Re: worktrees vs. alternates

[Martin Fick <mfick@codeaurora.org>]

On Wednesday, May 16, 2018 10:58:19 AM Konstantin Ryabitsev 
wrote:
> 
> 1. Find every repo mentioning the parent repository in
> their alternates 2. Repack them without the -l switch
> (which copies all the borrowed objects into those repos)
> 3. Once all child repos have been repacked this way, prune
> the parent repo (it's safe now)

This is probably only true if the repos are in read-only 
mode?  I suspect this is still racy on a busy server with no 
downtime.

> 4. Repack child repos again, this time with the -l flag,
> to get your savings back.
 
> I would heartily love a way to teach git-repack to
> recognize when an object it's borrowing from the parent
> repo is in danger of being pruned. The cheapest way of
> doing this would probably be to hardlink loose objects
> into its own objects directory and only consider "safe"
> objects those that are part of the parent repository's
> pack. This should make alternates a lot safer, just in
> case git-prune happens to run by accident.

I think that hard linking is generally a good approach to 
solving many of the "pruning" races left in git.

I have uploaded a "hard linking" proposal to jgit that could 
potentially solve a similar situation that is not alternate 
specific, and only for packfiles, with the intent of 
eventually also doing something similar for loose 
objects.  You can see this here: 

https://git.eclipse.org/r/c/122288/2

I think it would be good to fill in more of these pruning 
gaps!

-Martin

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