update: resurrect bare update from null parent to tip-most branch head

The situation is tricky if repository has no "default" branch, because "null"
revision belongs to non-existent "default" branch.

Before e1dd0de26557, bare update from null would bring us to the tip-most
non-closed branch head. e1dd0de26557 removed the special handling of missing
"default" branch since we wanted to stick to the uncommitted branch in that
case. But, if the parent is "null" revision, and if the missing branch is
"default", it shouldn't be an uncommitted branch. In this case, bare update
should bring us to the tip-most head as before.

This should fix the test breakage introduced by e1dd0de26557.

mercurial/destutil.py

@@ -1,418 +1,422 b''
 # destutil.py - Mercurial utility function for command destination
 #
 #  Copyright Matt Mackall <[e-mail]> and other
 #
 # This software may be used and distributed according to the terms of the
 # GNU General Public License version 2 or any later version.
 
 from __future__ import absolute_import
 
 from .i18n import _
 from . import (
     bookmarks,
     error,
     obsolete,
 )
 
 def _destupdatevalidate(repo, rev, clean, check):
     """validate that the destination comply to various rules
 
     This exists as its own function to help wrapping from extensions."""
     wc = repo[None]
     p1 = wc.p1()
     if not clean:
         # Check that the update is linear.
         #
         # Mercurial do not allow update-merge for non linear pattern
         # (that would be technically possible but was considered too confusing
         # for user a long time ago)
         #
         # See mercurial.merge.update for details
         if p1.rev() not in repo.changelog.ancestors([rev], inclusive=True):
             dirty = wc.dirty(missing=True)
             foreground = obsolete.foreground(repo, [p1.node()])
             if not repo[rev].node() in foreground:
                 if dirty:
                     msg = _("uncommitted changes")
                     hint = _("commit and merge, or update --clean to"
                              " discard changes")
                     raise error.UpdateAbort(msg, hint=hint)
                 elif not check:  # destination is not a descendant.
                     msg = _("not a linear update")
                     hint = _("merge or update --check to force update")
                     raise error.UpdateAbort(msg, hint=hint)
 
 def _destupdateobs(repo, clean, check):
     """decide of an update destination from obsolescence markers"""
     node = None
     wc = repo[None]
     p1 = wc.p1()
     movemark = None
 
     if p1.obsolete() and not p1.children():
         # allow updating to successors
         successors = obsolete.successorssets(repo, p1.node())
 
         # behavior of certain cases is as follows,
         #
         # divergent changesets: update to highest rev, similar to what
         #     is currently done when there are more than one head
         #     (i.e. 'tip')
         #
         # replaced changesets: same as divergent except we know there
         # is no conflict
         #
         # pruned changeset: no update is done; though, we could
         #     consider updating to the first non-obsolete parent,
         #     similar to what is current done for 'hg prune'
 
         if successors:
             # flatten the list here handles both divergent (len > 1)
             # and the usual case (len = 1)
             successors = [n for sub in successors for n in sub]
 
             # get the max revision for the given successors set,
             # i.e. the 'tip' of a set
             node = repo.revs('max(%ln)', successors).first()
             if bookmarks.isactivewdirparent(repo):
                 movemark = repo['.'].node()
     return node, movemark, None
 
 def _destupdatebook(repo, clean, check):
     """decide on an update destination from active bookmark"""
     # we also move the active bookmark, if any
     activemark = None
     node, movemark = bookmarks.calculateupdate(repo.ui, repo, None)
     if node is not None:
         activemark = node
     return node, movemark, activemark
 
 def _destupdatebranch(repo, clean, check):
     """decide on an update destination from current branch
 
