(#15049) Return only one selinuxfs path as string from mountinfo

[domcleal]

The block that parses /proc/self/mountinfo to find a selinuxfs filesystem would
return results as an array. On Ruby 1.8, interpolating this into a string for
File.exists? when one result was returned worked, while on Ruby 1.9 it
interpolated as ["/sys/fs/selinux"]/enforce so later failed.

This changes the block to return the single result string rather than an array.
This also fixes #11531 where multiple selinuxfs filesystems could be mounted,
as it returns only the first mountpoint.

[HAIL9000]

If this ends up not finding the line, it will return an empty array, and we'll end up with the same problem that we had before. In the case where the line is not found, it would be better to return an empty string, to avoid the problem with the array and File.exists?.

This could be more clearly handled if you split the finding of the line from the returning of the line.

[domcleal]

Good point, thanks. I've updated the branch, however I've returned /selinux as standard instead of an empty string, as it was the standard fallback. By changing the source /proc file used too, we've got much better compatibility (e.g. on RHEL 5).

[HAIL9000]

Awesome, thanks! That was better than my suggestion anyways. We'll try to get this merged in soon. :)

[domcleal]

You're right, I got my paths muddled. /proc/mounts is a symlink to self/mounts, but I've updated the branch now to use /proc/self/mounts consistently.