Junio C Hamano <gitster <at> pobox.com> writes:
> The "alternates" mechanism [...]

sorry for the somewhat late response - i found this thread only now.

at qt-project.org we have a somewhat peculiar setup: we have the qt4 repository,
and a bunch of qt5 repositories which resulted from a split. qt5 is under active
development, but qt4 is still maintained. that means that we need to cherry-pick
between those repositories quite a lot. for an optimal cherry-picking experience
one needs three-way-merging, which means we need shared object stores. which is
where the problems start:

my first approach was just a common objects/ directory with all repositories
symlinking into it. problems:
- the object store can never be garbage-collected. with a lot of heavy rebasing
and temporarily added remotes, it gets messy after a while.
- there is a constant risk of destroying the object store by inadvertently
running git gc - which is particularly likely with git-gui, as it seems to be
retarded enough to ignore the auto-gc setting.

so the second approach is the "bare aggregator repo" which adds all other repos
as remotes, and the other repos link back via alternates. problems:
- to actually share objects, one always needs to push to the aggregator
- tags having a shared namespace doesn't actually work, because the repos have
the same tags on different commits (they are independent repos, after all)
- one still cannot safely garbage-collect the aggregator, as the refs don't
include the stashes and the index, so rebasing may invalidate these more
transient objects.

i would re-propose hallvard's "volatile" alternates (at least i think that's
what he was talking about two weeks ago): they can be used to obtain objects,
but every object which is in any way referenced from the current clone must be
available locally (or from a "regular" alternate). that means that diffing, etc.
would get objects only temporarily, while cherry-picking would actually copy
(some of) the objects. this would make it possible to "cross-link" repositories,
safely and without any "3rd parties".



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