     This ignores closed branch heads.
     """
     wc = repo[None]
     movemark = node = None
     currentbranch = wc.branch()
     if currentbranch in repo.branchmap():
         heads = repo.branchheads(currentbranch)
         if heads:
             node = repo.revs('max(.::(%ln))', heads).first()
         if bookmarks.isactivewdirparent(repo):
             movemark = repo['.'].node()
+    elif currentbranch == 'default' and not wc.p1():
+        # "null" parent belongs to "default" branch, but it doesn't exist, so
+        # update to the tipmost non-closed branch head
+        node = repo.revs('max(head() and not closed())').first()
     else:
         node = repo['.'].node()
     return node, movemark, None
 
 def _destupdatebranchfallback(repo, clean, check):
     """decide on an update destination from closed heads in current branch"""
     wc = repo[None]
     currentbranch = wc.branch()
     movemark = None
     if currentbranch in repo.branchmap():
         # here, all descendant branch heads are closed
         heads = repo.branchheads(currentbranch, closed=True)
         assert heads, "any branch has at least one head"
         node = repo.revs('max(.::(%ln))', heads).first()
         assert node is not None, ("any revision has at least "
                                   "one descendant branch head")
         if bookmarks.isactivewdirparent(repo):
             movemark = repo['.'].node()
     else:
         # here, no "default" branch, and all branches are closed
         node = repo.lookup('tip')
         assert node is not None, "'tip' exists even in empty repository"
     return node, movemark, None
 
 # order in which each step should be evalutated
 # steps are run until one finds a destination
 destupdatesteps = ['evolution', 'bookmark', 'branch', 'branchfallback']
 # mapping to ease extension overriding steps.
 destupdatestepmap = {'evolution': _destupdateobs,
                      'bookmark': _destupdatebook,
                      'branch': _destupdatebranch,
                      'branchfallback': _destupdatebranchfallback,
                      }
 
 def destupdate(repo, clean=False, check=False):
     """destination for bare update operation
 
     return (rev, movemark, activemark)
 
     - rev: the revision to update to,
     - movemark: node to move the active bookmark from
                 (cf bookmark.calculate update),
     - activemark: a bookmark to activate at the end of the update.
     """
     node = movemark = activemark = None
 
     for step in destupdatesteps:
         node, movemark, activemark = destupdatestepmap[step](repo, clean, check)
         if node is not None:
             break
     rev = repo[node].rev()
 
     _destupdatevalidate(repo, rev, clean, check)
 
     return rev, movemark, activemark
 
 msgdestmerge = {
     # too many matching divergent bookmark
     'toomanybookmarks':
         {'merge':
             (_("multiple matching bookmarks to merge -"
                " please merge with an explicit rev or bookmark"),
              _("run 'hg heads' to see all heads")),
          'rebase':
             (_("multiple matching bookmarks to rebase -"
                " please rebase to an explicit rev or bookmark"),
              _("run 'hg heads' to see all heads")),
         },
     # no other matching divergent bookmark
     'nootherbookmarks':
         {'merge':
             (_("no matching bookmark to merge - "
                "please merge with an explicit rev or bookmark"),
              _("run 'hg heads' to see all heads")),
          'rebase':
             (_("no matching bookmark to rebase - "
                "please rebase to an explicit rev or bookmark"),
              _("run 'hg heads' to see all heads")),
         },
     # branch have too many unbookmarked heads, no obvious destination
     'toomanyheads':
         {'merge':
             (_("branch '%s' has %d heads - please merge with an explicit rev"),
              _("run 'hg heads .' to see heads")),
          'rebase':
             (_("branch '%s' has %d heads - please rebase to an explicit rev"),
              _("run 'hg heads .' to see heads")),
         },
     # branch have no other unbookmarked heads
     'bookmarkedheads':
         {'merge':
             (_("heads are bookmarked - please merge with an explicit rev"),
              _("run 'hg heads' to see all heads")),
          'rebase':
             (_("heads are bookmarked - please rebase to an explicit rev"),
              _("run 'hg heads' to see all heads")),
         },
     # branch have just a single heads, but there is other branches
     'nootherbranchheads':
         {'merge':
             (_("branch '%s' has one head - please merge with an explicit rev"),
              _("run 'hg heads' to see all heads")),
          'rebase':
             (_("branch '%s' has one head - please rebase to an explicit rev"),
              _("run 'hg heads' to see all heads")),
         },
     # repository have a single head
     'nootherheads':
         {'merge':
             (_('nothing to merge'),
             None),
          'rebase':
             (_('nothing to rebase'),
             None),
         },
     # repository have a single head and we are not on it
     'nootherheadsbehind':
         {'merge':
             (_('nothing to merge'),
              _("use 'hg update' instead")),
          'rebase':
             (_('nothing to rebase'),
              _("use 'hg update' instead")),
         },
     # We are not on a head
     'notatheads':
         {'merge':
             (_('working directory not at a head revision'),
              _("use 'hg update' or merge with an explicit revision")),
          'rebase':
             (_('working directory not at a head revision'),
              _("use 'hg update' or rebase to an explicit revision"))
         },
     'emptysourceset':
         {'merge':
             (_('source set is empty'),
              None),
          'rebase':
             (_('source set is empty'),
              None),
         },
     'multiplebranchessourceset':
         {'merge':
             (_('source set is rooted in multiple branches'),
              None),
          'rebase':
             (_('rebaseset is rooted in multiple named branches'),
              _('specify an explicit destination with --dest')),
         },
     }
 
