Re: DEVEL: Help with feature implementation

From: Antonio Russo <aerusso@aerusso.net>
Date: Jan 18, 2021, 18:19 UTC
Message-ID: <f29fadef-6497-403c-59e0-8bd7beec77af@aerusso.net>
In-Reply-To: <C8MHFL3YJDSF.1GLOUSV7BYS1X@localhost>

On 1/18/21 11:06 AM, Aiyee Bee wrote:

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> Hello everybody!
> 
> I want to implement a feature in git, and I'm looking for some help.
> 
> I wanted to add a history-simplification option to rev-list so that it
> doesn't simplify away any irrelevant commits if they have multiple
> relevant _children_, i.e. when they are the point where two relevant
> histories diverge.

I'm actually working on something that does the opposite---it ignores the fork point when drawing the graph. (I'm currently dog-fooding a partial implementation of this.)

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> Basically the effect I want for --show-forkpoints (named like --show-pulls):
> http://ix.io/2Ms6
> 
> But it seems there is no existing apparatus in the revision walker for
> deciding simplification on basis of *children*, am I correct? Admittedly
> my understanding of it is still a WIP, but I don't see anything that
> could help.

It's even worse than this. AFAICT, there are two code paths: one for when the git-commit-graph is enabled, and one when it is not. There's a special "revs->limited" option which lets you deal with the commits all at once, at the expense of losing all the speed improvements from the git-commit-graph efforts.

Aside: I want to get rid of that alternative path.  But I only have limited
time.

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> I was hoping that simply the flag CHILD_SHOWN could be checked, but it
> seems that's only set on boundary commits :(
> 
> This option would be pretty useful when used with some diffs, to see how
> much two forks have diverged. Currently if you use history simplification,
> the diffs for both divergent histories will be created against the last
> relevant commit instead of the last common ancestor, which creates pretty
> useless diffs with a lot of intersection between them.

Sounds neat!

> Are there any viable alternative means to do this that I can explore?

I too would like to hear about suggestions for this.

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> 
> Thanks!
> 
> FriendlyNeighborhoodShane
>