                 if len(repo.parents()) == 2:
                     repo.ui.debug('resuming interrupted rebase\n')
                 else:
                     try:
                         ui.setconfig('ui', 'forcemerge', opts.get('tool', ''),
                                      'rebase')
                         stats = rebasenode(repo, rev, p1, base, state,
                                            collapsef, target)
                         if stats and stats[3] > 0:
                             raise error.InterventionRequired(
                                 _('unresolved conflicts (see hg '
                                   'resolve, then hg rebase --continue)'))
                     finally:
                         ui.setconfig('ui', 'forcemerge', '', 'rebase')
                 if not collapsef:
                     merging = p2 != nullrev
                     editform = cmdutil.mergeeditform(merging, 'rebase')
                     editor = cmdutil.getcommiteditor(editform=editform, **opts)
                     newnode = concludenode(repo, rev, p1, p2, extrafn=extrafn,
                                            editor=editor)
                 else:
                     # Skip commit if we are collapsing
                     repo.dirstate.beginparentchange()
                     repo.setparents(repo[p1].node())
                     repo.dirstate.endparentchange()
                     newnode = None
                 # Update the state
                 if newnode is not None:
                     state[rev] = repo[newnode].rev()
                     ui.debug('rebased as %s\n' % short(newnode))
                 else:
                     ui.warn(_('note: rebase of %d:%s created no changes '
                               'to commit\n') % (rev, ctx))
                     if not collapsef:
                         skipped.add(rev)
                     state[rev] = p1
                     ui.debug('next revision set to %s\n' % p1)
             elif state[rev] == nullmerge:
                 ui.debug('ignoring null merge rebase of %s\n' % rev)
             elif state[rev] == revignored:
                 ui.status(_('not rebasing ignored %s\n') % desc)
             else:
                 ui.status(_('already rebased %s as %s\n') %
                           (desc, repo[state[rev]]))
 
         ui.progress(_('rebasing'), None)
         ui.note(_('rebase merging completed\n'))
 
         if collapsef and not keepopen:
             p1, p2, _base = defineparents(repo, min(state), target,
                                           state, targetancestors)
             editopt = opts.get('edit')
             editform = 'rebase.collapse'
             if collapsemsg:
                 commitmsg = collapsemsg
             else:
                 commitmsg = 'Collapsed revision'
                 for rebased in state:
                     if rebased not in skipped and state[rebased] > nullmerge:
                         commitmsg += '\n* %s' % repo[rebased].description()
                 editopt = True
             editor = cmdutil.getcommiteditor(edit=editopt, editform=editform)
             newnode = concludenode(repo, rev, p1, external, commitmsg=commitmsg,
                                    extrafn=extrafn, editor=editor)
             if newnode is None:
                 newrev = target
             else:
                 newrev = repo[newnode].rev()
             for oldrev in state.iterkeys():
                 if state[oldrev] > nullmerge:
                     state[oldrev] = newrev
 
         if 'qtip' in repo.tags():
             updatemq(repo, state, skipped, **opts)
 
         if currentbookmarks:
             # Nodeids are needed to reset bookmarks
             nstate = {}
             for k, v in state.iteritems():
                 if v > nullmerge:
                     nstate[repo[k].node()] = repo[v].node()
             # XXX this is the same as dest.node() for the non-continue path --
             # this should probably be cleaned up
             targetnode = repo[target].node()
 
         # restore original working directory
         # (we do this before stripping)
         newwd = state.get(originalwd, originalwd)
         if newwd < 0:
             # original directory is a parent of rebase set root or ignored
             newwd = originalwd
         if newwd not in [c.rev() for c in repo[None].parents()]:
             ui.note(_("update back to initial working directory parent\n"))
             hg.updaterepo(repo, newwd, False)
 
         if not keepf:
             collapsedas = None
             if collapsef:
                 collapsedas = newnode
             clearrebased(ui, repo, state, skipped, collapsedas)
 
         if currentbookmarks:
             updatebookmarks(repo, targetnode, nstate, currentbookmarks)
             if activebookmark not in repo._bookmarks:
                 # active bookmark was divergent one and has been deleted
                 activebookmark = None
 
         clearstatus(repo)
         ui.note(_("rebase completed\n"))
         util.unlinkpath(repo.sjoin('undo'), ignoremissing=True)
         if skipped:
             ui.note(_("%d revisions have been skipped\n") % len(skipped))
 
