diff --git a/mercurial/copies.py b/mercurial/copies.py
--- a/mercurial/copies.py
+++ b/mercurial/copies.py
@@ -306,7 +306,6 @@
 def _changesetforwardcopies(a, b, match):
     if a.rev() in (node.nullrev, b.rev()):
         return {}
-
     repo = a.repo()
     children = {}
     revinfo = _revinfogetter(repo)
@@ -314,6 +313,8 @@
 
     cl = repo.changelog
     missingrevs = cl.findmissingrevs(common=[a.rev()], heads=[b.rev()])
+    mrset = set(missingrevs)
+    roots = set()
     for r in missingrevs:
         for p in parents(r):
             if p == node.nullrev:
@@ -322,17 +323,23 @@
                 children[p] = [r]
             else:
                 children[p].append(r)
+            if p not in mrset:
+                roots.add(p)
+    if not roots:
+        # no common revision to track copies from
+        return {}
+    min_root = min(roots)
 
-    roots = set(children) - set(missingrevs)
-    work = [r for r in roots]
+    from_head = set(cl.reachableroots(min_root, [b.rev()], list(roots), includepath=True))
+
+    iterrevs = set(from_head)
+    iterrevs &= mrset
+    iterrevs.update(roots)
+    iterrevs.remove(b.rev())
     all_copies = dict((r, {}) for r in roots)
-    heapq.heapify(work)
     alwaysmatch = match.always()
-    while work:
-        r = heapq.heappop(work)
+    for r in sorted(iterrevs):
         copies = all_copies.pop(r)
-        if r == b.rev():
-            return copies
         for i, c in enumerate(children[r]):
             p1, p2, p1copies, p2copies, removed = revinfo(c)
             if r == p1:
@@ -358,7 +365,6 @@
                     del newcopies[f]
             othercopies = all_copies.get(c)
             if othercopies is None:
-                heapq.heappush(work, c)
                 all_copies[c] = newcopies
             else:
                 # we are the second parent to work on c, we need to merge our
@@ -378,7 +384,7 @@
                 else:
                     newcopies.update(othercopies)
                     all_copies[c] = newcopies
-    assert False
+    return all_copies[b.rev()]
 
 
 def _forwardcopies(a, b, base=None, match=None):
diff --git a/tests/test-copies.t b/tests/test-copies.t
--- a/tests/test-copies.t
+++ b/tests/test-copies.t
@@ -236,7 +236,7 @@
   $ hg debugpathcopies 1 2
   y -> x
   $ hg debugpathcopies 1 3
-  $ hg debugpathcopies 2 3
+  $ FOO=1 hg debugpathcopies 2 3
   x -> y
 
 Copy file from either side in a merge