 def _destmergebook(repo, action='merge', sourceset=None):
     """find merge destination in the active bookmark case"""
     node = None
     bmheads = repo.bookmarkheads(repo._activebookmark)
     curhead = repo[repo._activebookmark].node()
     if len(bmheads) == 2:
         if curhead == bmheads[0]:
             node = bmheads[1]
         else:
             node = bmheads[0]
     elif len(bmheads) > 2:
         msg, hint = msgdestmerge['toomanybookmarks'][action]
         raise error.ManyMergeDestAbort(msg, hint=hint)
     elif len(bmheads) <= 1:
         msg, hint = msgdestmerge['nootherbookmarks'][action]
         raise error.NoMergeDestAbort(msg, hint=hint)
     assert node is not None
     return node
 
 def _destmergebranch(repo, action='merge', sourceset=None, onheadcheck=True):
     """find merge destination based on branch heads"""
     node = None
 
     if sourceset is None:
         sourceset = [repo[repo.dirstate.p1()].rev()]
         branch = repo.dirstate.branch()
     elif not sourceset:
         msg, hint = msgdestmerge['emptysourceset'][action]
         raise error.NoMergeDestAbort(msg, hint=hint)
     else:
         branch = None
         for ctx in repo.set('roots(%ld::%ld)', sourceset, sourceset):
             if branch is not None and ctx.branch() != branch:
                 msg, hint = msgdestmerge['multiplebranchessourceset'][action]
                 raise error.ManyMergeDestAbort(msg, hint=hint)
             branch = ctx.branch()
 
     bheads = repo.branchheads(branch)
     onhead = repo.revs('%ld and %ln', sourceset, bheads)
     if onheadcheck and not onhead:
         # Case A: working copy if not on a head. (merge only)
         #
         # This is probably a user mistake We bailout pointing at 'hg update'
         if len(repo.heads()) <= 1:
             msg, hint = msgdestmerge['nootherheadsbehind'][action]
         else:
             msg, hint = msgdestmerge['notatheads'][action]
         raise error.Abort(msg, hint=hint)
     # remove heads descendants of source from the set
     bheads = list(repo.revs('%ln - (%ld::)', bheads, sourceset))
     # filters out bookmarked heads
     nbhs = list(repo.revs('%ld - bookmark()', bheads))
     if len(nbhs) > 1:
         # Case B: There is more than 1 other anonymous heads
         #
         # This means that there will be more than 1 candidate. This is
         # ambiguous. We abort asking the user to pick as explicit destination
         # instead.
         msg, hint = msgdestmerge['toomanyheads'][action]
         msg %= (branch, len(bheads) + 1)
         raise error.ManyMergeDestAbort(msg, hint=hint)
     elif not nbhs:
         # Case B: There is no other anonymous heads
         #
         # This means that there is no natural candidate to merge with.
         # We abort, with various messages for various cases.
         if bheads:
             msg, hint = msgdestmerge['bookmarkedheads'][action]
         elif len(repo.heads()) > 1:
             msg, hint = msgdestmerge['nootherbranchheads'][action]
             msg %= branch
         elif not onhead:
             # if 'onheadcheck == False' (rebase case),
             # this was not caught in Case A.
             msg, hint = msgdestmerge['nootherheadsbehind'][action]
         else:
             msg, hint = msgdestmerge['nootherheads'][action]
         raise error.NoMergeDestAbort(msg, hint=hint)
     else:
         node = nbhs[0]
     assert node is not None
     return node
 
 def destmerge(repo, action='merge', sourceset=None, onheadcheck=True):
     """return the default destination for a merge
 
