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compare: 91badeba32d31d4dcc695a8888e5b697b4c3d90c
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  • 4 commits
  • 3 files changed
  • 1 contributor

Commits on Oct 24, 2022

  1. builtin/repack.c: pass "out" to prepare_pack_objects 

    `builtin/repack.c`'s `prepare_pack_objects()` is used to prepare a set
of arguments to a `pack-objects` process which will generate a desired
pack.

A future patch will add an `--expire-to` option which allows `git
repack` to write a cruft pack containing the pruned objects out to a
separate repository. Prepare for this by teaching that function to write
packs to an arbitrary location specified by the caller.

All existing callers of `prepare_pack_objects()` will pass `packtmp` for
`out`, retaining the existing behavior.

Signed-off-by: Taylor Blau <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>
    @ttaylorr @gitster
    ttaylorr authored and gitster committed Oct 24, 2022
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  2. builtin/repack.c: pass "cruft_expiration" to write_cruft_pack 

    `builtin/repack.c`'s `write_cruft_pack()` is used to generate the cruft
pack when `--cruft` is supplied. It uses a static variable
"cruft_expiration" which is filled in by option parsing.

A future patch will add an `--expire-to` option which allows `git
repack` to write a cruft pack containing the pruned objects out to a
separate repository. In order to implement this functionality, some
callers will have to pass a value for `cruft_expiration` different than
the one filled out by option parsing.

Prepare for this by teaching `write_cruft_pack` to take a
"cruft_expiration" parameter, instead of reading a single static
variable.

The (sole) existing caller of `write_cruft_pack()` will pass the value
for "cruft_expiration" filled in by option parsing, retaining existing
behavior. This means that we can make the variable local to
`cmd_repack()`, and eliminate the static declaration.

Signed-off-by: Taylor Blau <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>
    @ttaylorr @gitster
    ttaylorr authored and gitster committed Oct 24, 2022
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  3. builtin/repack.c: write cruft packs to arbitrary locations 

    In the following commit, a new write_cruft_pack() caller will be added
which wants to write a cruft pack to an arbitrary location. Prepare for
this by adding a parameter which controls the destination of the cruft
pack.

For now, provide "packtmp" so that this commit does not change any
behavior.

Signed-off-by: Taylor Blau <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>
    @ttaylorr @gitster
    ttaylorr authored and gitster committed Oct 24, 2022
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  4. builtin/repack.c: implement --expire-to for storing pruned objects 

    When pruning objects with `--cruft`, `git repack` offers some
flexibility when selecting the set of which objects are pruned via the
`--cruft-expiration` option.

This is useful for expiring objects which are older than the grace
period, making races where to-be-pruned objects become reachable and
then ancestors of freshly pushed objects, leaving the repository in a
corrupt state after pruning substantially less likely [1].

But in practice, such races are impossible to avoid entirely, no matter
how long the grace period is. To prevent this race, it is often
advisable to temporarily put a repository into a read-only state. But in
practice, this is not always practical, and so some middle ground would
be nice.

This patch introduces a new option, `--expire-to`, which teaches `git
repack` to write an additional cruft pack containing just the objects
which were pruned from the repository. The caller can specify a
directory outside of the current repository as the destination for this
second cruft pack.

This makes it possible to prune objects from a repository, while still
holding onto a supplemental copy of them outside of the original
repository. Having this copy on-disk makes it substantially easier to
recover objects when the aforementioned race is encountered.

`--expire-to` is implemented in a somewhat convoluted manner, which is
to take advantage of the fact that the first time `write_cruft_pack()`
is called, it adds the name of the cruft pack to the `names` string
list. That means the second time we call `write_cruft_pack()`, objects
in the previously-written cruft pack will be excluded.

As long as the caller ensures that no objects are expired during the
second pass, this is sufficient to generate a cruft pack containing all
objects which don't appear in any of the new packs written by `git
repack`, including the cruft pack. In other words, all of the objects
which are about to be pruned from the repository.

It is important to note that the destination in `--expire-to` does not
necessarily need to be a Git repository (though it can be) Notably, the
expired packs do not contain all ancestors of expired objects. So if the
source repository contains something like:

              <unreachable>
             /
    C1 --- C2
      \
       refs/heads/master

where C2 is unreachable, but has a parent (C1) which is reachable, and
C2 would be pruned, then the expiry pack will contain only C2, not C1.

[1]: https://lore.kernel.org/git/[email protected]/

Signed-off-by: Taylor Blau <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>
    @ttaylorr @gitster
    ttaylorr authored and gitster committed Oct 24, 2022
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