linkerd check hangs if identity is unavailable

[adleong]

If using the edge-19.4.1 Linkerd CLI with an older Linkerd installed such as edge-19.3.2 which does not include the linkerd-identity deployment, linkerd check will hang forever waiting for linkerd-identity pods to become ready. This also prevents the use of other commands such as linkerd dashboard.

Steps to reproduce:

1. Install Linkerd edge-19.3.2 to a cluster
2. Use edge-19.4.1 CLI to run linkerd check

[siggy]

The linkerd check command should timeout after 5 minutes. The primary symptom users will observe is if they upgrade their cli and then run linkerd check, the missing identity deployment will exhibit this behavior. We should call out this case in the docs.

[adleong]

Can we improve this behavior by skipping the check for the identity pod if we know it won't be there? e.g. if the server version is too old? or if the identity deployment doesn't exist?

[siggy]

Skipping a check for the identity pod would solve this particular issue. This should probably motivate a larger discussion around how version-aware we want to make linkerd check, and also priority for such work.