gc: introduce `gc.recentObjectsHook`

This patch introduces a new multi-valued configuration option,
`gc.recentObjectsHook` as a means to mark certain objects as recent (and
thus exempt from garbage collection), regardless of their age.

When performing a garbage collection operation on a repository with
unreachable objects, Git makes its decision on what to do with those
object(s) based on how recent the objects are or not. Generally speaking,
unreachable-but-recent objects stay in the repository, and older objects
are discarded.

However, we have no convenient way to keep certain precious, unreachable
objects around in the repository, even if they have aged out and would
be pruned. Our options today consist of:

  - Point references at the reachability tips of any objects you
    consider precious, which may be undesirable or infeasible if there
    are many such objects.

  - Track them via the reflog, which may be undesirable since the
    reflog's lifetime is limited to that of the reference it's tracking
    (and callers may want to keep those unreachable objects around for
    longer).

  - Extend the grace period, which may keep around other objects that
    the caller *does* want to discard.

  - Manually modify the mtimes of objects you want to keep. If those
    objects are already loose, this is easy enough to do (you can just
    enumerate and `touch -m` each one).

    But if they are packed, you will either end up modifying the mtimes
    of *all* objects in that pack, or be forced to write out a loose
    copy of that object, both of which may be undesirable. Even worse,
    if they are in a cruft pack, that requires modifying its `*.mtimes`
    file by hand, since there is no exposed plumbing for this.

  - Force the caller to construct the pack of objects they want
    to keep themselves, and then mark the pack as kept by adding a
    ".keep" file. This works, but is burdensome for the caller, and
    having extra packs is awkward as you roll forward your cruft pack.

This patch introduces a new option to the above list via the
`gc.recentObjectsHook` configuration, which allows the caller to
specify a program (or set of programs) whose output is treated as a set
of objects to treat as recent, regardless of their true age.

The implementation is straightforward. Git enumerates recent objects via
`add_unseen_recent_objects_to_traversal()`, which enumerates loose and
packed objects, and eventually calls add_recent_object() on any objects
for which `want_recent_object()`'s conditions are met.

This patch modifies the recency condition from simply "is the mtime of
this object more recent than the cutoff?" to "[...] or, is this object
mentioned by at least one `gc.recentObjectsHook`?".

Depending on whether or not we are generating a cruft pack, this allows
the caller to do one of two things:

  - If generating a cruft pack, the caller is able to retain additional
    objects via the cruft pack, even if they would have otherwise been
    pruned due to their age.

  - If not generating a cruft pack, the caller is likewise able to
    retain additional objects as loose.

A potential alternative here is to introduce a new mode to alter the
contents of the reachable pack instead of the cruft one. One could
imagine a new option to `pack-objects`, say `--extra-reachable-tips`
that does the same thing as above, adding the visited set of objects
along the traversal to the pack.

But this has the unfortunate side-effect of altering the reachability
closure of that pack. If parts of the unreachable object graph mentioned
by one or more of the "extra reachable tips" programs is not closed,
then the resulting pack won't be either. This makes it impossible in the
general case to write out reachability bitmaps for that pack, since
closure is a requirement there.

Instead, keep these unreachable objects in the cruft pack (or set of
unreachable, loose objects) instead, to ensure that we can continue to
have a pack containing just reachable objects, which is always safe to
write a bitmap over.

Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 31 insertions, 0 deletions

diff --git a/t/t7701-repack-unpack-unreachable.sh b/t/t7701-repack-unpack-unreachable.sh
index ebb267855f..ba428c18a8 100755
--- a/t/t7701-repack-unpack-unreachable.sh
+++ b/t/t7701-repack-unpack-unreachable.sh
@@ -113,6 +113,37 @@ test_expect_success 'do not bother loosening old objects' '
 	test_must_fail git cat-file -p $obj2
 '
 
+test_expect_success 'gc.recentObjectsHook' '
+	obj1=$(echo one | git hash-object -w --stdin) &&
+	obj2=$(echo two | git hash-object -w --stdin) &&
+	obj3=$(echo three | git hash-object -w --stdin) &&
+	pack1=$(echo $obj1 | git pack-objects .git/objects/pack/pack) &&
+	pack2=$(echo $obj2 | git pack-objects .git/objects/pack/pack) &&
+	pack3=$(echo $obj3 | git pack-objects .git/objects/pack/pack) &&
+	git prune-packed &&
+
+	git cat-file -p $obj1 &&
+	git cat-file -p $obj2 &&
+	git cat-file -p $obj3 &&
+
+	git tag -a -m tag obj2-tag $obj2 &&
+	obj2_tag="$(git rev-parse obj2-tag)" &&
+
+	write_script precious-objects <<-EOF &&
+	echo $obj2_tag
+	EOF
+	git config gc.recentObjectsHook ./precious-objects &&
+
+	test-tool chmtime =-86400 .git/objects/pack/pack-$pack2.pack &&
+	test-tool chmtime =-86400 .git/objects/pack/pack-$pack3.pack &&
+	git repack -A -d --unpack-unreachable=1.hour.ago &&
+
+	git cat-file -p $obj1 &&
+	git cat-file -p $obj2 &&
+	git cat-file -p $obj2_tag &&
+	test_must_fail git cat-file -p $obj3
+'
+
 test_expect_success 'keep packed objects found only in index' '
 	echo my-unique-content >file &&
 	git add file &&