Ignore "unknown instance" error when rm instance

[threadproc]

Fixes #4112

This is especially useful when using Spot instances on EC2, as they may be removed at any time.

[GordonTheTurtle]

Please sign your commits following these rules:
https://github.com/moby/moby/blob/master/CONTRIBUTING.md#sign-your-work
The easiest way to do this is to amend the last commit:

$ git clone -b "4112-allow-remove-unknown" [email protected]:parryjacob/machine.git somewhere
$ cd somewhere
$ git commit --amend -s --no-edit
$ git push -f

Amending updates the existing PR. You DO NOT need to open a new one.

[nathanleclaire]

Good idea, just one suggestion:

How about instead of returning nil you return something like errors.New("Can't locate remote instance -- The instance may have been removed by another method")? I'd still consider a rm that doesn't actually do anything an error (non-zero exit, just like how rm i_dont_exist would return 1).

[threadproc]

@nathanleclaire The general motivation behind this is the idea that when this function returns an error, it cascades up the stack and causes docker-machine to not remove the instance from its local state when it doesn't exist remotely either. Generally, the idea is that rm should remove the machine locally and remotely, and if it exists locally but not remotely it isn't really an error and it should still remove it locally instead of throwing an error.

This circumstance is most common when using Amazon EC2 Spot Instances, when the remote machine being gone isn't exceptional and is expected.

This also supposedly brings the amazonec2 driver inline with the behaviour of other drivers like the digitalocean driver which apparently does not have this issue. (see related issue in the GitLab CI runner project: https://gitlab.com/gitlab-org/gitlab-ci-multi-runner/issues/1947#note_19955573)

[nathanleclaire]

OK -- seems reasonable -- how about just a log.Warn (Remote instance does not exist, proceeding with removing local reference) before the return nil then.

[threadproc]

@nathanleclaire I've added that, thanks for the feedback!

[nathanleclaire]

LGTM