`pax -rwl` doesn't correctly set mtime on directories

Originator:thakis
Number:rdar://FB8957230 Date Originated:2020 Dec 30
Status: Resolved:
Product:macos Product Version:11.0.1
Classification:bug Reproducible:yes
 
1. Create a directory containing a file:

  % mkdir foo
  % touch foo/file

2. Copy that directory to another directory using `rm -rf bar && mkdir bar && cd foo && pax -rwl . ../bar`:

  % (rm -rf bar && mkdir bar && cd foo && pax -rwl . ../bar)

3. Compare timestamps on both the directory (!) and the file. (`pax -p` copies mtime by default, see `man pax` under the `-p` flag.)

  % stat -f '%N %Fm' foo
  foo 1609297230.485880489
  % stat -f '%N %Fm' foo/*
  foo/file 1609294944.907726636

  % stat -f '%N %Fm' bar
  bar 1609297230.000000000
  % stat -f '%N %Fm' bar/file
  bar/file 1609294944.907726636

Expected: The mtimes on both file and directory match.
Actual: The mtime on file matches (but only because it's a hardlink, without `-l` it's wrong too). The mtime on the directory loses its fractional part.

This is a problem for mtime-based build systems. They'll think that the output of the copy has an older timestamp than the input and will re-run the copy on every build. Ninja 1.9+ (https://ninja-build.org/) is an example of a build system that reads high-resolution timestamps and that gets confused by this.

See https://github.com/nico/hack/blob/master/notes/copydir.md for some background around this bug report.

This reproduces on macOS 10.15.7 and macOS 11.0.1.

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