         if (activebookmark and
             repo['.'].node() == repo._bookmarks[activebookmark]):
                 bookmarks.setcurrent(repo, activebookmark)
 
     finally:
         release(lock, wlock)
 
 def externalparent(repo, state, targetancestors):
     """Return the revision that should be used as the second parent
     when the revisions in state is collapsed on top of targetancestors.
     Abort if there is more than one parent.
     """
     parents = set()
     source = min(state)
     for rev in state:
         if rev == source:
             continue
         for p in repo[rev].parents():
             if (p.rev() not in state
                         and p.rev() not in targetancestors):
                 parents.add(p.rev())
     if not parents:
         return nullrev
     if len(parents) == 1:
         return parents.pop()
     raise util.Abort(_('unable to collapse on top of %s, there is more '
                        'than one external parent: %s') %
                      (max(targetancestors),
                       ', '.join(str(p) for p in sorted(parents))))
 
 def concludenode(repo, rev, p1, p2, commitmsg=None, editor=None, extrafn=None):
     '''Commit the wd changes with parents p1 and p2. Reuse commit info from rev
     but also store useful information in extra.
     Return node of committed revision.'''
     try:
         repo.dirstate.beginparentchange()
         repo.setparents(repo[p1].node(), repo[p2].node())
         repo.dirstate.endparentchange()
         ctx = repo[rev]
         if commitmsg is None:
             commitmsg = ctx.description()
         extra = {'rebase_source': ctx.hex()}
         if extrafn:
             extrafn(ctx, extra)
 
         backup = repo.ui.backupconfig('phases', 'new-commit')
         try:
             targetphase = max(ctx.phase(), phases.draft)
             repo.ui.setconfig('phases', 'new-commit', targetphase, 'rebase')
             # Commit might fail if unresolved files exist
             newnode = repo.commit(text=commitmsg, user=ctx.user(),
                                   date=ctx.date(), extra=extra, editor=editor)
         finally:
             repo.ui.restoreconfig(backup)
 
         repo.dirstate.setbranch(repo[newnode].branch())
         return newnode
     except util.Abort:
         # Invalidate the previous setparents
         repo.dirstate.invalidate()
         raise
 
 def rebasenode(repo, rev, p1, base, state, collapse, target):
     'Rebase a single revision rev on top of p1 using base as merge ancestor'
     # Merge phase
     # Update to target and merge it with local
     if repo['.'].rev() != p1:
         repo.ui.debug(" update to %d:%s\n" % (p1, repo[p1]))
         merge.update(repo, p1, False, True, False)
     else:
         repo.ui.debug(" already in target\n")
     repo.dirstate.write()
     repo.ui.debug(" merge against %d:%s\n" % (rev, repo[rev]))
     if base is not None:
         repo.ui.debug("   detach base %d:%s\n" % (base, repo[base]))
     # When collapsing in-place, the parent is the common ancestor, we
     # have to allow merging with it.
     stats = merge.update(repo, rev, True, True, False, base, collapse,
                         labels=['dest', 'source'])
     if collapse:
         copies.duplicatecopies(repo, rev, target)
     else:
         # If we're not using --collapse, we need to
         # duplicate copies between the revision we're
         # rebasing and its first parent, but *not*
         # duplicate any copies that have already been
         # performed in the destination.
         p1rev = repo[rev].p1().rev()
         copies.duplicatecopies(repo, rev, p1rev, skiprev=target)
     return stats
 
 def nearestrebased(repo, rev, state):
     """return the nearest ancestors of rev in the rebase result"""
     rebased = [r for r in state if state[r] > nullmerge]
     candidates = repo.revs('max(%ld  and (::%d))', rebased, rev)
     if candidates:
         return state[candidates.first()]
     else:
         return None
 
 def defineparents(repo, rev, target, state, targetancestors):
     'Return the new parent relationship of the revision that will be rebased'
     parents = repo[rev].parents()
     p1 = p2 = nullrev
 
     p1n = parents[0].rev()
     if p1n in targetancestors:
         p1 = target
     elif p1n in state:
         if state[p1n] == nullmerge:
             p1 = target
         elif state[p1n] == revignored:
             p1 = nearestrebased(repo, p1n, state)
             if p1 is None:
                 p1 = target
         else:
             p1 = state[p1n]
     else: # p1n external
         p1 = target
         p2 = p1n
 
