[PATCH v2 0/3] Convert USE_NSEC to runtime config

Topic name: dk/use-nsec-runtime (applied)

Topic summary: Expose USE_NSEC as a runtime configuration, since
build-time is too early for distributing Git [1]. As a result, common
index-related options, like git-diff, are less likely to hit "racy git"
problems on supported filesystems.

[1]: https://git.github.io/rev_news/2026/07/31/edition-137/

Built on master (2c78326f81 (The 11th batch, 2026-08-05)).

Changes in v2:

- move Best-viewed-with trailer into message body as descriptive
  text.
- read core.useNanosec through struct repo instead of parsing
  config strings. The test suite passes locally this way, though that
  skipped 151 tests.
    - CI run: https://github.com/benknoble/git/actions/runs/31701945211

Original cover letter:

Hi all, this series follows up on the previous racy Git/USE_NSEC
conversations.

- The first patch is a mostly-unrelated documentation fix for Meson, but
  it came out of something I spotted while reviewing the outputs of the
  final (main) patch.
- The second patch is a preliminary no-op reorganization of
  repo_config_values_init.
- The third patch is the meat, converting USE_NSEC into core.useNanosec.

There is a small textual and semantic conflict with
'ty/repo-config-cleanups' in 'seen', since that branch removes the
comments in 'struct repo_config_values' which this series adds to. (The
semantic conflict is that, if we drop those comments, we should probably
not add them to repo_config_values_init like I do in patch 2.)

Todo: I haven't touched any tests; I saw a bunch of hits for "git grep
racy t" but wasn't sure how to fit this particular change in, especially
since it won't be equally valid on all systems? Advice welcome.

Todo: I wonder if "useNanosec" paints us into too much of a corner; that
is (slightly more abstractly), we are using *extended precision* in the
index. Maybe the name and documentation should reflect that, so we
aren't too committed to "nanoseconds"?
    - Some platforms could offer extended precision that is not as
      precise as nanoseconds
    - Some could offer precision _beyond_ nanoseconds
idk.

v1: <[email protected]>

[1/3] meson: expose knob for xmlto relative links in manuals
[2/3] environment: align repo_config_values_init with struct declaration
[3/3] core: convert build-time USE_NSEC into runtime core.useNanosec

 Documentation/config/core.adoc        |  6 ++++++
 Documentation/meson.build             |  7 ++++++-
 Documentation/technical/racy-git.adoc | 11 ++++++-----
 Makefile                              | 12 +-----------
 builtin/update-index.c                |  2 +-
 compat/posix.h                        |  1 -
 configure.ac                          |  6 ------
 environment.c                         | 25 ++++++++++++++++++-------
 environment.h                         |  1 +
 meson_options.txt                     |  2 ++
 read-cache.c                          | 16 ++++++++--------
 statinfo.c                            | 14 +++++++-------
 12 files changed, 56 insertions(+), 47 deletions(-)

Diff-intervalle contre v1 :
1:  d612de6c2d = 1:  d612de6c2d meson: expose knob for xmlto relative links in manuals
2:  5693baa992 = 2:  5693baa992 environment: align repo_config_values_init with struct declaration
3:  dbbd96d508 ! 3:  2d1424732a core: convert build-time USE_NSEC into runtime core.useNanosec
    @@ Commit message
         In addition, update the Racy Git documentation and other mentions of
         USE_NSEC in the code.
     
    -    Best-viewed-with: --ignore-space-change
    +    Due to the conversion from #ifdef to runtime check, using the flag
    +    "--ignore-space-change" may be particularly helpful when viewing changes
    +    from this patch.
     
     
      ## Notes (benknoble/commits) ##
    -    Repeating the benchmark from my original mail [1] shows an improvement
    -
    -    # git worktree add -d ../perf-test HEAD
    -    # hyperfine -N --warmup=10 './build/bin-wrappers/git diff'
    -    Benchmark 1: ./build/bin-wrappers/git diff
    -      Time (mean ± σ):       3.8 ms ±   0.4 ms    [User: 4.7 ms, System: 4.4 ms]
    -      Range (min … max):     3.2 ms …   5.6 ms    780 runs
    -    # (pushd ../perf-test && hyperfine -N --warmup=10 $OLDPWD/'./build/bin-wrappers/git diff')
    -    Benchmark 1: /home/benknoble/code/git/./build/bin-wrappers/git diff
    -      Time (mean ± σ):     217.5 ms ±   2.9 ms    [User: 202.1 ms, System: 23.4 ms]
    -      Range (min … max):   213.9 ms … 223.3 ms    13 runs
    -    # (pushd ../perf-test && hyperfine -N --warmup=10 $OLDPWD/'./build/bin-wrappers/git -c core.useNanosec=true diff')
    -    Benchmark 1: /home/benknoble/code/git/./build/bin-wrappers/git -c core.useNanosec=true diff
    -      Time (mean ± σ):       3.8 ms ±   0.4 ms    [User: 5.3 ms, System: 4.2 ms]
    -      Range (min … max):     3.2 ms …   6.9 ms    541 runs
    -
    -    [1]: <CALnO6CADMJSixqYvL1Yo8qKX5rWhKQ+2OoSEuPUh-yoeK9TseQ@mail.gmail.com>
    -
    -    Passing CI: https://github.com/benknoble/git/actions/runs/31104581195
    +    Related benchmarks: <https://lore.kernel.org/git/CALnO6CBm4g27mWBvD9m6yL0e5YZu3M9_zcUeLZk7QwTgnxMLQA@mail.gmail.com/>
    +    CI: <https://github.com/benknoble/git/actions/runs/31701945211>
     
      ## Documentation/config/core.adoc ##
     @@ Documentation/config/core.adoc: core.trustctime::
    @@ read-cache.c: static int ce_match_stat_basic(const struct cache_entry *ce, struc
      static int is_racy_stat(const struct index_state *istate,
      			const struct stat_data *sd)
      {
    -+	int use_nsec = 0;
    -+	repo_config_get_bool(the_repository, "core.useNanosec", &use_nsec);
    ++	int use_nsec = repo_config_values(istate->repo)->use_nanosec;
     +
      	return (istate->timestamp.sec &&
     -#ifdef USE_NSEC

base-commit: 2c78326f810173a4f3aefd8021f1e07575412481
-- 
2.55.0.699.gb54405d56f.dirty