Fallback to copy if chown/chgrp would land in different filesystems

[cem]

Summary

Add install hardlink mode, -l c, that falls back to copy on EXDEV.

Add prog/progs target, LINKS_OR_COPIES, which implies this installation mode.
(Just in case it should not totally replace LINKS.)

Use that mode for usr.sbin/chown.

Documentation is provided.

Test Plan

This is wholly untested.

Motivated by https://github.com/freebsd/freebsd/pull/133 .

[rpokala]

Out of curiosity, why specifically for chown / chgrp?

[cem]

In D14897#313257, @rpokala wrote:

Out of curiosity, why specifically for chown / chgrp?

They are the observed weird case of being "links" in different directories — usr/sbin for chown, but usr/bin for chgrp. This causes problems for ZFS users who hyper-partition their installation filesystems (in order to set nosetuid+noexec etc by default, but reeanble it for specific directories like /bin, /sbin, and /usr/[s]bin).

For more detail, see the linked github PR.

[brooks]

I'm not a fan of this or the mixed link support, but this seems like an ok solution to the problem. I'd mostly prefer to burn the extra 10k of disk space and do copy all the time.

[cem]

In D14897#313261, @brooks wrote:

I'm not a fan of this or the mixed link support, but this seems like an ok solution to the problem. I'd mostly prefer to burn the extra 10k of disk space and do copy all the time.

Copy all the time sounds totally fine to me as well.

[bcr]

Looks good to me from manpages.

[cem]

I don't intend to commit this. I don't think it's necessarily a bad idea, I just don't particularly care about it and don't have time to verify (to my own satisfaction) that it doesn't break anything.

[emaste]

This is perhaps in general a reasonable approach. I think it would be fine for the xinstall change to go in, and for bsd.links.mk to just use this mode instead of -l h. But that said, I do see that both BSD tar and GNU tar do not fallback, they just fail, and prior to switching to install we just used ln -f which would fail

[se]

Instead of LINKS_OR_COPIES we could use LINKS_OR_SYMLINKS to try a hard link and if that fails a symbolic link.

I had used SYMLINKS instead of LINKS in chown/Makefile for a long time, but meanwhile have recreated /usr/bin and /usr/sbin on a single file system.

I consider falling back to a sym-link the better approach (compared to a file copy) in case of a failed hard-link.

(Instead of SYMLINKS I'd rather use RSYMLINKS now, BTW.)