[Bug 3984] New: Merge with ambiguous ancestor does the wrong thing

[mercurial-bugs at selenic.com]

http://bz.selenic.com/show_bug.cgi?id=3984

          Priority: normal
            Bug ID: 3984
                CC: mercurial-devel at selenic.com
          Assignee: bugzilla at selenic.com
           Summary: Merge with ambiguous ancestor does the wrong thing
          Severity: bug
    Classification: Unclassified
                OS: Linux
          Reporter: telliott at apptio.com
          Hardware: PC
            Status: UNCONFIRMED
           Version: 2.6
         Component: Mercurial
           Product: Mercurial

(I don't _think_ this is the same thing as 1327, but correct me if I'm wrong)

Mercurial version 2.6 (+ AFAIK, we tested with 2.6.0, 2.6.1, and 2.6.2) cannot
determine the common ancestor, whereas 2.2.2 and 2.4.2 can.

For the repro steps below, the following is the result in 2.2.2:
> hg log -r "ancestor(I, J)"
changeset: D
parent:    B

> hg up I
> hg merge J

Does exactly what we want it to.  Causes the changes in J to be merged into
default.

In 2.6.x, the log command does nothing, and the attempt to merge causes 22
thousand files (that were deleted or have been moved) to be re-added to the
repository in default.

Repro steps for the last time this occurred:
--------------------------------------------
(I can come up with a simple repository with changesets that exhibits the
problem, but it's too much work I don't have time for right now)

- Have a repository with a bunch of changes, two branches (foo, default).
- Ensure the branches are merged together at some point.  I'll assume the
following initial state for the example:

default:      A ---+ C  (Merge of B and A)
foo branch:   B --/

Person 1:  
1. Make a change to foo branch (D)
2. Merge that change to default (E)

default:      A ---+ C ----+ E 
foo branch:   B --/-- D --/


Person 2:  (Have NOT pulled 1's changes yet)
1. Make a change to foo branch (F)
2. Merge that change to default (G)
3. Try to push.  Ooops, multiple heads!  Pull person 1's changes.
4. Merge B & D on Foo (H)
5. Merge C and E on default (I)

default:      A -----+ C ---+ G --+ I
                    /      /  E -/
                   /      /
foo branch:   B --/-- F ----+ H
                      D ---/

(Note that person 2 did NOT merge H and I into default.  Bad person 2!  Even
though it's no-op, they should have done it)

Person 3:  (Up to date with all of the above)
1. Make a change to foo branch (J)
2. Merge that change to default (K)

default:      I -------+ K
foo branch:   H -- J -/

*** This merge into K does the WRONG thing, it adds 22 thousand files back to
the
repository.

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