rebase: do not invent successor to skipped changeset

When rebase results in an empty a changeset it is "skipped" and no related
changeset is created at all. When we added obsolescence support to rebase (in
fc2a6114f0a0) it seemed a good idea to use its parent successor as the
successors for such dropped changesets. (see old version of the altered test).
This option was chosen because it seems a good way to hint about were the
dropped changeset "intended" to be. Such hint would have been used by automatic
evolution mechanism to rebase potential unstable children.

However, field testing of this version are not conclusive. It very often leads
to the creation of (totally unfounded) evolution divergence. This changeset
changes this behavior and mark skipped changesets as pruned (obsolete without
successors). This prevents the issue and seems semantically better probably a
win for obsolescence reading tool.

See example bellow for details:

User Babar has five changesets of interest:
- O, its current base of development.
- U, the new upstream
- A and C, some development changesets
- B another development changeset independent from A

    O - A - B - C
      \
        U

Babar decides that B is more critical than the A and C and rebase it first

  $ hg rebase --rev B --dest U

B is now obsolete (in lower case bellow). Rebase result, B', is its
successors.(note, C is unstable)

    O - A - b - C
      \
        U - B'

Babar is now done with B', and want to rebase the rest of its history:

  $ hg rebase --source A --dest B'


hg rebase process A, B and C. B is skipped as all its changes are already contained
in B'.

    O - U - B' - A' - C'

Babar have the expected result graph wise, obsolescence marker are as follow:

    B -> B' (from first rebase)
    A -> A' (from second rebase)
    C -> C' (from second rebase)
    B -> ?? (from second rebase)

Before this changeset, the last marker is `B -> A'`. This cause two issues:

- This is semantically wrong. B have nothing to do with A'
- B has now two successors sets: (B',) and (A',). We detect a divergent
  rewriting. The B' and A' are reported as "divergent" to Babar, confusion
  ensues. In addition such divergent situation (divergent changeset are children
  to each other) is tricky to solve.

With this changeset the last marker is `B -> ø`:

- This is semantically better.
- B has a single successors set (B',)

This scenario is added to the tests suite.

hgext/rebase.py

@@ -1,737 +1,741 b''
 # 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
 from mercurial.commands import templateopts
 from mercurial.node import nullrev
 from mercurial.lock import release
 from mercurial.i18n import _
 import os, errno
 
 nullmerge = -2
 
 cmdtable = {}
 command = cmdutil.command(cmdtable)
 testedwith = 'internal'
 
 @command('rebase',
     [('s', 'source', '',
      _('rebase from the specified changeset'), _('REV')),
     ('b', 'base', '',
      _('rebase from the base of the specified changeset '
        '(up to greatest common ancestor of base and dest)'),
      _('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)')),
     ('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.
 
     If you don't specify a destination changeset (``-d/--dest``),
     rebase uses the tipmost head of the current named branch 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.
 
     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 latest (tipmost) head 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.
 
     Returns 0 on success, 1 if nothing to rebase.
     """
     originalwd = target = None
     external = nullrev
     state = {}
     skipped = set()
     targetancestors = set()
 
     editor = None
     if opts.get('edit'):
         editor = cmdutil.commitforceeditor
 
     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)
         extrafn = opts.get('extrafn') # internal, used by e.g. hgsubversion
         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 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'))
 
             (originalwd, target, state, skipped, collapsef, keepf,
                                 keepbranchesf, external) = restorestatus(repo)
             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.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 = repo.revs('%lr', revf)
             elif srcf:
                 src = scmutil.revrange(repo, [srcf])
                 rebaseset = repo.revs('(%ld)::', src)
             else:
                 base = scmutil.revrange(repo, [basef or '.'])
                 rebaseset = repo.revs(
                     '(children(ancestor(%ld, %d)) and ::(%ld))::',
                     base, dest, base)
             if rebaseset:
                 root = min(rebaseset)
             else:
                 root = None
 
             if not rebaseset:
                 repo.ui.debug('base is ancestor of destination\n')
                 result = None
             elif (not (keepf or obsolete._enabled)
                   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'))
             elif 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'))
             else:
                 result = buildstate(repo, dest, rebaseset, collapsef)
 
             if not result:
                 # Empty state built, nothing to rebase
                 ui.status(_('nothing to rebase\n'))
                 return 1
             else:
                 originalwd, target, state = result
                 if collapsef:
                     targetancestors = repo.changelog.ancestors([target],
                                                                inclusive=True)
                     external = checkexternal(repo, state, targetancestors)
 
