[PATCH] meson: wire up USE_NSEC build knob

[PATCH] meson: wire up USE_NSEC build knob

[D. Ben Knoble]

Autotools-style builds permit enabling USE_NSEC for cases where that's
desired; the equivalent knob is missing from meson-based builds.

Signed-off-by: D. Ben Knoble <ben.knoble+github@gmail.com>
---
 meson.build       | 4 ++++
 meson_options.txt | 2 ++
 2 files changed, 6 insertions(+)

diff --git a/meson.build b/meson.build
index 3247697f74..85a11119c5 100644
--- a/meson.build
+++ b/meson.build
@@ -838,6 +838,10 @@ if help_format_opt != 'man'
     libgit_c_args += '-DDEFAULT_HELP_FORMAT="' + help_format_opt + '"'
 endif
 
+if get_option('nanosec')
+  libgit_c_args += '-DUSE_NSEC'
+endif
+
 libgit_include_directories = [ '.' ]
 libgit_dependencies = [ ]
 
diff --git a/meson_options.txt b/meson_options.txt
index d936ada098..1bc75278a8 100644
--- a/meson_options.txt
+++ b/meson_options.txt
@@ -21,6 +21,8 @@ option('runtime_prefix', type: 'boolean', value: false,
   description: 'Resolve ancillary tooling and support files relative to the location of the runtime binary instead of hard-coding them into the binary.')
 option('sane_tool_path', type: 'array', value: [],
   description: 'An array of paths to pick up tools from in case the normal tools are broken or lacking.')
+option('nanosec', type: 'boolean', value: false,
+  description: 'Care about sub-second file mtimes and ctimes.')
 
 # Build information compiled into Git and other parts like documentation.
 option('build_date', type: 'string', value: '',

base-commit: 0c8ab3ebcc76981376809c8fe632d0fe18e93347
-- 
2.55.0.rc0.738.g0c8ab3ebcc.dirty

Re: [PATCH] meson: wire up USE_NSEC build knob

[Junio C Hamano]

"D. Ben Knoble" <ben.knoble+github@gmail.com> writes:

> Autotools-style builds permit enabling USE_NSEC for cases where that's
> desired; the equivalent knob is missing from meson-based builds.

With or without autoconf, Makefile based build can use USE_NSEC.  It
is a welcome addition to the other side of thw world.  I do not know
if 'meson setup -Dnanosec=true' is a name that is easy to discover,
though.

Will queue.  Thanks.

Re: [PATCH] meson: wire up USE_NSEC build knob

[D. Ben Knoble]

On Sat, Jun 20, 2026 at 9:01 PM Junio C Hamano <gitster@pobox.com> wrote:
>
> "D. Ben Knoble" <ben.knoble+github@gmail.com> writes:
>
> > Autotools-style builds permit enabling USE_NSEC for cases where that's
> > desired; the equivalent knob is missing from meson-based builds.
>
> With or without autoconf, Makefile based build can use USE_NSEC.

Thanks. I almost wrote "Make-based," but I wasn't sure how we
preferred to describe it.

> It
> is a welcome addition to the other side of thw world.  I do not know
> if 'meson setup -Dnanosec=true' is a name that is easy to discover,
> though.
>
> Will queue.  Thanks.

Agreed for the name. Alternatives welcome.

Re: [PATCH] meson: wire up USE_NSEC build knob

[Patrick Steinhardt]

On Sat, Jun 20, 2026 at 06:01:25PM -0700, Junio C Hamano wrote:
> "D. Ben Knoble" <ben.knoble+github@gmail.com> writes:
> 
> > Autotools-style builds permit enabling USE_NSEC for cases where that's
> > desired; the equivalent knob is missing from meson-based builds.
> 
> With or without autoconf, Makefile based build can use USE_NSEC.  It
> is a welcome addition to the other side of thw world.  I do not know
> if 'meson setup -Dnanosec=true' is a name that is easy to discover,
> though.

I think the name itself is fine. As is the case for other options, it
can be discovered rather easily by just running `meson setup` in the
source directory, which gives you an overview of all available build
options.