     (or raise exception about why it can't pick one)
 
     :action: the action being performed, controls emitted error message
     """
     if repo._activebookmark:
         node = _destmergebook(repo, action=action, sourceset=sourceset)
     else:
         node = _destmergebranch(repo, action=action, sourceset=sourceset,
                                 onheadcheck=onheadcheck)
     return repo[node].rev()
 
 histeditdefaultrevset = 'reverse(only(.) and not public() and not ::merge())'
 
 def desthistedit(ui, repo):
     """Default base revision to edit for `hg histedit`."""
     # Avoid cycle: scmutil -> revset -> destutil
     from . import scmutil
 
     default = ui.config('histedit', 'defaultrev', histeditdefaultrevset)
     if default:
         revs = scmutil.revrange(repo, [default])
         if revs:
             # The revset supplied by the user may not be in ascending order nor
             # take the first revision. So do this manually.
             revs.sort()
             return revs.first()
 
     return None
 
 def _statusotherbook(ui, repo):
     bmheads = repo.bookmarkheads(repo._activebookmark)
     curhead = repo[repo._activebookmark].node()
     if repo.revs('%n and parents()', curhead):
         # we are on the active bookmark
         bmheads = [b for b in bmheads if curhead != b]
         if bmheads:
             msg = _('%i other divergent bookmarks for "%s"\n')
             ui.status(msg % (len(bmheads), repo._activebookmark))
 
 def _statusotherbranchheads(ui, repo):
     currentbranch = repo.dirstate.branch()
     allheads = repo.branchheads(currentbranch, closed=True)
     heads = repo.branchheads(currentbranch)
     if repo.revs('%ln and parents()', allheads):
         # we are on a head, even though it might be closed
         #
         #  on closed otherheads
         #  ========= ==========
         #      o        0       all heads for current branch are closed
         #               N       only descendant branch heads are closed
         #      x        0       there is only one non-closed branch head
         #               N       there are some non-closed branch heads
         #  ========= ==========
         otherheads = repo.revs('%ln - parents()', heads)
         if repo['.'].closesbranch():
             ui.warn(_('no open descendant heads on branch "%s", '
                         'updating to a closed head\n') %
                       (currentbranch))
             if otherheads:
                 ui.warn(_('(committing will reopen the head, '
                             'use `hg heads .` to see %i other heads)\n') %
                           (len(otherheads)))
             else:
                 ui.warn(_('(committing will reopen branch "%s")\n') %
                           (currentbranch))
         elif otherheads:
             ui.status(_('%i other heads for branch "%s"\n') %
                       (len(otherheads), currentbranch))
 
 def statusotherdests(ui, repo):
     """Print message about other head"""
     # XXX we should probably include a hint:
     # - about what to do
     # - how to see such heads
     if repo._activebookmark:
         _statusotherbook(ui, repo)
     else:
         _statusotherbranchheads(ui, repo)

tests/test-newbranch.t

@@ -1,393 +1,465 b''
   $ branchcache=.hg/cache/branch2
 
   $ listbranchcaches() {
   >    for f in .hg/cache/branch2*;
   >       do echo === $f ===;
   >       cat $f;
   >     done;
   > }
   $ purgebranchcaches() {
   >     rm .hg/cache/branch2*
   > }
 
   $ hg init t
   $ cd t
 
   $ hg branches
   $ echo foo > a
   $ hg add a
   $ hg ci -m "initial"
   $ hg branch foo
   marked working directory as branch foo
   (branches are permanent and global, did you want a bookmark?)
   $ hg branch
   foo
   $ hg ci -m "add branch name"
   $ hg branch bar
   marked working directory as branch bar
   $ hg ci -m "change branch name"
 
