[MacPorts] #58729: legacy-support missing futimens() (and utimensat)

[MacPorts]

#58729: legacy-support missing futimens() (and utimensat)
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  Reporter:  RJVB            |      Owner:  kencu
      Type:  enhancement     |     Status:  assigned
  Priority:  Normal          |  Milestone:
 Component:  ports           |    Version:
Resolution:                  |   Keywords:
      Port:  legacy-support  |
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Changes (by Ionic):

 * cc: Ionic (added)


Comment:

 Emulating that should be safe.

 POSIX/SUS specifies:

 >For `futimens`() and `utimensat`(), the times argument is an array of two
 **timespec** structures. The first array member represents the date and
 time of last access, and the second member represents the date and time of
 last modification. The times in the **timespec** structure are measured in
 seconds and nanoseconds since the Epoch. The file's relevant timestamp
 shall be set to the greatest value supported by the file system that is
 not greater than the specified time.
 >
 >If the `tv_nsec` field of a **timespec** structure has the special value
 UTIME_NOW, the file's relevant timestamp shall be set to the greatest
 value supported by the file system that is not greater than the current
 time. If the `tv_nsec` field has the special value UTIME_OMIT, the file's
 relevant timestamp shall not be changed. In either case, the `tv_sec`
 field shall be ignored.
 >
 >If the `times` argument is a null pointer, both the access and
 modification timestamps shall be set to the greatest value supported by
 the file system that is not greater than the current time.

 Thus, SUS does have provisions for file systems not supporting such low
 resolutions. That argument is moot.

 I've looked up the relevant source code in 10.13. That would be
 [http://web.archive.org/web/20210129201556/https://opensource.apple.com/source/xnu/xnu-4570.71.2/bsd/sys/stat.h.auto.html
 include/sys/stat.h] and
 [http://web.archive.org/web/20210129201720/https://opensource.apple.com/source/xnu/xnu-4570.71.2/libsyscall/wrappers/utimensat.c.auto.html
 utimensat.c].

 Ironically, if the comment is right and I read the code correctly, macOS
 10.13 does support nanosecond-based time resolution for
 `futimens`/`utimensat` only if passed-in directly, but not for the special
 `NOW` value, since `gettimeofday` cannot return those. Given that, I think
 that emulating it with microseconds isn't a big deal.

 Further, we don't even really need to emulate it. We can take the code
 verbatim for 10.6 - 10.12. That'll be APSL 2.0, but I guess that's fine.

 For 10.6 and below, things are a bit more complicated.

 `utimensat` looks less problematic, since it uses `setattrlistat`, which
 should be supported even by older operating systems.

 `futimens` uses `fsetattrlist`, which is only available on 10.6+. This
 said, I guess we could likewise just copy its code, since it's likely
 based on `setattrlistat`.

 I'd have to look deeper at older systems to find out if all support
 `ATTR_BIT_MAP_COUNT`, but I cannot imagine that they don't.

 All in all, it sounds very doable.

 Thoughts?

-- 
Ticket URL: <https://trac.macports.org/ticket/58729#comment:11>
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