     if len(parents) == 2 and parents[1].rev() not in targetancestors:
         p2n = parents[1].rev()
         # interesting second parent
         if p2n in state:
             if p1 == target: # p1n in targetancestors or external
                 p1 = state[p2n]
             elif state[p2n] == revignored:
                 p2 = nearestrebased(repo, p2n, state)
                 if p2 is None:
                     # no ancestors rebased yet, detach
                     p2 = target
             else:
                 p2 = state[p2n]
         else: # p2n external
             if p2 != nullrev: # p1n external too => rev is a merged revision
                 raise util.Abort(_('cannot use revision %d as base, result '
                         'would have 3 parents') % rev)
             p2 = p2n
     repo.ui.debug(" future parents are %d and %d\n" %
                             (repo[p1].rev(), repo[p2].rev()))
 
     if rev == min(state):
         # Case (1) initial changeset of a non-detaching rebase.
         # Let the merge mechanism find the base itself.
         base = None
     elif not repo[rev].p2():
         # Case (2) detaching the node with a single parent, use this parent
         base = repo[rev].p1().rev()
     else:
-        # In case of merge, we need to pick the right parent as merge base.
+        # Assuming there is a p1, this is the case where there also is a p2.
+        # We are thus rebasing a merge and need to pick the right merge base.
         #
         # Imagine we have:
-        # - M: currently rebase revision in this step
+        # - M: current rebase revision in this step
         # - A: one parent of M
-        # - B: second parent of M
+        # - B: other parent of M
         # - D: destination of this merge step (p1 var)
         #
-        # If we are rebasing on D, D is the successors of A or B. The right
-        # merge base is the one D succeed to. We pretend it is B for the rest
-        # of this comment
+        # Consider the case where D is a descendant of A or B and the other is
+        # 'outside'. In this case, the right merge base is the D ancestor.
+        #
+        # An informal proof, assuming A is 'outside' and B is the D ancestor:
         #
         # If we pick B as the base, the merge involves:
         # - changes from B to M (actual changeset payload)
         # - changes from B to D (induced by rebase) as D is a rebased
         #   version of B)
         # Which exactly represent the rebase operation.
         #
-        # If we pick the A as the base, the merge involves
+        # If we pick A as the base, the merge involves:
         # - changes from A to M (actual changeset payload)
         # - changes from A to D (with include changes between unrelated A and B
         #   plus changes induced by rebase)
         # Which does not represent anything sensible and creates a lot of
-        # conflicts.
+        # conflicts. A is thus not the right choice - B is.
+        #
+        # Note: The base found in this 'proof' is only correct in the specified
+        # case. This base does not make sense if is not D a descendant of A or B
+        # or if the other is not parent 'outside' (especially not if the other
+        # parent has been rebased). The current implementation does not
+        # make it feasible to consider different cases separately. In these
+        # other cases we currently just leave it to the user to correctly
+        # resolve an impossible merge using a wrong ancestor.
         for p in repo[rev].parents():
             if state.get(p.rev()) == p1:
                 base = p.rev()
                 break
         else: # fallback when base not found
             base = None
 
             # Raise because this function is called wrong (see issue 4106)
             raise AssertionError('no base found to rebase on '
                                  '(defineparents called wrong)')
     return p1, p2, base
 
 def isagitpatch(repo, patchname):
     'Return true if the given patch is in git format'
     mqpatch = os.path.join(repo.mq.path, patchname)
     for line in patch.linereader(file(mqpatch, 'rb')):
         if line.startswith('diff --git'):
             return True
     return False
 
 def updatemq(repo, state, skipped, **opts):
     'Update rebased mq patches - finalize and then import them'
     mqrebase = {}
     mq = repo.mq
     original_series = mq.fullseries[:]
     skippedpatches = set()
 
     for p in mq.applied:
         rev = repo[p.node].rev()
         if rev in state:
             repo.ui.debug('revision %d is an mq patch (%s), finalize it.\n' %
                                         (rev, p.name))
             mqrebase[rev] = (p.name, isagitpatch(repo, p.name))
         else:
             # Applied but not rebased, not sure this should happen
             skippedpatches.add(p.name)
 
     if mqrebase:
         mq.finish(repo, mqrebase.keys())
 
