om health: check access to private caches

[shivaraj-bh]

As of now we verify the presence of the cache in nix config’s substituters. But the presence alone is not enough to verify this for private caches.

[srid]

How do we do the verification?

[shivaraj-bh]

This is actually simple to do by checking the exit-code of running nix store ping --store <cache-url>. If it is a private cache, it just fails with error: '<cache-url>' does not appear to be a binary cache. It could also mean that the user might have entered incorrect url, but if we ignore that, this seems to be the simplest way to verify.

[shivaraj-bh]

Note: Implementing the above would also mean we would be running more nix commands in the om develop preShell.

[srid]

That makes sense. Currently, we only test trusted-substituters, and not if the cache is actually accessible (requires netrc for private cache).

So the solution is to check for both trusted-substituters and whether the cache can be accessed.