Re: git merge banch w/ different submodule revision

[Elijah Newren <newren@gmail.com>]

On Thu, Apr 26, 2018 at 3:49 AM, Middelschulte, Leif
<Leif.Middelschulte@klsmartin.com> wrote:
> Hi,
>
> we're using git-flow as a basic development workflow. However, doing so revealed unexpected merge-behavior by git.
>
> Assume the following setup:
>
> - Repository `S` is sourced by repository `p` as submodule `s`
> - Repository `p` has two branches: `feature_x` and `develop`
> - The revisions sourced via the submodule have a linear history
>
>
> * 1c1d38f (feature_x) update submodule revision to b17e9d9
> | * 3290e69 (HEAD -> develop) update submodule revision to 0598394
> |/
> * cd5e1a5 initial submodule revision
>
>
> Problem case: Merge either branch into the other
>
> Expected behavior: Merge conflict.
>
> Actual behavior: Auto merge without conflicts.
>
> Note 1: A merge conflict does occur, if the sourced revisions do *not* have a linear history
>
> Did I get something wrong about how git resolves merges? Shouldn't git be like: "hey, you're trying to merge two different contents for the same line" (the submodule's revision)

Hard to say without saying what commit was referenced for the
submodule in the merge-bases for the two repositories you have.  In
the basic case..

If branch A and branch B have different commits checked out in the
submodule, say:
   A: deadbeef
   B: ba5eba11

then it's not clear whether there's a conflict or not.  The merge-base
(the common point of history) matters.  So, for example if the
original version (which I'll refer to as 'O") had:
  O: deadbeef

then you would say, "Oh, branch A made no change to this submodule but
B did.  So let's go with what B has."  Conversely, of O had ba5eba11,
then you'd go the other way.

But, there is some further smarts in that if either A or B point at
commits that contain the other in their history and both contain the
commit that O points at, then you can just do a fast-forward update to
the newest.


You didn't tell us how the merge-base (cd5e1a5 from the diagram you
gave) differed in your example here between the two repositories.  In
fact, the non-linear case could have several merge-bases, in which
case they all become potentially relevant (as does their merge-bases
since at that point you'll trigger the recursive portion of
merge-recursive).  Giving us that info might help us point out what
happened, though if either the fast-forward logic comes into play or
the recursive logic gets in the mix, then we may need you to provide a
testcase (or access to the repo in question) in order to explain it
and/or determine if you've found a bug.

Does that help?

Elijah