         # We must start import from the newest revision
         for rev in sorted(mqrebase, reverse=True):
             if rev not in skipped:
                 name, isgit = mqrebase[rev]
                 repo.ui.note(_('updating mq patch %s to %s:%s\n') %
                              (name, state[rev], repo[state[rev]]))
                 mq.qimport(repo, (), patchname=name, git=isgit,
                                 rev=[str(state[rev])])
             else:
                 # Rebased and skipped
                 skippedpatches.add(mqrebase[rev][0])
 
         # Patches were either applied and rebased and imported in
         # order, applied and removed or unapplied. Discard the removed
         # ones while preserving the original series order and guards.
         newseries = [s for s in original_series
                      if mq.guard_re.split(s, 1)[0] not in skippedpatches]
         mq.fullseries[:] = newseries
         mq.seriesdirty = True
         mq.savedirty()
 
 def updatebookmarks(repo, targetnode, nstate, originalbookmarks):
     'Move bookmarks to their correct changesets, and delete divergent ones'
     marks = repo._bookmarks
     for k, v in originalbookmarks.iteritems():
         if v in nstate:
             # update the bookmarks for revs that have moved
             marks[k] = nstate[v]
             bookmarks.deletedivergent(repo, [targetnode], k)
 
     marks.write()
 
 def storestatus(repo, originalwd, target, state, collapse, keep, keepbranches,
                 external, activebookmark):
     'Store the current status to allow recovery'
     f = repo.opener("rebasestate", "w")
     f.write(repo[originalwd].hex() + '\n')
     f.write(repo[target].hex() + '\n')
     f.write(repo[external].hex() + '\n')
     f.write('%d\n' % int(collapse))
     f.write('%d\n' % int(keep))
     f.write('%d\n' % int(keepbranches))
     f.write('%s\n' % (activebookmark or ''))
     for d, v in state.iteritems():
         oldrev = repo[d].hex()
         if v >= 0:
             newrev = repo[v].hex()
         elif v == revtodo:
             # To maintain format compatibility, we have to use nullid.
             # Please do remove this special case when upgrading the format.
             newrev = hex(nullid)
         else:
             newrev = v
         f.write("%s:%s\n" % (oldrev, newrev))
     f.close()
     repo.ui.debug('rebase status stored\n')
 
 def clearstatus(repo):
     'Remove the status files'
     util.unlinkpath(repo.join("rebasestate"), ignoremissing=True)
 
 def restorestatus(repo):
     'Restore a previously stored status'
     try:
         keepbranches = None
         target = None
         collapse = False
         external = nullrev
         activebookmark = None
         state = {}
         f = repo.opener("rebasestate")
         for i, l in enumerate(f.read().splitlines()):
             if i == 0:
                 originalwd = repo[l].rev()
             elif i == 1:
                 target = repo[l].rev()
             elif i == 2:
                 external = repo[l].rev()
             elif i == 3:
                 collapse = bool(int(l))
             elif i == 4:
                 keep = bool(int(l))
             elif i == 5:
                 keepbranches = bool(int(l))
             elif i == 6 and not (len(l) == 81 and ':' in l):
                 # line 6 is a recent addition, so for backwards compatibility
                 # check that the line doesn't look like the oldrev:newrev lines
                 activebookmark = l
             else:
                 oldrev, newrev = l.split(':')
                 if newrev in (str(nullmerge), str(revignored)):
                     state[repo[oldrev].rev()] = int(newrev)
                 elif newrev == nullid:
                     state[repo[oldrev].rev()] = revtodo
                     # Legacy compat special case
                 else:
                     state[repo[oldrev].rev()] = repo[newrev].rev()
 
         if keepbranches is None:
             raise util.Abort(_('.hg/rebasestate is incomplete'))
 
         skipped = set()
         # recompute the set of skipped revs
         if not collapse:
             seen = set([target])
             for old, new in sorted(state.items()):
                 if new != revtodo and new in seen:
                     skipped.add(old)
                 seen.add(new)
         repo.ui.debug('computed skipped revs: %s\n' %
                       (' '.join(str(r) for r in sorted(skipped)) or None))
         repo.ui.debug('rebase status resumed\n')
         return (originalwd, target, state, skipped,
                 collapse, keep, keepbranches, external, activebookmark)
     except IOError, err:
         if err.errno != errno.ENOENT:
             raise
         raise util.Abort(_('no rebase in progress'))
 
 def inrebase(repo, originalwd, state):
     '''check whether the working dir is in an interrupted rebase'''
     parents = [p.rev() for p in repo.parents()]
     if originalwd in parents:
         return True
 
     for newrev in state.itervalues():
         if newrev in parents:
             return True
 
