Re: git merge banch w/ different submodule revision

From: Elijah Newren <newren@gmail.com>
Date: Apr 28, 2018, 00:24 UTC
Message-ID: <CABPp-BGX-hQYdqfNQZ42313VVhKd7GzgUJqvgwOj=0TEO5UQpQ@mail.gmail.com>
In-Reply-To: <1524825269.2227.5.camel@klsmartin.com>

Hi,

On Fri, Apr 27, 2018 at 3:37 AM, Middelschulte, Leif <Leif.Middelschulte@klsmartin.com> wrote:

> Am Donnerstag, den 26.04.2018, 17:19 -0700 schrieb Elijah Newren:
>> On Thu, Apr 26, 2018 at 3:49 AM, Middelschulte, Leif
>> <Leif.Middelschulte@klsmartin.com> wrote:

<snip>

Show 7 quoted lines

>> > 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

Let me just note that I don't actually use submodules myself, and rarely run across them, so as far as users expect submodules should behave I may have to defer to others. But it was particularly this sentence of yours that caught my attention and got me to respond. I may have misunderstood which repository had the non-linear history, but...

<snip>

>> 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.

This particular paragraph, is relevant to your example; more details below.

Show 15 quoted lines

>> 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.
>
> I placed two reositories here: https://gitlab.com/foss-contributions/git-examples/network/develop
> The access should be public w/o login.
>
> If you prefer the examples to be placed somewhere else, let me know.

So the only thing I see here is a single repository, which contains a submodule with linear history. (unless I was grabbing it wrong; I just tried `git clone --recurse-submodules https://gitlab.com/foss-contributions/git-examples`) Do you also have an example with non-linear history demonstrating your claim that it behaves differently, for comparison?

Anyway, in this case you had both branches updating the submodule to something newer (to a fast-forward update of what it previously was), but one side advanced it further than the other side did (in particular, to what turned out to be a fast-forward update of what the other branch used). That means the whole fast-forwarding logic of commit 68d03e4a6e44 ("Implement automatic fast-forward merge for submodules", 2010-07-07)) came into play.

I would expect that a different example involving non-linear history would behave the same, if both sides update the submodule in a fashion that is just fast-forwarding and one commit contains the other in its history. I'm curious if you have a counter example.