25e5ceb9ee
The goal of this commit was to fix a const warning when compiling
with new versions of glibc, but ended up untangling a much deeper
problem.
The find_pseudo_merge() function does a bsearch() on the "commits"
pointer of a pseudo_merge_map. This pointer ultimately comes from memory
mapped from the on-disk bitmap file, and is thus not writable.
The "commits" array is correctly marked const, but the result from
bsearch() is returned directly as a non-const pseudo_merge_commit
struct. Since new versions of glibc annotate bsearch() in a way that
detects the implicit loss of const, the compiler now warns.
My first instinct was that we should be returning a const struct. That
requires apply_pseudo_merges_for_commit() to mark its local pointer as
const. But that doesn't work! If the offset field has the high-bit set,
we look it up in the extended table via nth_pseudo_merge_ext(). And that
function then feeds our const struct to read_pseudo_merge_commit_at(),
which writes into it by byte-swapping from the on-disk mmap.
But I think this points to a larger problem with find_pseudo_merge(). It
is not just that the return value is missing const, but it is missing
that byte-swapping! And we know that byte-swapping is needed here,
because the comparator we use for bsearch() also calls our
read_pseudo_merge_commit_at() helper.
So I think the interface is all wrong here. We should not be returning a
pointer to a struct which was cast from on-disk data. We should be
filling in a caller-provided struct using the bytes we found,
byte-swapping the values.
That of course raises the dual question: how did this ever work, and
does it work now? The answer to the first part is: this code does not
seem to be triggered in the test suite at all. If we insert a BUG("foo")
call into apply_pseudo_merges_for_commit(), it never triggers.
So I think there is something wrong or missing from the test setup, and
this bears further investigation. Sadly the answer to the second part
("does it work now") is still "no idea". I _think_ this takes us in a
positive direction, but my goal here is mainly to quiet the compiler
warning. Further bug-hunting on this experimental feature can be done
separately.
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
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README.md
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.adoc to get started, then see
Documentation/giteveryday.adoc for a useful minimum set of commands, and
Documentation/git-<commandname>.adoc 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.adoc
(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 and Documentation/CodingGuidelines).
Those wishing to help with error message, usage and informational message
string translations (localization l10) should see po/README.md
(a po file is a Portable Object file that holds the translations).
To subscribe to the list, send an email to git+subscribe@vger.kernel.org (see https://subspace.kernel.org/subscribing.html for details). The mailing list archives are available at https://lore.kernel.org/git/, https://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