     return False
 
 def abort(repo, originalwd, target, state):
     'Restore the repository to its original state'
     dstates = [s for s in state.values() if s >= 0]
     immutable = [d for d in dstates if not repo[d].mutable()]
     cleanup = True
     if immutable:
         repo.ui.warn(_("warning: can't clean up immutable changesets %s\n")
                      % ', '.join(str(repo[r]) for r in immutable),
                      hint=_('see hg help phases for details'))
         cleanup = False
 
     descendants = set()
     if dstates:
         descendants = set(repo.changelog.descendants(dstates))
     if descendants - set(dstates):
         repo.ui.warn(_("warning: new changesets detected on target branch, "
                        "can't strip\n"))
         cleanup = False
 
     if cleanup:
         # Update away from the rebase if necessary
         if inrebase(repo, originalwd, state):
             merge.update(repo, originalwd, False, True, False)
 
         # Strip from the first rebased revision
         rebased = filter(lambda x: x >= 0 and x != target, state.values())
         if rebased:
             strippoints = [c.node()  for c in repo.set('roots(%ld)', rebased)]
             # no backup of rebased cset versions needed
             repair.strip(repo.ui, repo, strippoints)
 
     clearstatus(repo)
     repo.ui.warn(_('rebase aborted\n'))
     return 0
 
 def buildstate(repo, dest, rebaseset, collapse):
     '''Define which revisions are going to be rebased and where
 
     repo: repo
     dest: context
     rebaseset: set of rev
     '''
 
     # This check isn't strictly necessary, since mq detects commits over an
     # applied patch. But it prevents messing up the working directory when
     # a partially completed rebase is blocked by mq.
     if 'qtip' in repo.tags() and (dest.node() in
                             [s.node for s in repo.mq.applied]):
         raise util.Abort(_('cannot rebase onto an applied mq patch'))
 
     roots = list(repo.set('roots(%ld)', rebaseset))
     if not roots:
         raise util.Abort(_('no matching revisions'))
     roots.sort()
     state = {}
     detachset = set()
     for root in roots:
         commonbase = root.ancestor(dest)
         if commonbase == root:
             raise util.Abort(_('source is ancestor of destination'))
         if commonbase == dest:
             samebranch = root.branch() == dest.branch()
             if not collapse and samebranch and root in dest.children():
                 repo.ui.debug('source is a child of destination\n')
                 return None
 
         repo.ui.debug('rebase onto %d starting from %s\n' % (dest, root))
         state.update(dict.fromkeys(rebaseset, revtodo))
         # Rebase tries to turn <dest> into a parent of <root> while
         # preserving the number of parents of rebased changesets:
         #
         # - A changeset with a single parent will always be rebased as a
         #   changeset with a single parent.
         #
         # - A merge will be rebased as merge unless its parents are both
         #   ancestors of <dest> or are themselves in the rebased set and
         #   pruned while rebased.
         #
         # If one parent of <root> is an ancestor of <dest>, the rebased
         # version of this parent will be <dest>. This is always true with
         # --base option.
         #
         # Otherwise, we need to *replace* the original parents with
         # <dest>. This "detaches" the rebased set from its former location
         # and rebases it onto <dest>. Changes introduced by ancestors of
         # <root> not common with <dest> (the detachset, marked as
         # nullmerge) are "removed" from the rebased changesets.
         #
         # - If <root> has a single parent, set it to <dest>.
         #
         # - If <root> is a merge, we cannot decide which parent to
         #   replace, the rebase operation is not clearly defined.
         #
         # The table below sums up this behavior:
         #
         # +------------------+----------------------+-------------------------+
         # |                  |     one parent       |  merge                  |
         # +------------------+----------------------+-------------------------+
         # | parent in        | new parent is <dest> | parents in ::<dest> are |
         # | ::<dest>         |                      | remapped to <dest>      |
         # +------------------+----------------------+-------------------------+
         # | unrelated source | new parent is <dest> | ambiguous, abort        |
         # +------------------+----------------------+-------------------------+
         #
         # The actual abort is handled by `defineparents`
         if len(root.parents()) <= 1:
             # ancestors of <root> not ancestors of <dest>
             detachset.update(repo.changelog.findmissingrevs([commonbase.rev()],
                                                             [root.rev()]))
     for r in detachset:
         if r not in state:
             state[r] = nullmerge
     if len(roots) > 1:
         # If we have multiple roots, we may have "hole" in the rebase set.
         # Rebase roots that descend from those "hole" should not be detached as
         # other root are. We use the special `revignored` to inform rebase that
         # the revision should be ignored but that `defineparents` should search
         # a rebase destination that make sense regarding rebased topology.
         rebasedomain = set(repo.revs('%ld::%ld', rebaseset, rebaseset))
         for ignored in set(rebasedomain) - set(rebaseset):
             state[ignored] = revignored
     return repo['.'].rev(), dest.rev(), state
 
