Lazy init

[kolyshkin]

With the following proggy:

package main

import 	_ "github.com/opencontainers/selinux/go-selinux"

func main() {}

we can easily measure the overhead of merely importing (not actually using) the package:

$ GODEBUG=inittrace=1 go run a.go 2>&1 | grep go-selinux
# github.com/opencontainers/selinux/go-selinux
init github.com/opencontainers/selinux/go-selinux @0.87 ms, 0.15 ms clock, 18328 bytes, 152 allocs

This PR tries to minimize that by:

• removing regex usage;
• initializing some stuff in a lazy (on demand) manner, rather than during initialization time

With these patches, we have

init github.com/opencontainers/selinux/go-selinux @0.53 ms, 0 ms clock, 160 bytes, 8 allocs

which eliminates most of the overhead.

Besides, modified readConfig and loadLabels show some performance and memory improvements as well:

[kir@kir-rhat go-selinux]$ benchstat before after
name          old time/op    new time/op    delta
ReadConfig-4    4.86µs ± 1%    3.46µs ± 2%  -28.82%  (p=0.002 n=6+6)
LoadLabels-4    12.7µs ± 3%     6.8µs ± 1%  -46.24%  (p=0.002 n=6+6)

name          old alloc/op   new alloc/op   delta
ReadConfig-4    5.08kB ± 0%    4.35kB ± 0%  -14.33%  (p=0.002 n=6+6)
LoadLabels-4    6.49kB ± 0%    6.10kB ± 0%   -6.07%  (p=0.000 n=6+5)

name          old allocs/op  new allocs/op  delta
ReadConfig-4      20.0 ± 0%       8.0 ± 0%  -60.00%  (p=0.002 n=6+6)
LoadLabels-4      45.0 ± 0%      45.0 ± 0%     ~     (all equal)

See individual commit messages for details.

[vrothberg]

LGTM, nice catch

[thaJeztah]

left some questions / suggestions / thoughts 😅

[rhatdan]

LGTM, but need to answer @thaJeztah feedback.

[thaJeztah]

LGTM

thanks! it's both cleaner and more performant, which is always nice!

[thaJeztah]

@rhatdan ptal; ready to go if it still LGTY

[rhatdan]

Rebase and we can merge, and I will cut a new release.

[kolyshkin]

rebased

[thaJeztah]

still LGTM, thanks!

[rhatdan]

LGTM