 Branch shadowing:
 
   $ hg branch default
   abort: a branch of the same name already exists
   (use 'hg update' to switch to it)
   [255]
 
   $ hg branch -f default
   marked working directory as branch default
 
   $ hg ci -m "clear branch name"
   created new head
 
 There should be only one default branch head
 
   $ hg heads .
   changeset:   3:1c28f494dae6
   tag:         tip
   user:        test
   date:        Thu Jan 01 00:00:00 1970 +0000
   summary:     clear branch name
   
 Merging and branches
 
   $ hg co foo
   0 files updated, 0 files merged, 0 files removed, 0 files unresolved
   $ hg branch
   foo
 
  set existing branch name fails unless force - setting existing parent branch works without force:
 
   $ hg branch bar
   abort: a branch of the same name already exists
   (use 'hg update' to switch to it)
   [255]
 
   $ hg branch -f bar
   marked working directory as branch bar
 
   $ hg branch foo
   marked working directory as branch foo
 
   $ echo bleah > a
   $ hg ci -m "modify a branch"
 
   $ hg merge default
   0 files updated, 0 files merged, 0 files removed, 0 files unresolved
   (branch merge, don't forget to commit)
 
   $ hg branch
   foo
 
  set existing branch name where branch head is ancestor:
 
   $ hg branch bar
   abort: a branch of the same name already exists
   (use 'hg update' to switch to it)
   [255]
 
  set (other) parent branch as branch name
 
   $ hg branch default
   marked working directory as branch default
 
  set (first) parent branch as branch name
 
   $ hg branch foo
   marked working directory as branch foo
 
   $ hg ci -m "merge"
 
   $ hg log -G -T '{rev}:{node|short} {branch} {desc}\n'
   @    5:530046499edf foo merge
   |\
   | o  4:adf1a74a7f7b foo modify a branch
   | |
   o |  3:1c28f494dae6 default clear branch name
   | |
   o |  2:c21617b13b22 bar change branch name
   |/
   o  1:6c0e42da283a foo add branch name
   |
   o  0:db01e8ea3388 default initial
   
   $ hg branches
   foo                            5:530046499edf
   default                        3:1c28f494dae6 (inactive)
   bar                            2:c21617b13b22 (inactive)
 
   $ hg branches -q
   foo
   default
   bar
 
 Test for invalid branch cache:
 
   $ hg rollback
   repository tip rolled back to revision 4 (undo commit)
   working directory now based on revisions 4 and 3
 
   $ cp ${branchcache}-served .hg/bc-invalid
 
   $ hg log -r foo
   changeset:   4:adf1a74a7f7b
   branch:      foo
   tag:         tip
   parent:      1:6c0e42da283a
   user:        test
   date:        Thu Jan 01 00:00:00 1970 +0000
   summary:     modify a branch
   
   $ cp .hg/bc-invalid $branchcache
 
   $ hg --debug log -r foo
   changeset:   4:adf1a74a7f7b4cd193d12992f5d0d6a004ed21d6
   branch:      foo
   tag:         tip
   phase:       draft
   parent:      1:6c0e42da283a56b5edc5b4fadb491365ec7f5fa8
   parent:      -1:0000000000000000000000000000000000000000
   manifest:    1:8c342a37dfba0b3d3ce073562a00d8a813c54ffe
   user:        test
   date:        Thu Jan 01 00:00:00 1970 +0000
   files:       a
   extra:       branch=foo
   description:
   modify a branch
   
   
   $ purgebranchcaches
   $ echo corrupted > $branchcache
 
   $ hg log -qr foo
   4:adf1a74a7f7b
 
   $ listbranchcaches
   === .hg/cache/branch2 ===
   corrupted
   === .hg/cache/branch2-served ===
   adf1a74a7f7b4cd193d12992f5d0d6a004ed21d6 4
   c21617b13b220988e7a2e26290fbe4325ffa7139 o bar
   1c28f494dae69a2f8fc815059d257eccf3fcfe75 o default
   adf1a74a7f7b4cd193d12992f5d0d6a004ed21d6 o foo
 
