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, 15 insertions, 0 deletions

diff --git a/Documentation/config/gc.txt b/Documentation/config/gc.txt
index 7f95c866e1..ca47eb2008 100644
--- a/Documentation/config/gc.txt
+++ b/Documentation/config/gc.txt
@@ -130,6 +130,21 @@ or rebase occurring.  Since these changes are not part of the current
 project most users will want to expire them sooner, which is why the
 default is more aggressive than `gc.reflogExpire`.
 
+gc.recentObjectsHook::
+	When considering whether or not to remove an object (either when
+	generating a cruft pack or storing unreachable objects as
+	loose), use the shell to execute the specified command(s).
+	Interpret their output as object IDs which Git will consider as
+	"recent", regardless of their age. By treating their mtimes as
+	"now", any objects (and their descendants) mentioned in the
+	output will be kept regardless of their true age.
++
+Output must contain exactly one hex object ID per line, and nothing
+else. Objects which cannot be found in the repository are ignored.
+Multiple hooks are supported, but all must exit successfully, else the
+operation (either generating a cruft pack or unpacking unreachable
+objects) will be halted.
+
 gc.rerereResolved::
 	Records of conflicted merge you resolved earlier are
 	kept for this many days when 'git rerere gc' is run.