Setting file times on symlinks

[joshtriplett]

Proposal

Problem statement

I'd like to be able to set the file times on a symlink.

The standard library supports setting file times on an open file, using File::set_times. However, this doesn't work portably for a symlink. Changing a symlink's timestamps requires calling utimensat or lutimes depending on the system.

Motivating examples or use cases

This allows unpacking an archive with timestamps preserved, or syncing a directory between systems with timestamps preserved.

Solution sketch

// in `std::fs`
pub fn set_times_nofollow<P: AsRef<Path>>(path: P, times: FileTimes) -> Result<()>

(Modeled after std::fs::chroot and File::set_times.)

Alternatives

I'd love to implement this by opening the symlink as a file descriptor and setting the timestamp using set_times, but both steps of that are not possible in a portable way. I think this is the only reasonable approach.

What happens now?

This issue contains an API change proposal (or ACP) and is part of the libs-api team feature lifecycle. Once this issue is filed, the libs-api team will review open proposals as capability becomes available. Current response times do not have a clear estimate, but may be up to several months.

Possible responses

The libs team may respond in various different ways. First, the team will consider the problem (this doesn't require any concrete solution or alternatives to have been proposed):

• We think this problem seems worth solving, and the standard library might be the right place to solve it.
• We think that this probably doesn't belong in the standard library.

Second, if there's a concrete solution:

• We think this specific solution looks roughly right, approved, you or someone else should implement this. (Further review will still happen on the subsequent implementation PR.)
• We're not sure this is the right solution, and the alternatives or other materials don't give us enough information to be sure about that. Here are some questions we have that aren't answered, or rough ideas about alternatives we'd want to see discussed.

[ChrisDenton]

I'd love to implement this by opening the symlink as a file descriptor and setting the timestamp using set_times, but both steps of that are not possible in a portable way. I think this is the only reasonable approach.

Does that need to be mutually exclusive with std::fs::set_symlink_times? Couldn't we do both for platforms that support it but error on the former if it's not supported?

[joshtriplett]

It's not mutually exclusive, but the fd-based version would be platform-specific and non-portable. On Linux, at least, it'd require O_NOFOLLOW | O_PATH.

We could, theoretically, add an open_symlink function to wrap that, and I'm all for that.

That's a more substantial overhaul. If someone wants to propose it, I'd certainly support it. But right now, I'd like to propose the set_symlink_times function, which will work more portably on more versions of more OSes.

[joshtriplett]

Renamed to set_times_nofollow based on discussion in @rust-lang/libs-api meeting, for consistency with other _nofollow methods, and to reflect that it will work on a non-symlink as well.

[Amanieu]

We discussed this in the @rust-lang/libs-api meeting and are happy to accept this. std::fs::set_times should also be added for consistency (even though it is also available on File).

[joshtriplett]

Closing in favor of the tracking issue.