 def clearrebased(ui, repo, state, skipped, collapsedas=None):
     """dispose of rebased revision at the end of the rebase
 
     If `collapsedas` is not None, the rebase was a collapse whose result if the
     `collapsedas` node."""
     if obsolete.isenabled(repo, obsolete.createmarkersopt):
         markers = []
         for rev, newrev in sorted(state.items()):
             if newrev >= 0:
                 if rev in skipped:
                     succs = ()
                 elif collapsedas is not None:
                     succs = (repo[collapsedas],)
                 else:
                     succs = (repo[newrev],)
                 markers.append((repo[rev], succs))
         if markers:
             obsolete.createmarkers(repo, markers)
     else:
         rebased = [rev for rev in state if state[rev] > nullmerge]
         if rebased:
             stripped = []
             for root in repo.set('roots(%ld)', rebased):
                 if set(repo.changelog.descendants([root.rev()])) - set(state):
                     ui.warn(_("warning: new changesets detected "
                               "on source branch, not stripping\n"))
                 else:
                     stripped.append(root.node())
             if stripped:
                 # backup the old csets by default
                 repair.strip(ui, repo, stripped, "all")
 
 
 def pullrebase(orig, ui, repo, *args, **opts):
     'Call rebase after pull if the latter has been invoked with --rebase'
     if opts.get('rebase'):
         if opts.get('update'):
             del opts['update']
             ui.debug('--update and --rebase are not compatible, ignoring '
                      'the update flag\n')
 
         movemarkfrom = repo['.'].node()
         revsprepull = len(repo)
         origpostincoming = commands.postincoming
         def _dummy(*args, **kwargs):
             pass
         commands.postincoming = _dummy
         try:
             orig(ui, repo, *args, **opts)
         finally:
             commands.postincoming = origpostincoming
         revspostpull = len(repo)
         if revspostpull > revsprepull:
             # --rev option from pull conflict with rebase own --rev
             # dropping it
             if 'rev' in opts:
                 del opts['rev']
             rebase(ui, repo, **opts)
             branch = repo[None].branch()
             dest = repo[branch].rev()
             if dest != repo['.'].rev():
                 # there was nothing to rebase we force an update
                 hg.update(repo, dest)
                 if bookmarks.update(repo, [movemarkfrom], repo['.'].node()):
                     ui.status(_("updating bookmark %s\n")
                               % repo._bookmarkcurrent)
     else:
         if opts.get('tool'):
             raise util.Abort(_('--tool can only be used with --rebase'))
         orig(ui, repo, *args, **opts)
 
 def summaryhook(ui, repo):
     if not os.path.exists(repo.join('rebasestate')):
         return
     try:
         state = restorestatus(repo)[2]
     except error.RepoLookupError:
         # i18n: column positioning for "hg summary"
         msg = _('rebase: (use "hg rebase --abort" to clear broken state)\n')
         ui.write(msg)
         return
     numrebased = len([i for i in state.itervalues() if i >= 0])
     # i18n: column positioning for "hg summary"
     ui.write(_('rebase: %s, %s (rebase --continue)\n') %
              (ui.label(_('%d rebased'), 'rebase.rebased') % numrebased,
               ui.label(_('%d remaining'), 'rebase.remaining') %
               (len(state) - numrebased)))
 
 def uisetup(ui):
     'Replace pull with a decorator to provide --rebase option'
     entry = extensions.wrapcommand(commands.table, 'pull', pullrebase)
     entry[1].append(('', 'rebase', None,
                      _("rebase working directory to branch head")))
     entry[1].append(('t', 'tool', '',
                      _("specify merge tool for rebase")))
     cmdutil.summaryhooks.add('rebase', summaryhook)
     cmdutil.unfinishedstates.append(
         ['rebasestate', False, False, _('rebase in progress'),
          _("use 'hg rebase --continue' or 'hg rebase --abort'")])