         if keepbranchesf:
             assert not extrafn, 'cannot use both keepbranches and extrafn'
             def extrafn(ctx, extra):
                 extra['branch'] = ctx.branch()
             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 = repo._bookmarkcurrent
         if activebookmark:
             bookmarks.unsetcurrent(repo)
 
         sortedstate = sorted(state)
         total = len(sortedstate)
         pos = 0
         for rev in sortedstate:
             pos += 1
             if state[rev] == -1:
                 ui.progress(_("rebasing"), pos, ("%d:%s" % (rev, repo[rev])),
                             _('changesets'), total)
                 storestatus(repo, originalwd, target, state, collapsef, keepf,
                                                     keepbranchesf, external)
                 p1, p2 = defineparents(repo, rev, target, state,
                                                         targetancestors)
                 if len(repo.parents()) == 2:
                     repo.ui.debug('resuming interrupted rebase\n')
                 else:
                     try:
                         ui.setconfig('ui', 'forcemerge', opts.get('tool', ''))
                         stats = rebasenode(repo, rev, p1, state, collapsef)
                         if stats and stats[3] > 0:
                             raise util.Abort(_('unresolved conflicts (see hg '
                                         'resolve, then hg rebase --continue)'))
                     finally:
                         ui.setconfig('ui', 'forcemerge', '')
                 cmdutil.duplicatecopies(repo, rev, target)
                 if not collapsef:
                     newrev = concludenode(repo, rev, p1, p2, extrafn=extrafn,
                                           editor=editor)
                 else:
                     # Skip commit if we are collapsing
                     repo.setparents(repo[p1].node())
                     newrev = None
                 # Update the state
                 if newrev is not None:
                     state[rev] = repo[newrev].rev()
                 else:
                     if not collapsef:
                         ui.note(_('no changes, revision %d skipped\n') % rev)
                         ui.debug('next revision set to %s\n' % p1)
                         skipped.add(rev)
                     state[rev] = p1
 
         ui.progress(_('rebasing'), None)
         ui.note(_('rebase merging completed\n'))
 
         if collapsef and not keepopen:
             p1, p2 = defineparents(repo, min(state), target,
                                                         state, targetancestors)
             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()
                 commitmsg = ui.edit(commitmsg, repo.ui.username())
             newrev = concludenode(repo, rev, p1, external, commitmsg=commitmsg,
                                   extrafn=extrafn, editor=editor)
 
         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()
 
         if not keepf:
             collapsedas = None
             if collapsef:
                 collapsedas = newrev
-            clearrebased(ui, repo, state, collapsedas)
+            clearrebased(ui, repo, state, skipped, collapsedas)
 
         if currentbookmarks:
             updatebookmarks(repo, nstate, currentbookmarks, **opts)
 
         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['tip'].node() == repo._bookmarks[activebookmark]):
                 bookmarks.setcurrent(repo, activebookmark)
 
     finally:
         release(lock, wlock)
 
 def checkexternal(repo, state, targetancestors):
     """Check whether one or more external revisions need to be taken in
     consideration. In the latter case, abort.
     """
     external = nullrev
     source = min(state)
     for rev in state:
         if rev == source:
             continue
         # Check externals and fail if there are more than one
         for p in repo[rev].parents():
             if (p.rev() not in state
                         and p.rev() not in targetancestors):
                 if external != nullrev:
                     raise util.Abort(_('unable to collapse, there is more '
                             'than one external parent'))
                 external = p.rev()
     return external
 
 def concludenode(repo, rev, p1, p2, commitmsg=None, editor=None, extrafn=None):
     'Commit the changes and store useful information in extra'
     try:
         repo.setparents(repo[p1].node(), repo[p2].node())
         ctx = repo[rev]
         if commitmsg is None:
             commitmsg = ctx.description()
         extra = {'rebase_source': ctx.hex()}
         if extrafn:
             extrafn(ctx, extra)
         # Commit might fail if unresolved files exist
         newrev = repo.commit(text=commitmsg, user=ctx.user(),
                              date=ctx.date(), extra=extra, editor=editor)
         repo.dirstate.setbranch(repo[newrev].branch())
         targetphase = max(ctx.phase(), phases.draft)
         # retractboundary doesn't overwrite upper phase inherited from parent
         newnode = repo[newrev].node()
         if newnode:
             phases.retractboundary(repo, targetphase, [newnode])
         return newrev
     except util.Abort:
         # Invalidate the previous setparents
         repo.dirstate.invalidate()
         raise
 
