git log --merges doesn't show commits as expected

git log --merges doesn't show commits as expected

[Dominik Rauch]

Hello!
 
(This is my first post to this mailing list and I couldn't find a FAQ section - please excuse me, if I haven't followed all the established posting guidelines yet.)

I have the following repository tree:

C
|\
| B
| /
A

Commit A: Parents=(). Initial commit. Contains file foo with content "ABC".
Commit B: Parents=(A). Represents a commit on some feature branch. Contains file foo with content "XYZ".
Commit C: Parents=(A, B). Represents a merge commit of a feature branch back to the main branch. Contains file foo with content "XYZ".

I expected "git log --merges foo" to show C, however, the log is empty! Specifying "--full-history" results in the correct history, therefore I assume, I misunderstand Git's default history simplification algorithm. Unfortunately, the example in the Git docs at [1] does not contain the very same situation (although it is probably one of the most common situations...).

Does anybody know why I don't see the log output I expect? I'm confused...even if the log output is correct, I don't think it follows the principle of least surprise...

Side note: specifying "--first-parent" also results in commit C being shown.

Best regards,
Dominik

PS: This is a cross post of [2], somebody noted it could be a bug, which is why I decided to post to this mailing list.

[1] https://git-scm.com/docs/git-log#_history_simplification 
[2] http://stackoverflow.com/q/33695763/

Re: git log --merges doesn't show commits as expected

[Jeff King]

On Fri, Nov 13, 2015 at 04:21:36PM +0000, Dominik Rauch wrote:

> (This is my first post to this mailing list and I couldn't find a FAQ
> section - please excuse me, if I haven't followed all the established
> posting guidelines yet.)

This is the right place. Welcome. :)

> I have the following repository tree:
> 
> C
> |\
> | B
> | /
> A
> 
> Commit A: Parents=(). Initial commit. Contains file foo with content "ABC".
> Commit B: Parents=(A). Represents a commit on some feature branch. Contains file foo with content "XYZ".
> Commit C: Parents=(A, B). Represents a merge commit of a feature branch back to the main branch. Contains file foo with content "XYZ".
> 
> I expected "git log --merges foo" to show C, however, the log is
> empty! Specifying "--full-history" results in the correct history,
> therefore I assume, I misunderstand Git's default history
> simplification algorithm. Unfortunately, the example in the Git docs
> at [1] does not contain the very same situation (although it is
> probably one of the most common situations...).

I don't think "--merges" is relevant to the simplification here. By
asking for "foo", that turns on history simplification. Since the merge
at C took one side directly, that means we can cull the parent link that
leads to A (its foo=ABC did not contribute to the final outcome). And
then C is TREESAME to its only remaining parent (they both have
foo=XYZ), so it can be removed, leaving only commit B to be passed to
the next stage.

And then we apply "--merges", and see that B is not a merge, and so
do not show it (but we still traverse it, of course).

In theory we could apply "--merges" first (by simplifying history to
shrink any chains of non-merges to a single point). But I don't think
there's any way to do that currently with git.

-Peff