builtin/pack-objects.c: freshen objects from existing cruft packs

Once an object is written into a cruft pack, we can only freshen it by writing a new loose or packed copy of that object with a more recent mtime. Prior to 61568efa95 (builtin/pack-objects.c: support `--max-pack-size` with `--cruft`, 2023-08-28), we typically had at most one cruft pack in a repository at any given time. So freshening unreachable objects was straightforward when already rewriting the cruft pack (and its *.mtimes file). But 61568efa95 changes things: 'pack-objects' now supports writing multiple cruft packs when invoked with `--cruft` and the `--max-pack-size` flag. Cruft packs are rewritten until they reach some size threshold, at which point they are considered "frozen", and will only be modified in a pruning GC, or if the threshold itself is adjusted. Prior to this patch, however, this process breaks down when we attempt to freshen an object packed in an earlier cruft pack, and that cruft pack is larger than the threshold and thus will survive the repack. When this is the case, it is impossible to freshen objects in cruft pack(s) when those cruft packs are larger than the threshold. This is because we would avoid writing them in the new cruft pack entirely, for a couple of reasons. 1. When enumerating packed objects via 'add_objects_in_unpacked_packs()' we pass the SKIP_IN_CORE_KEPT_PACKS, which is used to avoid looping over the packs we're going to retain (which are marked as kept in-core by 'read_cruft_objects()'). This means that we will avoid enumerating additional packed copies of objects found in any cruft packs which are larger than the given size threshold. Thus there is no opportunity to call 'create_object_entry()' whatsoever. 2. We likewise will discard the loose copy (if one exists) of any unreachable object packed in a cruft pack that is larger than the threshold. Here our call path is 'add_unreachable_loose_objects()', which uses the 'add_loose_object()' callback. That function will eventually land us in 'want_object_in_pack()' (via 'add_cruft_object_entry()'), and we'll discard the object as it appears in one of the packs which we marked as kept in-core. This means in effect that it is impossible to freshen an unreachable object once it appears in a cruft pack larger than the given threshold. Instead, we should pack an additional copy of an unreachable object we want to freshen even if it appears in a cruft pack, provided that the cruft copy has an mtime which is before the mtime of the copy we are trying to pack/freshen. This is sub-optimal in the sense that it requires keeping an additional copy of unreachable objects upon freshening, but we don't have a better alternative without the ability to make in-place modifications to existing *.mtimes files. In order to implement this, we have to adjust the behavior of 'want_found_object()'. When 'pack-objects' is told that we're *not* going to retain any cruft packs (i.e. the set of packs marked as kept in-core does not contain a cruft pack), the behavior is unchanged. But when there *is* at least one cruft pack that we're holding onto, it is no longer sufficient to reject a copy of an object found in that cruft pack for that reason alone. In this case, we only want to reject a candidate object when copies of that object either: - exists in a non-cruft pack that we are retaining, regardless of that pack's mtime, or - exists in a cruft pack with an mtime at least as recent as the copy we are debating whether or not to pack, in which case freshening would be redundant. To do this, keep track of whether or not we have any cruft packs in our in-core kept list with a new 'ignore_packed_keep_in_core_has_cruft' flag. When we end up in this new special case, we replace a call to 'has_object_kept_pack()' to 'want_cruft_object_mtime()', and only reject objects when we have a copy in an existing cruft pack with at least as recent an mtime as our candidate (in which case "freshening" would be redundant). Signed-off-by: Taylor Blau <me@ttaylorr.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>