Patrick

Re: [PATCH] meson: wire up USE_NSEC build knob

[Jeff King]

On Sat, Jun 20, 2026 at 12:00:24PM -0400, D. Ben Knoble wrote:

> Autotools-style builds permit enabling USE_NSEC for cases where that's
> desired; the equivalent knob is missing from meson-based builds.

Seems reasonable. This is not changing the defaults at all, but just
bringing meson's options to parity with the Makefile.

I'm not still not sure if turning on USE_NSEC is a good idea. There's
some discussion in Documentation/technical/racy-git.adoc:

  With `USE_NSEC`
  compile-time option, `st_mtim.tv_nsec` and `st_ctim.tv_nsec`
  members are also compared. On Linux, this is not enabled by default
  because in-core timestamps can have finer granularity than
  on-disk timestamps, resulting in meaningless changes when an
  inode is evicted from the inode cache.  See commit 8ce13b0
  of git://git.kernel.org/pub/scm/linux/kernel/git/tglx/history.git
  ([PATCH] Sync in core time granularity with filesystems,
  2005-01-04). This patch is included in kernel 2.6.11 and newer, but
  only fixes the issue for file systems with exactly 1 ns or 1 s
  resolution. Other file systems are still broken in current Linux
  kernels (e.g. CEPH, CIFS, NTFS, UDF), see
  https://lore.kernel.org/lkml/5577240D.7020309@gmail.com/

That's the most succinct description of the problem I've seen, but I
have no idea how widely it still applies. Kernel 2.6.11 is quite old
now, but I could believe that other filesystems (especially network
ones) still exhibit the issue.

So I guess if we wanted to go further it would take some digging as to
how each platform behaves, and then flipping the config.make.uname knob
for ones where it can be argued that the behavior is always reasonable.

But that's all outside the scope of your patch here.

-Peff

Re: [PATCH] meson: wire up USE_NSEC build knob

[Patrick Steinhardt]

On Sun, Jun 21, 2026 at 01:49:34PM -0400, Jeff King wrote:
> On Sat, Jun 20, 2026 at 12:00:24PM -0400, D. Ben Knoble wrote:
> 
> > Autotools-style builds permit enabling USE_NSEC for cases where that's
> > desired; the equivalent knob is missing from meson-based builds.
> 
> Seems reasonable. This is not changing the defaults at all, but just
> bringing meson's options to parity with the Makefile.

I was originally wondering whether I should recommend that Meson can
auto-discover the availability of nanoseconds. But your below remarks
make me question that.

> I'm not still not sure if turning on USE_NSEC is a good idea. There's
> some discussion in Documentation/technical/racy-git.adoc:
> 
>   With `USE_NSEC`
>   compile-time option, `st_mtim.tv_nsec` and `st_ctim.tv_nsec`
>   members are also compared. On Linux, this is not enabled by default
>   because in-core timestamps can have finer granularity than
>   on-disk timestamps, resulting in meaningless changes when an
>   inode is evicted from the inode cache.  See commit 8ce13b0
>   of git://git.kernel.org/pub/scm/linux/kernel/git/tglx/history.git
>   ([PATCH] Sync in core time granularity with filesystems,
>   2005-01-04). This patch is included in kernel 2.6.11 and newer, but
>   only fixes the issue for file systems with exactly 1 ns or 1 s
>   resolution. Other file systems are still broken in current Linux
>   kernels (e.g. CEPH, CIFS, NTFS, UDF), see
>   https://lore.kernel.org/lkml/5577240D.7020309@gmail.com/
> 
> That's the most succinct description of the problem I've seen, but I
> have no idea how widely it still applies. Kernel 2.6.11 is quite old
> now, but I could believe that other filesystems (especially network
> ones) still exhibit the issue.
> 
> So I guess if we wanted to go further it would take some digging as to
> how each platform behaves, and then flipping the config.make.uname knob
> for ones where it can be argued that the behavior is always reasonable.

Yeah, it would be nice indeed to figure out whether these concerns still
apply. If they do, I would argue that it might even make sense to remove
the build option completely. It doesn't really make sense in my opinion
to have a build option that nobody uses and that is subtly broken when
enabled.

> But that's all outside the scope of your patch here.

Kind of, I guess. If we figure that this mechanism is still subtly broken
then I'd argue that it doesn't make sense to expose the option via
Meson.

Patrick