 Push should update the branch cache:
 
   $ hg init ../target
 
 Pushing just rev 0:
 
   $ hg push -qr 0 ../target
 
   $ (cd ../target/; listbranchcaches)
   === .hg/cache/branch2-base ===
   db01e8ea3388fd3c7c94e1436ea2bd6a53d581c5 0
   db01e8ea3388fd3c7c94e1436ea2bd6a53d581c5 o default
 
 Pushing everything:
 
   $ hg push -qf ../target
 
   $ (cd ../target/; listbranchcaches)
   === .hg/cache/branch2-base ===
   adf1a74a7f7b4cd193d12992f5d0d6a004ed21d6 4
   c21617b13b220988e7a2e26290fbe4325ffa7139 o bar
   1c28f494dae69a2f8fc815059d257eccf3fcfe75 o default
   adf1a74a7f7b4cd193d12992f5d0d6a004ed21d6 o foo
 
 Update with no arguments: tipmost revision of the current branch:
 
   $ hg up -q -C 0
   $ hg up -q
   $ hg id
   1c28f494dae6
 
   $ hg up -q 1
   $ hg up -q
   $ hg id
   adf1a74a7f7b (foo) tip
 
   $ hg branch foobar
   marked working directory as branch foobar
 
   $ hg up
   0 files updated, 0 files merged, 0 files removed, 0 files unresolved
 
 Fast-forward merge:
 
   $ hg branch ff
   marked working directory as branch ff
 
   $ echo ff > ff
   $ hg ci -Am'fast forward'
   adding ff
 
   $ hg up foo
   0 files updated, 0 files merged, 1 files removed, 0 files unresolved
 
   $ hg merge ff
   1 files updated, 0 files merged, 0 files removed, 0 files unresolved
   (branch merge, don't forget to commit)
 
   $ hg branch
   foo
   $ hg commit -m'Merge ff into foo'
   $ hg parents
   changeset:   6:185ffbfefa30
   branch:      foo
   tag:         tip
   parent:      4:adf1a74a7f7b
   parent:      5:1a3c27dc5e11
   user:        test
   date:        Thu Jan 01 00:00:00 1970 +0000
   summary:     Merge ff into foo
   
   $ hg manifest
   a
   ff
 
 
 Test merging, add 3 default heads and one test head:
 
   $ cd ..
   $ hg init merges
   $ cd merges
   $ echo a > a
   $ hg ci -Ama
   adding a
 
   $ echo b > b
   $ hg ci -Amb
   adding b
 
   $ hg up 0
   0 files updated, 0 files merged, 1 files removed, 0 files unresolved
   $ echo c > c
   $ hg ci -Amc
   adding c
   created new head
 
   $ hg up 0
   0 files updated, 0 files merged, 1 files removed, 0 files unresolved
   $ echo d > d
   $ hg ci -Amd
   adding d
   created new head
 
   $ hg up 0
   0 files updated, 0 files merged, 1 files removed, 0 files unresolved
   $ hg branch test
   marked working directory as branch test
   (branches are permanent and global, did you want a bookmark?)
   $ echo e >> e
   $ hg ci -Ame
   adding e
 
   $ hg log
   changeset:   4:3a1e01ed1df4
   branch:      test
   tag:         tip
   parent:      0:cb9a9f314b8b
   user:        test
   date:        Thu Jan 01 00:00:00 1970 +0000
   summary:     e
   
   changeset:   3:980f7dc84c29
   parent:      0:cb9a9f314b8b
   user:        test
   date:        Thu Jan 01 00:00:00 1970 +0000
   summary:     d
   
   changeset:   2:d36c0562f908
   parent:      0:cb9a9f314b8b
   user:        test
   date:        Thu Jan 01 00:00:00 1970 +0000
   summary:     c
   
   changeset:   1:d2ae7f538514
   user:        test
   date:        Thu Jan 01 00:00:00 1970 +0000
   summary:     b
   
   changeset:   0:cb9a9f314b8b
   user:        test
   date:        Thu Jan 01 00:00:00 1970 +0000
   summary:     a
   