 def rebasenode(repo, rev, p1, state, collapse):
     'Rebase a single revision'
     # Merge phase
     # Update to target and merge it with local
     if repo['.'].rev() != repo[p1].rev():
         repo.ui.debug(" update to %d:%s\n" % (repo[p1].rev(), 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" % (repo[rev].rev(), repo[rev]))
     base = None
     if repo[rev].rev() != repo[min(state)].rev():
         base = repo[rev].p1().node()
     # When collapsing in-place, the parent is the common ancestor, we
     # have to allow merging with it.
     return merge.update(repo, rev, True, True, False, base, collapse)
 
 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
         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]
             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()))
     return p1, p2
 
 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.debug('import mq patch %d (%s)\n' % (state[rev], name))
                 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, nstate, originalbookmarks, **opts):
     'Move bookmarks to their correct changesets'
     marks = repo._bookmarks
     for k, v in originalbookmarks.iteritems():
         if v in nstate:
             if nstate[v] != nullmerge:
                 # update the bookmarks for revs that have moved
                 marks[k] = nstate[v]
 
     marks.write()
 
 def storestatus(repo, originalwd, target, state, collapse, keep, keepbranches,
                                                                 external):
     '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))
     for d, v in state.iteritems():
         oldrev = repo[d].hex()
         if v != nullmerge:
             newrev = repo[v].hex()
         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:
         target = None
         collapse = False
         external = nullrev
         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))
             else:
                 oldrev, newrev = l.split(':')
                 if newrev != str(nullmerge):
                     state[repo[oldrev].rev()] = repo[newrev].rev()
                 else:
                     state[repo[oldrev].rev()] = int(newrev)
         skipped = set()
         # recompute the set of skipped revs
         if not collapse:
             seen = set([target])
             for old, new in sorted(state.items()):
                 if new != nullrev and new in seen:
                     skipped.add(old)
                 seen.add(new)
         repo.ui.debug('computed skipped revs: %s\n' % skipped)
         repo.ui.debug('rebase status resumed\n')
         return (originalwd, target, state, skipped,
                 collapse, keep, keepbranches, external)
     except IOError, err:
         if err.errno != errno.ENOENT:
             raise
         raise util.Abort(_('no rebase in progress'))
 
 def abort(repo, originalwd, target, state):
     'Restore the repository to its original state'
     dstates = [s for s in state.values() if s != nullrev]
     immutable = [d for d in dstates if not repo[d].mutable()]
     if immutable:
         raise util.Abort(_("can't abort rebase due to immutable changesets %s")
                          % ', '.join(str(repo[r]) for r in immutable),
                          hint=_('see hg help phases for details'))
 
     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 abort\n"))
         return -1
     else:
         # Strip from the first rebased revision
         merge.update(repo, repo[originalwd].rev(), False, True, False)
         rebased = filter(lambda x: x > -1 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, roots))
         state.update(dict.fromkeys(rebaseset, nullrev))
         # 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
     return repo['.'].rev(), dest.rev(), state
 
-def clearrebased(ui, repo, state, collapsedas=None):
+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._enabled:
         markers = []
         for rev, newrev in sorted(state.items()):
             if newrev >= 0:
-                if collapsedas is not None:
-                    newrev = collapsedas
-                markers.append((repo[rev], (repo[newrev],)))
+                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()
         cmdutil.bailifchanged(repo)
         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 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")))

tests/test-rebase-obsolete.t

@@ -1,331 +1,387 b''
 ==========================
 Test rebase with obsolete
 ==========================
 
 Enable obsolete
 
   $ cat > ${TESTTMP}/obs.py << EOF
   > import mercurial.obsolete
   > mercurial.obsolete._enabled = True
   > EOF
   $ cat >> $HGRCPATH << EOF
   > [ui]
   > logtemplate= {rev}:{node|short} {desc|firstline}
   > [phases]
   > publish=False
   > [extensions]'
   > rebase=
   > 
   > obs=${TESTTMP}/obs.py
   > EOF
 
