Add a `--with-env` flag to `limactl shell`

[jandubois]

Lima tries to create the illusion that containers are running locally by mounting directories and forwarding ports. But there is no mechanism to propagate environment variables to commands executed within the VM.

This is especially needed for nerdctl which cannot run on the host but must be executed inside the VM (see #412).

I propose to add a --with-env feature to limactl shell to pass along all environment variables explicitly on the commandline. Running nerdctl -e FOO ... will then become

limactl shell default FOO=foo … nerdctl -e FOO ...

We will have an internal block list that will exclude a set of variables, like PATH, SSH_* etc.

The user can specify their own blocklist instead:

export LIMA_SHELLENV_BLOCK="PATH,SSH_*"

Exact syntax TBD, but I think it can be simply a list of names, with a * as a wildcard. I don't think we need full regex syntax, but could change my mind if somebody shows a real usecase that can't be done with a simpler form.

If the blocklist starts with a + it will be added to the builtin block list; otherwise it will replace it.

Alternatively the user can specify LIMA_SHELLENV_ALLOW which enumerates all allowed env variables instead (maybe no wildcards allowed?).

The builtin block list will be included in the limactl info output.

nerdctl.lima will become limactl shell --with-env default nerdctl "$@".

Thoughts?

[AkihiroSuda]

SGTM

Bikeshedding on the flag name: --propagate-env, --inherit-env, --inherit, ...

[afbjorklund]

The sudo command has --preserve-env, for what it is worth (with capital -E)

EDIT: Unintuitively, since the configuration is called env_keep, but whatever.

[jandubois]

Bikeshedding on the flag name: --propagate-env, --inherit-env, --inherit, ...

I dislike --propagate-env. --inherit-env is fine (but --with-env is shorter). --inherit by itself is ambiguous to me.

The sudo command has --preserve-env, for what it is worth

Yes, that works too. sudo also allows --preserve-env=FOO,BAR which we could support as well. It would be equivalent to LIMA_SHELLENV_ALLOW=FOO,BAR limatctl shell ….

I think if both LIMA_SHELLENV_BLOCK and LIMA_SHELLENV_ALLOW are set simultaneously, then the former should be ignored (with a warning).

[jandubois]

Yes, that works too. sudo also allows --preserve-env=FOO,BAR which we could support as well. It would be equivalent to LIMA_SHELLENV_ALLOW=FOO,BAR limatctl shell ….

Actually, it would not really work well because we would need to treat --preserve-env=FOO differently from --preserve-env FOO. In the latter case FOO would be the instance name:

limactl shell --preserve-env FOO ls

This does not match how argument parsing works for any other option (except booleans).