runc: remove --criu option

[kolyshkin]

This was introduced in an initial commit, back in the day when criu was
a highly experimental thing. Today it's not; most users who need it have
it packaged by their distro vendor.

The usual way to run a binary is to look it up in directories listed in
$PATH. This is flexible enough and allows for multiple scenarios (custom
binaries, extra binaries, etc.). This is the way criu should be run.

Make --criu a hidden option (thus removing it from help). Remove the
option from man pages, integration tests, etc. Remove all traces of
CriuPath from data structures.

Add a warning that --criu is ignored and will be removed.

This used to be part of #3316.

[kolyshkin]

This should be done 5 years ago but I guess better late than never.

runc also uses ps, busctl, systemctl, newuidmap/newgidmap etc, but do not provide any options to configure those -- $PATH is sufficient and flexible enough.

[AkihiroSuda]

Can be another PR, but maybe we should have docs/deprecation.md in addition to docs/experimental.md ?

[kolyshkin]

I don't know. It seems easy enough to search CHANGELOG.md for eprecat.

[kolyshkin]

Now, an alternative would be to honor --criu but warn that it will be removed. I'd rather not do it, because:

• most probably no one is using this option;
• whoever uses it can employ a quick and easy fix:

-runc --criu /some/custom/criu/dir/criu ...
+PATH=/some/custom/criu/dir:$PATH runc

(or a two-liner if criu binary is not named criu).

The only question is, should using --criu result in an error (rather than a warning).

[rhatdan]

LGTM

[thaJeztah]

I guess this is fine, as the flag is still present (so things won't hard-fail).

I stumbled upon this option in containerd, which (I think) may be using this flag; https://github.com/containerd/containerd/blob/77d53d2d230c3bcd3f02e6f493019a72905c875b/cmd/containerd-shim/main_unix.go#L60

Originally added in containerd/containerd@ab0cb4e, and I can see the --criu flag being set in containerd/go-runc; https://github.com/containerd/go-runc/blob/1c99ade6c04ed41bc4c9a8ac12b81b5fd4e14bcc/runc.go#L730-L732

So that code probably should be removed, and the option deprecated on containerd's side as well, @AkihiroSuda

[thaJeztah]

Opened containerd/go-runc#82 and containerd/containerd#6496

[thaJeztah]

LGTM

[thaJeztah]

@AkihiroSuda @cyphar good to go?

[kolyshkin]

@adrianreber PTAL