# HG changeset patch
# User Mads Kiilerich <madski@unity3d.com>
# Date 2014-10-03 00:20:36
# Node ID f4c60fafac5413c44cafab386000ca7456309aca
# Parent  18f6bdd1e3fc81972eb52398a295921380a98271

compare: workaround unexpected Mercurial behaviour when finding ancestor of null rev

ancestor(id(0000000000000000000000000000000000000000),7) == 7
- that caused weird pull request diffs and lots of grief when (accidentally)
creating a pull request from the null revision.

Add special handling for the case of null revisions.

diff --git a/kallithea/controllers/compare.py b/kallithea/controllers/compare.py
--- a/kallithea/controllers/compare.py
+++ b/kallithea/controllers/compare.py
@@ -88,10 +88,14 @@ class CompareController(BaseRepoControll
             else:
                 hgrepo = other_repo._repo
 
-            ancestors = hgrepo.revs("ancestor(id(%s), id(%s))", org_rev, other_rev)
-            if ancestors:
-                # pick arbitrary ancestor - but there is usually only one
-                ancestor = hgrepo[ancestors[0]].hex()
+            if org_repo.EMPTY_CHANGESET in (org_rev, other_rev):
+                # work around unexpected Mercurial behaviour
+                ancestor = org_repo.EMPTY_CHANGESET
+            else:
+                ancestors = hgrepo.revs("ancestor(id(%s), id(%s))", org_rev, other_rev)
+                if ancestors:
+                    # FIXME: picks arbitrary ancestor - but there is usually only one
+                    ancestor = hgrepo[ancestors[0]].hex()
 
             other_revs = hgrepo.revs("ancestors(id(%s)) and not ancestors(id(%s)) and not id(%s)",
                                      other_rev, org_rev, org_rev)