 Setup rebase canonical repo
 
   $ hg init base
   $ cd base
   $ hg unbundle "$TESTDIR/bundles/rebase.hg"
   adding changesets
   adding manifests
   adding file changes
   added 8 changesets with 7 changes to 7 files (+2 heads)
   (run 'hg heads' to see heads, 'hg merge' to merge)
   $ hg up tip
   3 files updated, 0 files merged, 0 files removed, 0 files unresolved
   $ hg log -G
   @  7:02de42196ebe H
   |
   | o  6:eea13746799a G
   |/|
   o |  5:24b6387c8c8c F
   | |
   | o  4:9520eea781bc E
   |/
   | o  3:32af7686d403 D
   | |
   | o  2:5fddd98957c8 C
   | |
   | o  1:42ccdea3bb16 B
   |/
   o  0:cd010b8cd998 A
   
   $ cd ..
 
 simple rebase
 ---------------------------------
 
   $ hg clone base simple
   updating to branch default
   3 files updated, 0 files merged, 0 files removed, 0 files unresolved
   $ cd simple
   $ hg up 32af7686d403
   3 files updated, 0 files merged, 2 files removed, 0 files unresolved
   $ hg rebase -d eea13746799a
   $ hg log -G
   @  10:8eeb3c33ad33 D
   |
   o  9:2327fea05063 C
   |
   o  8:e4e5be0395b2 B
   |
   | o  7:02de42196ebe H
   | |
   o |  6:eea13746799a G
   |\|
   | o  5:24b6387c8c8c F
   | |
   o |  4:9520eea781bc E
   |/
   o  0:cd010b8cd998 A
   
   $ hg log --hidden -G
   @  10:8eeb3c33ad33 D
   |
   o  9:2327fea05063 C
   |
   o  8:e4e5be0395b2 B
   |
   | o  7:02de42196ebe H
   | |
   o |  6:eea13746799a G
   |\|
   | o  5:24b6387c8c8c F
   | |
   o |  4:9520eea781bc E
   |/
   | x  3:32af7686d403 D
   | |
   | x  2:5fddd98957c8 C
   | |
   | x  1:42ccdea3bb16 B
   |/
   o  0:cd010b8cd998 A
   
   $ hg debugobsolete
   42ccdea3bb16d28e1848c95fe2e44c000f3f21b1 e4e5be0395b2cbd471ed22a26b1b6a1a0658a794 0 {'date': '*', 'user': 'test'} (glob)
   5fddd98957c8a54a4d436dfe1da9d87f21a1b97b 2327fea05063f39961b14cb69435a9898dc9a245 0 {'date': '*', 'user': 'test'} (glob)
   32af7686d403cf45b5d95f2d70cebea587ac806a 8eeb3c33ad33d452c89e5dcf611c347f978fb42b 0 {'date': '*', 'user': 'test'} (glob)
 
 
   $ cd ..
 
 empty changeset
 ---------------------------------
 
   $ hg clone base empty
   updating to branch default
   3 files updated, 0 files merged, 0 files removed, 0 files unresolved
   $ cd empty
   $ hg up eea13746799a
   1 files updated, 0 files merged, 1 files removed, 0 files unresolved
 
 We make a copy of both the first changeset in the rebased and some other in the
 set.
 
   $ hg graft 42ccdea3bb16 32af7686d403
   grafting revision 1
   grafting revision 3
   $ hg rebase  -s 42ccdea3bb16 -d .
   $ hg log -G
   @  10:5ae4c968c6ac C
   |
   o  9:08483444fef9 D
   |
   o  8:8877864f1edb B
   |
   | o  7:02de42196ebe H
   | |
   o |  6:eea13746799a G
   |\|
   | o  5:24b6387c8c8c F
   | |
   o |  4:9520eea781bc E
   |/
   o  0:cd010b8cd998 A
   
   $ hg log --hidden -G
   @  10:5ae4c968c6ac C
   |
   o  9:08483444fef9 D
   |
   o  8:8877864f1edb B
   |
   | o  7:02de42196ebe H
   | |
   o |  6:eea13746799a G
   |\|
   | o  5:24b6387c8c8c F
   | |
   o |  4:9520eea781bc E
   |/
   | x  3:32af7686d403 D
   | |
   | x  2:5fddd98957c8 C
   | |
   | x  1:42ccdea3bb16 B
   |/
   o  0:cd010b8cd998 A
   
