Subject: Re: [RFC/PATCH] log: add log.firstparent option

From: Jeff King <[email-masked]>
Date: Fri, 24 Jul 2015 00:34:16 -0700

On Thu, Jul 23, 2015 at 11:07:58PM -0700, Jacob Keller wrote:

> I think some projects definitely benefit from the first-parent setup,
> and it could be valuable, but I do tend to agree with Junio here that
> the mess is always helpful. If may be helpful if people's commit
> messages on that mess are good, but generally those that don't take
> the time to rebase local work and re-express the commit messages are
> not going to leave insightful messages the first time. However, have
> the ability to view history this way is still possibly valuable.

I think a really simple example is something like:

  1. somebody implements as feature. It needs to handle cases a, b, and
     c, but it only handles case a. Therefore it is buggy.

  2. During review, somebody notices case b, and a new commit is made to
     fix it. Nobody notices case c.

  3. The topic is merged.

  4. Much later, somebody notices the system is buggy and hunts in the
     history.

In a "clean" history, the patches from steps 1 and 2 are squashed. While
reading the history, you see only "implement feature X", and no mention
of the bug and its fix. But even if the person writes a terrible commit
message for step (2), even seeing it pulled out into its own diff shows
the exact nature of the already-seen bug, and may make it more obvious
to realize that case (c) is a problem.

I realize that's kind of vague. Another way to think about it is: in a
squashing workflow like git.git, any time you have to turn to the
mailing list to read the original sequence of re-rolls, you would have
been better off if that information were in git. That's a minority case,
but I certainly have turned to it (in some cases, the "fix" from our
step 2 above actually introduces the new bug, and it's nice to see the
reasoning that went into it :) ).

Not that I am advocating for git.git to move to such a workflow. I think
on balance the "clean" history is nicer to work with. I am only arguing
that keeping the messy history is not without value; there are some
cases where it is nice to have (and we keep it in the list archive,
which is a minor pain to access compared to git).

-Peff