Re: worktrees vs. alternates

[Jeff King <peff@peff.net>]

On Wed, May 16, 2018 at 10:58:19AM -0400, Konstantin Ryabitsev wrote:

> The parent repo is not keeping track of any other repositories that may
> be using it for alternates, which is why you basically:
> 
> 1. never run auto-gc in the parent repo
> 2. repack it manually using -Ad to keep loose objects that other repos
> may be borrowing (but we don't know if they are)
> 3. never prune the parent repo, because this may delete objects other
> repos are borrowing
> 
> Very infrequently you may consider this extra set of maintenance steps:
> 
> 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)
> 4. Repack child repos again, this time with the -l flag, to get your
> savings back.

You can also do periodic maintenance like:

  1. Copy each ref in the forked repositories into the parent repository
     (e.g., giving each child that borrows from the parent its own
     hierarchy in refs/remotes/<child>/*).

  2. Repack the parent as normal. It will retain any objects referenced
     by the children (because they are now referenced by it).

But note that:

  1. It's not atomic with respect to updates in the child repos (but
     then, neither is the single-repo case!).

  2. It doesn't know about reflogs or the index in the child
     repositories.

This is more or less how we use alternates at GitHub.

> 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.

If you set:

  git config core.repositoryformatversion 1
  git config extensions.preciousObjects true

in the parent, git-prune (repack -d) will refuse to run. That doesn't
solve the problem of how to repack, but it can help prevent accidental
misuse.

-Peff