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In an upcoming commit, 'git repack' will want to create a pack comprised
of all of the objects in some packs (the included packs) excluding any
objects in some other packs (the excluded packs).
This caller could iterate those packs themselves and feed the objects it
finds to 'git pack-objects' directly over stdin, but this approach has a
few downsides:
- It requires every caller that wants to drive 'git pack-objects' in
this way to implement pack iteration themselves. This forces the
caller to think about details like what order objects are fed to
pack-objects, which callers would likely rather not do.
- If the set of objects in included packs is large, it requires
sending a lot of data over a pipe, which is inefficient.
- The caller is forced to keep track of the excluded objects, too, and
make sure that it doesn't send any objects that appear in both
included and excluded packs.
But the biggest downside is the lack of a reachability traversal.
Because the caller passes in a list of objects directly, those objects
don't get a namehash assigned to them, which can have a negative impact
on the delta selection process, causing 'git pack-objects' to fail to
find good deltas even when they exist.
The caller could formulate a reachability traversal themselves, but the
only way to drive 'git pack-objects' in this way is to do a full
traversal, and then remove objects in the excluded packs after the
traversal is complete. This can be detrimental to callers who care
about performance, especially in repositories with many objects.
Introduce 'git pack-objects --stdin-packs' which remedies these four
concerns.
'git pack-objects --stdin-packs' expects a list of pack names on stdin,
where 'pack-xyz.pack' denotes that pack as included, and
'^pack-xyz.pack' denotes it as excluded. The resulting pack includes all
objects that are present in at least one included pack, and aren't
present in any excluded pack.
To address the delta selection problem, 'git pack-objects --stdin-packs'
works as follows. First, it assembles a list of objects that it is going
to pack, as above. Then, a reachability traversal is started, whose tips
are any commits mentioned in included packs. Upon visiting an object, we
find its corresponding object_entry in the to_pack list, and set its
namehash parameter appropriately.
To avoid the traversal visiting more objects than it needs to, the
traversal is halted upon encountering an object which can be found in an
excluded pack (by marking the excluded packs as kept in-core, and
passing --no-kept-objects=in-core to the revision machinery).
This can cause the traversal to halt early, for example if an object in
an included pack is an ancestor of ones in excluded packs. But stopping
early is OK, since filling in the namehash fields of objects in the
to_pack list is only additive (i.e., having it helps the delta selection
process, but leaving it blank doesn't impact the correctness of the
resulting pack).
Even still, it is unlikely that this hurts us much in practice, since
the 'git repack --geometric' caller (which is introduced in a later
commit) marks small packs as included, and large ones as excluded.
During ordinary use, the small packs usually represent pushes after a
large repack, and so are unlikely to be ancestors of objects that
already exist in the repository.
(I found it convenient while developing this patch to have 'git
pack-objects' report the number of objects which were visited and got
their namehash fields filled in during traversal. This is also included
in the below patch via trace2 data lines).
Suggested-by: Jeff King <peff@peff.net>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Git - fast, scalable, distributed revision control system
Git is a fast, scalable, distributed revision control system with an unusually rich command set that provides both high-level operations and full access to internals.
Git is an Open Source project covered by the GNU General Public License version 2 (some parts of it are under different licenses, compatible with the GPLv2). It was originally written by Linus Torvalds with help of a group of hackers around the net.
Please read the file INSTALL for installation instructions.
Many Git online resources are accessible from https://git-scm.com/ including full documentation and Git related tools.
See Documentation/gittutorial.txt to get started, then see
Documentation/giteveryday.txt for a useful minimum set of commands, and
Documentation/git-<commandname>.txt for documentation of each command.
If git has been correctly installed, then the tutorial can also be
read with man gittutorial or git help tutorial, and the
documentation of each command with man git-<commandname> or git help <commandname>.
CVS users may also want to read Documentation/gitcvs-migration.txt
(man gitcvs-migration or git help cvs-migration if git is
installed).
The user discussion and development of Git take place on the Git mailing list -- everyone is welcome to post bug reports, feature requests, comments and patches to git@vger.kernel.org (read Documentation/SubmittingPatches for instructions on patch submission). To subscribe to the list, send an email with just "subscribe git" in the body to majordomo@vger.kernel.org. The mailing list archives are available at https://lore.kernel.org/git/, http://marc.info/?l=git and other archival sites.
Issues which are security relevant should be disclosed privately to the Git Security mailing list git-security@googlegroups.com.
The maintainer frequently sends the "What's cooking" reports that list the current status of various development topics to the mailing list. The discussion following them give a good reference for project status, development direction and remaining tasks.
The name "git" was given by Linus Torvalds when he wrote the very first version. He described the tool as "the stupid content tracker" and the name as (depending on your mood):
- random three-letter combination that is pronounceable, and not actually used by any common UNIX command. The fact that it is a mispronunciation of "get" may or may not be relevant.
- stupid. contemptible and despicable. simple. Take your pick from the dictionary of slang.
- "global information tracker": you're in a good mood, and it actually works for you. Angels sing, and a light suddenly fills the room.
- "goddamn idiotic truckload of sh*t": when it breaks