Patchwork [2,of,2,V2] update: allow dirty update to foreground (successors)

diff --git a/mercurial/merge.py b/mercurial/merge.py
--- a/mercurial/merge.py
+++ b/mercurial/merge.py
@@ -5,10 +5,11 @@ 
 # This software may be used and distributed according to the terms of the
 # GNU General Public License version 2 or any later version.
 
 from node import nullid, nullrev, hex, bin
 from i18n import _
+from mercurial import obsolete
 import error, util, filemerge, copies, subrepo, worker, dicthelpers
 import errno, os, shutil
 
 class mergestate(object):
     '''track 3-way merge state of individual files'''
@@ -695,21 +696,30 @@  def update(repo, node, branchmerge, forc
                 if wc.sub(s).dirty():
                     raise util.Abort(_("outstanding uncommitted changes in "
                                        "subrepository '%s'") % s)
 
         elif not overwrite:
-            if pa == p1 or pa == p2: # linear
-                pass # all good
-            elif wc.dirty(missing=True):
-                raise util.Abort(_("crosses branches (merge branches or use"
-                                   " --clean to discard changes)"))
-            elif onode is None:
-                raise util.Abort(_("crosses branches (merge branches or update"
-                                   " --check to force update)"))
-            else:
-                # Allow jumping branches if clean and specific rev given
-                pa = p1
+            if pa not in (p1, p2):  # nolinear
+                dirty = wc.dirty(missing=True)
+                if dirty or onode is None:
+                    # Branching is a bit strange to ensure we do the minimal
+                    # amount of call to obsolete.background.
+                    foreground = obsolete.foreground(repo, [p1.node()])
+                    # note: the <node> variable contains a random identifier
+                    if repo[node].node() in foreground:
+                        pa = p1  # allow updating to successors
+                    elif dirty:
+                        msg = _("crosses branches (merge branches or use"
+                                " --clean to discard changes)")
+                        raise util.Abort(msg)
+                    else:  # node is none
+                        msg = _("crosses branches (merge branches or update"
+                                " --check to force update)")
+                        raise util.Abort(msg)
+                else:
+                    # Allow jumping branches if clean and specific rev given
+                    pa = p1
 
         ### calculate phase
         actions = calculateupdates(repo, wc, p2, pa,
                                    branchmerge, force, partial, mergeancestor)
 
diff --git a/tests/test-update-branches.t b/tests/test-update-branches.t
--- a/tests/test-update-branches.t
+++ b/tests/test-update-branches.t
@@ -162,5 +162,65 @@  Cases are run as shown in that table, ro
   $ revtest '-cC dirty linear'  dirty 1 2 -cC
   abort: cannot specify both -c/--check and -C/--clean
   parent=1
   M foo
 
+Test obsolescence behavior
+---------------------------------------------------------------------
+
+successors should be taken in account when checking head destination
+
+  $ cat << EOF >> $HGRCPATH
+  > [extensions]
+  > obs=$TESTTMP/obs.py
+  > [ui]
+  > logtemplate={rev}:{node|short} {desc|firstline}
+  > EOF
+  $ cat > $TESTTMP/obs.py << EOF
+  > import mercurial.obsolete
+  > mercurial.obsolete._enabled = True
+  > EOF
+
+Test no-argument update to a successor of an obsoleted changeset
+
+  $ hg log -G
+  o  5:ff252e8273df 5
+  |
+  o  4:d047485b3896 4
+  |
+  | o  3:6efa171f091b 3
+  | |
+  | | o  2:bd10386d478c 2
+  | |/
+  | @  1:0786582aa4b1 1
+  |/
+  o  0:60829823a42a 0
+  
+  $ hg status
+  M foo
+
+We add simple obsolescence marker between 3 and 4 (indirect successors)
+
+  $ hg id --debug -i -r 3
+  6efa171f091b00a3c35edc15d48c52a498929953
+  $ hg id --debug -i -r 4
+  d047485b3896813b2a624e86201983520f003206
+  $ hg debugobsolete 6efa171f091b00a3c35edc15d48c52a498929953 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
+  $ hg debugobsolete aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa d047485b3896813b2a624e86201983520f003206
+
+Test that 5 is detected as a valid destination from 3
+  $ hg up --quiet --hidden 3
+  $ hg up 5
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+
+Test that 5 is detected as a valid destination from 1
+  $ hg up --quiet 0          # we should be able to update to 3 directly
+  $ hg up --quiet --hidden 3 # but not implemented yet.
+  $ hg up 5
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+
+Test that 5 is not detected as a valid destination from 2
+  $ hg up --quiet 0
+  $ hg up --quiet 2
+  $ hg up 5
+  abort: crosses branches (merge branches or use --clean to discard changes)
+  [255]