uid_t is an unsigned 32-bit integer in linux right now.

MockRunAsUidGidRestoreUidError mocks an error from the calls that restore the original euid/egid.

RunAsUidGid starts a goroutine, pins it to the OS thread, sets euid and egid, and runs the function; after the function returns, it restores euid and egid.

FlagID can be passed to chown-ish functions to mean "no change", and can be returned from getuid-ish functions to mean "not found".

As of Go 1.9, the O_PATH constant does not seem to be declared uniformly over all archtiectures.

GroupID is the type of the system's group identifiers (in C, gid_t).

UserID is the type of the system's user identifiers (in C, uid_t).