   $ hg debugobsolete
-  42ccdea3bb16d28e1848c95fe2e44c000f3f21b1 08483444fef91d6224f6655ee586a65d263ad34c 0 {'date': '*', 'user': 'test'} (glob)
+  42ccdea3bb16d28e1848c95fe2e44c000f3f21b1 0 {'date': '*', 'user': 'test'} (glob)
+  5fddd98957c8a54a4d436dfe1da9d87f21a1b97b 5ae4c968c6aca831df823664e706c9d4aa34473d 0 {'date': '*', 'user': 'test'} (glob)
+  32af7686d403cf45b5d95f2d70cebea587ac806a 0 {'date': '*', 'user': 'test'} (glob)
+
+
+More complex case were part of the rebase set were already rebased
+
+  $ hg rebase --rev 'desc(D)' --dest 'desc(H)'
+  $ hg debugobsolete
+  42ccdea3bb16d28e1848c95fe2e44c000f3f21b1 0 {'date': '*', 'user': 'test'} (glob)
   5fddd98957c8a54a4d436dfe1da9d87f21a1b97b 5ae4c968c6aca831df823664e706c9d4aa34473d 0 {'date': '*', 'user': 'test'} (glob)
-  32af7686d403cf45b5d95f2d70cebea587ac806a 5ae4c968c6aca831df823664e706c9d4aa34473d 0 {'date': '*', 'user': 'test'} (glob)
+  32af7686d403cf45b5d95f2d70cebea587ac806a 0 {'date': '*', 'user': 'test'} (glob)
+  08483444fef91d6224f6655ee586a65d263ad34c cbc07f26687521cecfc9a141bf5ecfc0fd2b8531 0 {'date': '* *', 'user': 'test'} (glob)
+  $ hg log -G
+  @  11:cbc07f266875 D
+  |
+  | o  10:5ae4c968c6ac C
+  | |
+  | x  9:08483444fef9 D
+  | |
+  | o  8:8877864f1edb B
+  | |
+  o |  7:02de42196ebe H
+  | |
+  | o  6:eea13746799a G
+  |/|
+  o |  5:24b6387c8c8c F
+  | |
+  | o  4:9520eea781bc E
+  |/
+  o  0:cd010b8cd998 A
+  
+  $ hg rebase --source 'desc(B)' --dest 'tip'
+  $ hg debugobsolete
+  42ccdea3bb16d28e1848c95fe2e44c000f3f21b1 0 {'date': '* *', 'user': 'test'} (glob)
+  5fddd98957c8a54a4d436dfe1da9d87f21a1b97b 5ae4c968c6aca831df823664e706c9d4aa34473d 0 {'date': '* *', 'user': 'test'} (glob)
+  32af7686d403cf45b5d95f2d70cebea587ac806a 0 {'date': '* *', 'user': 'test'} (glob)
+  08483444fef91d6224f6655ee586a65d263ad34c cbc07f26687521cecfc9a141bf5ecfc0fd2b8531 0 {'date': '* *', 'user': 'test'} (glob)
+  8877864f1edb05d0e07dc4ba77b67a80a7b86672 b1861c79d66ec3aa1b607ac3c9fb819e38b12238 0 {'date': '* *', 'user': 'test'} (glob)
+  08483444fef91d6224f6655ee586a65d263ad34c 0 {'date': '* *', 'user': 'test'} (glob)
+  5ae4c968c6aca831df823664e706c9d4aa34473d dd4be135457a404ce5541de427ae1d98a28f4acd 0 {'date': '* *', 'user': 'test'} (glob)
+  $ hg log --rev 'divergent()'
+  $ hg log -G
+  @  13:dd4be135457a C
+  |
+  o  12:b1861c79d66e B
+  |
+  o  11:cbc07f266875 D
+  |
+  o  7:02de42196ebe H
+  |
+  | o  6:eea13746799a G
+  |/|
+  o |  5:24b6387c8c8c F
+  | |
+  | o  4:9520eea781bc E
+  |/
+  o  0:cd010b8cd998 A
 
 
   $ cd ..
 
 collapse rebase
 ---------------------------------
 
   $ hg clone base collapse
   updating to branch default
   3 files updated, 0 files merged, 0 files removed, 0 files unresolved
   $ cd collapse
   $ hg rebase  -s 42ccdea3bb16 -d eea13746799a --collapse
   $ hg log -G
   @  8:4dc2197e807b Collapsed revision
   |
   | o  7:02de42196ebe H
   | |
   o |  6:eea13746799a G
   |\|
   | o  5:24b6387c8c8c F
   | |
   o |  4:9520eea781bc E
   |/
   o  0:cd010b8cd998 A
   
