proposal: os: add Lchtimes to change only a symlink's times without following it

[Isolus]

Looks like there's no way to change access and modification times of a symlink.
We only have os.Chtimes which follows the symlink.

Since there is os.Chown and os.Lchown to change the uid and gid, I think there should also be Lchtimes. The implementation can be similar to the one of os.Chtimes. (E. g. on linux only the flag AT_SYMLINK_NOFOLLOW has to be added.)

[odeke-em]

Thank you for the feature request @Isolus and welcome to the Go project!

There exists lutimes on Darwin since 2006, and same for Linux https://linux.die.net/man/3/lutimes but it is missing on many platforms as per https://www.gnu.org/software/gnulib/manual/html_node/lutimes.html so perhaps we'll have to do something else as you've suggested.

I've tagged this as a proposal and it'll get reviewed over the cycle by others and myself too but for starters I shall kindly page @kevinburke @rsc @ianlancetaylor

[ianlancetaylor]

I think that a specialized operation like this, one that is not available at all on many systems, belongs in the golang.org/x/sys/unix package, not the os package.

[Isolus]

But isn't it a little unintuitive that Lstat (the no follow companion) to Stat is included in the os package. Therefore I can retrieve the times with the os package, but I can't write them?

I'm currently adding tar archiver functionality to an application and everything but a Lchtimes (or Lutimes) function for symlink handling (i. e. Lchmod and Lstat) is there (in the standard library). So my first thought was that this function was just forgotten.

[ianlancetaylor]

It's a little unintuitive that there are operating systems that provide lstat but not lutimes. But we can't pretend that that is not the case.

[bcmills]

CC @bradfitz @tklauser for the relationship to x/sys/unix

[tklauser]

I agree with Ian that we should not pretend to support lutimes where it is not supported. Rather we should add it to x/sys/unix where supported - either via SYS_LUTIMES as on some darwin or by wrapping utimensat e.g. on Linux and the BSDs.

[gopherbot]

Change https://golang.org/cl/164662 mentions this issue: unix: add Lutimes

[rsc]

I see this was merged into x/sys/unix. That seems like the right place to stop. Package os is for generally portable things (despite our imperfect execution of that plan), and this is a very special case.

[guonaihong]

Great, just need this function to implement the touch -h option.