How to recover after criss-cross merge?

[Manuel Jacob]

Hi,

In the PyPy repository [1] we have three branches (which are interesting 
for this post): "default", "py3k", "py3.3".  "py3k" branched from 
"default" and we regularly merge "default" into "py3k".  "py3.3" 
branched from "py3k" and we regularly merge "py3k" into "py3.3".

Everything worked fine until someone merged directly from "default" to 
"py3.3" (changeset 4fa19970ddeb), skipping "py3k".  When merging from 
"py3k" into "py3.3" the next time (changeset 88aafcb7c318) we ran into 
problems.  It showed many "ambiguous merge - picked m action" warnings.  
Also, some files (e.g. lib-python/3/_abcoll.py) which were removed in 
the "py3.3" branch (but still existent in "py3k") re-appeared without 
warning.

A possible solution is to close the current "py3.3" branch head and redo 
(i.e. grafting or rebasing with --keep) all changes from just before the 
merge from "default" into "py3.3" (except the problematic merge itself).

I'd prefer to only redo all changes from just before the merge from 
"py3k" into "py3.3".  I tested (locally) redoing this merge (changeset 
88aafcb7c318) with aeafe30eac60 as a merge base (by passing "--config 
merge.preferancestor=aeafe30eac60").  This changeset aeafe30eac60 is the 
last commit from "py3k" which got merged into "py3.3" before 
88aafcb7c318.  It seems to do exactly what I want.  However the next 
time I merged "py3k" into "py3.3" (after in turn merging "default" into 
"py3k") I ran into problems again (ambiguous merge warnings and 
re-appearing files).  Is there a way to permanently fix the problem?

-Manuel

[1] https://bitbucket.org/pypy/pypy/