Racy Git problems persist today, manifesting themselves in the
performance of commands like "git diff" in new worktrees [1]. We have
long had a build knob "USE_NSEC" to tell Git to use in-core nanosecond
precision when available, which mitigates most if not all racy issues,
but most builds we know about don't use it. In part, that's because
someone distributing Git can't safely enable it at compile-time if they
don't know exactly what platforms their distribution will be used on.
[1]: https://lore.kernel.org/git/CALnO6CADMJSixqYvL1Yo8qKX5rWhKQ+2OoSEuPUh-yoeK9TseQ@mail.gmail.com
These days, most platforms are likely to be safe for the USE_NSEC code.
Regardless, we want to give users the ability to benefit from it. This
requires exposing the compile-time gated code as a runtime option.
In addition, update the Racy Git documentation and other mentions of
USE_NSEC in the code.
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.
Signed-off-by: D. Ben Knoble <ben.knoble@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
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