Replace a 'git repack --cruft -d' with the wrapper 'git gc --cruft' to
exercise some logic in builtin/gc.c that adds the '--cruft' option to
the underlying 'git repack' command.
Signed-off-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
We don't bother to freshen objects stored in a cruft pack individually
by updating the `.mtimes` file. This is because we can't portably `mmap`
and write into the middle of a file (i.e., to update the mtime of just
one object). Instead, we would have to rewrite the entire `.mtimes` file
which may incur some wasted effort especially if there a lot of cruft
objects and they are freshened infrequently.
Instead, force the freshening code to avoid an optimizing write by
writing out the object loose and letting it pick up a current mtime.
This works because we prefer the mtime of the loose copy of an object
when both a loose and packed one exist (whether or not the packed copy
comes from a cruft pack or not).
This could certainly do with a test and/or be included earlier in this
series/PR, but I want to wait until after I have a chance to clean up
the overly-repetitive nature of the cruft pack tests in general.
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Expose the new `git repack --cruft` mode from `git gc` via a new opt-in
flag. When invoked like `git gc --cruft`, `git gc` will avoid exploding
unreachable objects as loose ones, and instead create a cruft pack and
`.mtimes` file.
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
When using cruft packs, the following race can occur when a geometric
repack that writes a MIDX bitmap takes place afterwords:
- First, create an unreachable object and do an all-into-one cruft
repack which stores that object in the repository's cruft pack.
- Then make that object reachable.
- Finally, do a geometric repack and write a MIDX bitmap.
Assuming that we are sufficiently unlucky as to select a commit from the
MIDX which reaches that object for bitmapping, then the `git
multi-pack-index` process will complain that that object is missing.
The reason is because we don't include cruft packs in the MIDX when
doing a geometric repack. Since the "make that object reachable" doesn't
necessarily mean that we'll create a new copy of that object in one of
the packs that will get rolled up as part of a geometric repack, it's
possible that the MIDX won't see any copies of that now-reachable
object.
Of course, it's desirable to avoid including cruft packs in the MIDX
because it causes the MIDX to store a bunch of objects which are likely
to get thrown away. But excluding that pack does open us up to the above
race.
This patch demonstrates the bug, and resolves it by including cruft
packs in the MIDX even when doing a geometric repack.
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
In servers which set the pack.window configuration to a large value, we
can wind up spending quite a lot of time finding new bases when breaking
delta chains between reachable and unreachable objects while generating
a cruft pack.
Introduce a handful of `repack.cruft*` configuration variables to
control the parameters used by pack-objects when generating a cruft
pack.
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Expose a way to split the contents of a repository into a main and cruft
pack when doing an all-into-one repack with `git repack --cruft -d`, and
a complementary configuration variable.
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
In a previous patch, pack-objects learned how to generate a cruft pack
so long as no objects are dropped.
This patch teaches pack-objects to handle the case where a non-never
`--cruft-expiration` value is passed. This case is slightly more
complicated than before, because we want pack-objects to save
unreachable objects which would have been pruned when there is another
recent (i.e., non-prunable) unreachable object which reaches the other.
We'll call these objects "unreachable but reachable-from-recent".
Here is how pack-objects handles `--cruft-expiration`:
- Instead of adding all objects outside of the kept pack(s) into the
packing list, only handle the ones whose mtime is within the grace
period.
- Construct a reachability traversal whose tips are the
unreachable-but-recent objects.
- Then, walk along that traversal, stopping if we reach an object in
the kept pack. At each step along the traversal, we add the object
we are visiting to the packing list.
In the majority of these cases, any object we visit in this traversal
will already be in our packing list. But we will sometimes encounter
reachable-from-recent cruft objects, which we want to retain even if
they aged out of the grace period.
The most subtle point of this process is that we actually don't need to
bother to update the rescued object's mtime. Even though we will write
an .mtimes file with a value that is older than the expiration window,
it will continue to survive cruft repacks so long as any objects which
reach it haven't aged out.
That is, a future repack will also exclude that object from the initial
packing list, only to discover it later on when doing the reachability
traversal.
Finally, stopping early once an object is found in a kept pack is safe
to do because the kept packs ordinarily represent which packs will
survive after repacking. Assuming that it _isn't_ safe to halt a
traversal early would mean that there is some ancestor object which is
missing, which implies repository corruption (i.e., the complete set of
reachable objects isn't present).
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Teach `pack-objects` how to generate a cruft pack when no objects are
dropped (i.e., `--cruft-expiration=never`). Later patches will teach
`pack-objects` how to generate a cruft pack that prunes objects.
When generating a cruft pack which does not prune objects, we want to
collect all unreachable objects into a single pack (noting and updating
their mtimes as we accumulate them). Ordinary use will pass the result
of a `git repack -A` as a kept pack, so when this patch says "kept
pack", readers should think "reachable objects".
Generating a non-expiring cruft packs works as follows:
- Callers provide a list of every pack they know about, and indicate
which packs are about to be removed.
- All packs which are going to be removed (we'll call these the
redundant ones) are marked as kept in-core.
Any packs the caller did not mention (but are known to the
`pack-objects` process) are also marked as kept in-core. Packs not
mentioned by the caller are assumed to be unknown to them, i.e.,
they entered the repository after the caller decided which packs
should be kept and which should be discarded.
Since we do not want to include objects in these "unknown" packs
(because we don't know which of their objects are or aren't
reachable), these are also marked as kept in-core.
- Then, we enumerate all objects in the repository, and add them to
our packing list if they do not appear in an in-core kept pack.
This results in a new cruft pack which contains all known objects that
aren't included in the kept packs. When the kept pack is the result of
`git repack -A`, the resulting pack contains all unreachable objects.
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
