isolators: add rlimit and linux rlmit extension isolators

[philips]

We should add the subset of rlimits that make sense from POSIX as isolators: http://pubs.opengroup.org/onlinepubs/009695399/functions/setrlimit.html

rlimit/open-files
rlimit/new-file-size
rlimit/cpu-limit
rlimit/core-size

We should probably consider adding the Linux specific ones too. See ulimit -a

core file size
data seg size
scheduling priority
file size
pending signals
max locked memory
max memory size
open files
pipe size
POSIX message queues
real-time priority
stack size
cpu time
max user processes
virtual memory
file locks

[b]

Why not include all the POXIS rlimit resources?

The spec does not currently distinguish between mandatory and optional isolators. Is that intentional?

[jonboulle]

Why not include all the POXIS rlimit resources?

We could consider it, but there's currently no intention for the spec to approximate POSIX

The spec does not currently distinguish between mandatory and optional isolators. Is that intentional?

Does this section not address it?

An executor MAY ignore isolators that it does not understand and run the container without them.
But, an executor MUST make information about which isolators were ignored, enforced or modified available to the user. 
An executor MAY implement a "strict mode" where an image cannot run unless all isolators are in place.

[b]

I'm not proposing approximating POSIX, only suggesting that if all systems can be expected to have the POSIX rlimit resources (which they can) and all of them are potentially useful (which they are) why not include them all?

The text only describes what to do if an executor doesn't understand an isolator the container wants. Specifying mandatory isolators provides a contract on which developers can rely for what isolators are guaranteed to be available. As I said in another issue, the strict mode question should be up to the container, not the executor.

[philips]

@b Sorry, this bug isn't very clear. What I am saying is we should add the posix ones as posix/rlimits/open-files and the non-posix linux ones as os/linux/rlmits/foobar.

[b]

+1 on that!

Sorry for misunderstanding.