spectral added a comment.
In https://phab.mercurial-scm.org/D2729#45664, @indygreg wrote:
> I'm sorry, but we cannot ship this as is.
> The reason is mtime based build systems, like GNU make.
> We can't have version control modifying files without bumping their mtime
because this invalidates the target freshness checks of mtime-based build
If the user does an `mv` in the shell, at least on Linux, it preserves mtime.
If they do a `cp`, it doesn't (the file gets the current timestamp). This
includes when overwriting a file. I think I'd be fine with mimicking this
behavior (only preserve mtime on `hg mv`) if that would make this safer or
easier to reason about.
> If we want to preserve mtime on file copy or move, I believe it is safe to
do that if and only if the destination file didn't already exist. But if the
destination exists, we need to ensure the mtime of the new file is greater than
the mtime of the old file.
`hg mv` and `hg cp` require `--after` if the destination file already exists;
in those cases, we don't seem to touch the working directory at all (including
not modifying the mtime, even with my patch). With `--after`, this is purely a
VCS operation that afaict "shouldn't" have any effect on build systems, so I
think we're safe here for that concern?
> Reading this patch, I /think/ the previous behavior was buggy in edge cases
because we never ensured the mtime of the replacement was newer than the
existing file. In 99.99% of cases, it will be because the existing file was
created sometime in the past. But if bad clocks or other wonky things are in
play, there's no guarantee that *wall clock now* is greater than the mtime of
the existing destination file. The correct thing to do in this situation is
read the mtime of the existing file and ensure the mtime of the new file is at
least 1s greater than the previous mtime (1s because not all filesystems
preserve microsecond or millisecond mtime granularity).
To: spectral, #hg-reviewers, durin42
Cc: indygreg, durin42, mercurial-devel
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