   $ hg log --hidden -G
   @  8:4dc2197e807b Collapsed revision
   |
   | o  7:02de42196ebe H
   | |
   o |  6:eea13746799a G
   |\|
   | o  5:24b6387c8c8c F
   | |
   o |  4:9520eea781bc E
   |/
   | x  3:32af7686d403 D
   | |
   | x  2:5fddd98957c8 C
   | |
   | x  1:42ccdea3bb16 B
   |/
   o  0:cd010b8cd998 A
   
   $ hg id --debug
   4dc2197e807bae9817f09905b50ab288be2dbbcf tip
   $ hg debugobsolete
   42ccdea3bb16d28e1848c95fe2e44c000f3f21b1 4dc2197e807bae9817f09905b50ab288be2dbbcf 0 {'date': '*', 'user': 'test'} (glob)
   5fddd98957c8a54a4d436dfe1da9d87f21a1b97b 4dc2197e807bae9817f09905b50ab288be2dbbcf 0 {'date': '*', 'user': 'test'} (glob)
   32af7686d403cf45b5d95f2d70cebea587ac806a 4dc2197e807bae9817f09905b50ab288be2dbbcf 0 {'date': '*', 'user': 'test'} (glob)
 
   $ cd ..
 
 Rebase set has hidden descendants
 ---------------------------------
 
 We rebase a changeset which has a hidden changeset. The hidden changeset must
 not be rebased.
 
   $ hg clone base hidden
   updating to branch default
   3 files updated, 0 files merged, 0 files removed, 0 files unresolved
   $ cd hidden
   $ hg rebase -s 5fddd98957c8 -d eea13746799a
   $ hg rebase -s 42ccdea3bb16 -d 02de42196ebe
   $ hg log -G
   @  10:7c6027df6a99 B
   |
   | o  9:cf44d2f5a9f4 D
   | |
   | o  8:e273c5e7d2d2 C
   | |
   o |  7:02de42196ebe H
   | |
   | o  6:eea13746799a G
   |/|
   o |  5:24b6387c8c8c F
   | |
   | o  4:9520eea781bc E
   |/
   o  0:cd010b8cd998 A
   
   $ hg log --hidden -G
   @  10:7c6027df6a99 B
   |
   | o  9:cf44d2f5a9f4 D
   | |
   | o  8:e273c5e7d2d2 C
   | |
   o |  7:02de42196ebe H
   | |
   | o  6:eea13746799a G
   |/|
   o |  5:24b6387c8c8c F
   | |
   | o  4:9520eea781bc E
   |/
   | x  3:32af7686d403 D
   | |
   | x  2:5fddd98957c8 C
   | |
   | x  1:42ccdea3bb16 B
   |/
   o  0:cd010b8cd998 A
   
   $ hg debugobsolete
   5fddd98957c8a54a4d436dfe1da9d87f21a1b97b e273c5e7d2d29df783dce9f9eaa3ac4adc69c15d 0 {'date': '*', 'user': 'test'} (glob)
   32af7686d403cf45b5d95f2d70cebea587ac806a cf44d2f5a9f4297a62be94cbdd3dff7c7dc54258 0 {'date': '*', 'user': 'test'} (glob)
   42ccdea3bb16d28e1848c95fe2e44c000f3f21b1 7c6027df6a99d93f461868e5433f63bde20b6dfb 0 {'date': '*', 'user': 'test'} (glob)
 
 Test that rewriting leaving instability behind is allowed
 ---------------------------------------------------------------------
 
   $ hg log -r 'children(8)'
   9:cf44d2f5a9f4 D (no-eol)
   $ hg rebase -r 8
   $ hg log -G
   @  11:0d8f238b634c C
   |
   o  10:7c6027df6a99 B
   |
   | o  9:cf44d2f5a9f4 D
   | |
   | x  8:e273c5e7d2d2 C
   | |
   o |  7:02de42196ebe H
   | |
   | o  6:eea13746799a G
   |/|
   o |  5:24b6387c8c8c F
   | |
   | o  4:9520eea781bc E
   |/
   o  0:cd010b8cd998 A
   
 
 
 Test multiple root handling
 ------------------------------------
 
   $ hg rebase --dest 4 --rev '7+11+9'
   $ hg log -G
   @  14:00891d85fcfc C
   |
   | o  13:102b4c1d889b D
   |/
   | o  12:bfe264faf697 H
   |/
   | o  10:7c6027df6a99 B
   | |
   | x  7:02de42196ebe H
   | |
   +---o  6:eea13746799a G
   | |/
   | o  5:24b6387c8c8c F
   | |
   o |  4:9520eea781bc E
   |/
   o  0:cd010b8cd998 A
   
+  $ cd ..