Just for clarification, is it only the ETCDCTL_API got unset in lock exec? Or every env starting ETCDCTL_?

I just tried on 3.3 release where etcdctl API is defaulted to v2. The following command will not work because ETCDCTL_API is unset:

$ ETCDCTL_API=3 ./bin/etcdctl lock mylock ./bin/etcdctl put foo bar
No help topic for 'put'
Error:  exit status 3

Other ETCDCTL_ env variables seem to work.

$ ETCDCTL_ENDPOINTS=http://127.0.0.1:2379 ETCDCTL_API=3 etcdctl lock mylock env | grep ETCDCTL
ETCDCTL_ENDPOINTS=http://127.0.0.1:2379

I guess it is fine to just document the behavior clearly. But it is definitely a little wired if user wants to execute a v3 command upon lock acquisition.

@jingyih thanks and yes it's only ETCDCTL_API that etcdctl will unset [1]. Besides that, ETCTCTL_ are only allowed to use with etcdctl flags. So for example, ETCDCTL_X gives this error:
2019-08-26 22:03:41.908806 W | pkg/flags: unrecognized environment variable ETCDCTL_X=3
[1] https://github.com/etcd-io/etcd/blob/master/etcdctl/main.go#L33

Safe merge so going for it :)