Patchwork [5,of,7] destutil: allow to specify an explicit source for the merge

diff --git a/mercurial/destutil.py b/mercurial/destutil.py
--- a/mercurial/destutil.py
+++ b/mercurial/destutil.py
@@ -183,13 +183,23 @@  msgdestmerge = {
     'notatheads':
         {'merge':
             (_('working directory not at a head revision'),
              _("use 'hg update' or merge with an explicit revision"))
         },
+    'emptysourceset':
+        {'merge':
+            (_('source set is empty'),
+             None)
+        },
+    'multiplebranchessourceset':
+        {'merge':
+            (_('source set is rooted in multiple branches'),
+             None)
+        },
     }
 
-def _destmergebook(repo, action='merge'):
+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:
@@ -204,30 +214,42 @@  def _destmergebook(repo, action='merge')
         msg, hint = msgdestmerge['nootherbookmarks'][action]
         raise error.Abort(msg, hint=hint)
     assert node is not None
     return node
 
-def _destmergebranch(repo, action='merge'):
+def _destmergebranch(repo, action='merge', sourceset=None):
     """find merge destination based on branch heads"""
     node = None
-    parent = repo.dirstate.p1()
-    branch = repo.dirstate.branch()
+
+    if sourceset is None:
+        sourceset = [repo[repo.dirstate.p1()].rev()]
+        branch = repo.dirstate.branch()
+    elif not sourceset:
+        msg, hint = msgdestmerge['emptysourceset'][action]
+        raise error.Abort(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.Abort(msg, hint=hint)
+            branch = ctx.branch()
+
     bheads = repo.branchheads(branch)
-
-    if parent not in bheads:
-        # Case A: working copy if not on a head.
+    if not repo.revs('%ld and %ln', sourceset, bheads):
+        # 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 current head from the set
-    bheads = [bh for bh in bheads if bh != parent]
+    bheads = list(repo.revs('%ln - (%ld::)', bheads, sourceset))
     # filters out bookmarked heads
-    nbhs = [bh for bh in bheads if not repo[bh].bookmarks()]
+    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
@@ -251,21 +273,21 @@  def _destmergebranch(repo, action='merge
     else:
         node = nbhs[0]
     assert node is not None
     return node
 
-def destmerge(repo, action='merge'):
+def destmerge(repo, action='merge', sourceset=None):
     """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)
+        node = _destmergebook(repo, action=action, sourceset=sourceset)
     else:
-        node = _destmergebranch(repo, action=action)
+        node = _destmergebranch(repo, action=action, sourceset=sourceset)
     return repo[node].rev()
 
 histeditdefaultrevset = 'reverse(only(.) and not public() and not ::merge())'
 
 def desthistedit(ui, repo):
diff --git a/mercurial/revset.py b/mercurial/revset.py
--- a/mercurial/revset.py
+++ b/mercurial/revset.py
@@ -539,12 +539,14 @@  def _destupdate(repo, subset, x):
     return subset & baseset([destutil.destupdate(repo, **args)[0]])
 
 @predicate('_destmerge')
 def _destmerge(repo, subset, x):
     # experimental revset for merge destination
-    getargs(x, 0, 0, _("_mergedefaultdest takes no arguments"))
-    return subset & baseset([destutil.destmerge(repo)])
+    sourceset = None
+    if x is not None:
+        sourceset = getset(repo, fullreposet(repo), x)
+    return subset & baseset([destutil.destmerge(repo, sourceset=sourceset)])
 
 @predicate('adds(pattern)', safe=True)
 def adds(repo, subset, x):
     """Changesets that add a file matching pattern.
 
diff --git a/tests/test-merge-default.t b/tests/test-merge-default.t
--- a/tests/test-merge-default.t
+++ b/tests/test-merge-default.t
@@ -114,5 +114,38 @@  Test experimental destination revset
   $ hg log -r '_destmerge()'
   abort: branch 'foobranch' has one head - please merge with an explicit rev
   (run 'hg heads' to see all heads)
   [255]
 
+(on a branch with a two heads)
+
+  $ hg up 5
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ echo f >> a
+  $ hg commit -mf
+  created new head
+  $ hg log -r '_destmerge()'
+  changeset:   6:e88e33f3bf62
+  parent:      5:a431fabd6039
+  parent:      3:ea9ff125ff88
+  user:        test
+  date:        Thu Jan 01 00:00:00 1970 +0000
+  summary:     m2
+  
+
+(from the other head)
+
+  $ hg log -r '_destmerge(e88e33f3bf62)'
+  changeset:   8:b613918999e2
+  tag:         tip
+  parent:      5:a431fabd6039
+  user:        test
+  date:        Thu Jan 01 00:00:00 1970 +0000
+  summary:     f
+  
+
+(from unrelated branch)
+
+  $ hg log -r '_destmerge(foobranch)'
+  abort: branch 'foobranch' has one head - please merge with an explicit rev
+  (run 'hg heads' to see all heads)
+  [255]