 Implicit merge with test branch as parent:
 
   $ hg merge
   abort: branch 'test' has one head - please merge with an explicit rev
   (run 'hg heads' to see all heads)
   [255]
   $ hg up -C default
   1 files updated, 0 files merged, 1 files removed, 0 files unresolved
 
 Implicit merge with default branch as parent:
 
   $ hg merge
   abort: branch 'default' has 3 heads - please merge with an explicit rev
   (run 'hg heads .' to see heads)
   [255]
 
 3 branch heads, explicit merge required:
 
   $ hg merge 2
   1 files updated, 0 files merged, 0 files removed, 0 files unresolved
   (branch merge, don't forget to commit)
   $ hg ci -m merge
 
 2 branch heads, implicit merge works:
 
   $ hg merge
   1 files updated, 0 files merged, 0 files removed, 0 files unresolved
   (branch merge, don't forget to commit)
 
   $ cd ..
 
 We expect that bare update on new branch, updates to parent
 
   $ hg init bareupdateonnewbranch
   $ cd bareupdateonnewbranch
   $ hg update
   0 files updated, 0 files merged, 0 files removed, 0 files unresolved
   $ touch a
   $ hg commit -A -m "a"
   adding a
   $ touch b
   $ hg commit -A -m "b"
   adding b
   $ touch c
   $ hg commit -A -m "c"
   adding c
   $ hg update -r 1
   0 files updated, 0 files merged, 1 files removed, 0 files unresolved
   $ hg log -G
   o  changeset:   2:991a3460af53
   |  tag:         tip
   |  user:        test
   |  date:        Thu Jan 01 00:00:00 1970 +0000
   |  summary:     c
   |
   @  changeset:   1:0e067c57feba
   |  user:        test
   |  date:        Thu Jan 01 00:00:00 1970 +0000
   |  summary:     b
   |
   o  changeset:   0:3903775176ed
      user:        test
      date:        Thu Jan 01 00:00:00 1970 +0000
      summary:     a
   
   $ hg branch dev
   marked working directory as branch dev
   (branches are permanent and global, did you want a bookmark?)
   $ hg update
   0 files updated, 0 files merged, 0 files removed, 0 files unresolved
   $ hg summary
   parent: 1:0e067c57feba 
    b
   branch: dev
   commit: (new branch)
   update: (current)
   phases: 3 draft
+
+  $ cd ..
+
+We need special handling for repositories with no "default" branch because
+"null" revision belongs to non-existent "default" branch.
+
+  $ hg init nodefault
+  $ cd nodefault
+  $ hg branch -q foo
+  $ touch 0
+  $ hg ci -Aqm0
+  $ touch 1
+  $ hg ci -Aqm1
+  $ hg update -qr0
+  $ hg branch -q bar
+  $ touch 2
+  $ hg ci -Aqm2
+  $ hg update -qr0
+  $ hg branch -q baz
+  $ touch 3
+  $ hg ci -Aqm3
+  $ hg ci --close-branch -m 'close baz'
+  $ hg update -q null
+  $ hg log -GT'{rev} {branch}\n'
+  _  4 baz
+  |
+  o  3 baz
+  |
+  | o  2 bar
+  |/
+  | o  1 foo
+  |/
+  o  0 foo
+  
+
+ a) updating from "null" should bring us to the tip-most branch head as
+ there is no "default" branch:
+
+  $ hg update -q null
+  $ hg id -bn
+  -1 default
+  $ hg update
+  2 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ hg id -bn
+  2 bar
+
+ b) but if we are at uncommitted "default" branch, we should stick to the
+ current revision:
+
+  $ hg update -q 0
+  $ hg branch default
+  marked working directory as branch default
+  $ hg id -bn
+  0 default
+  $ hg update
+  0 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ hg id -bn
+  0 default
+
+ c) also, if we have uncommitted branch at "null", we should stick to it:
+
+  $ hg update -q null
+  $ hg branch new
+  marked working directory as branch new
+  $ hg id -bn
+  -1 new
+  $ hg update
+  0 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ hg id -bn
+  -1 new
+
+  $ cd ..