Feature: connectivity check

[gnarula]

This PR adds a new protocol that checks connectivity with all other nodes in the roster. The results of the check are cached by default and controlled using the CONNECTIVITY_TTL environment variable.

Still a WIP. Need to remove time.Sleep in the tests

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🙅‍ Friendly checklist:

• [ ] 0. Code comments are added (or updated) when/where needed and explain the WHY of the code.
• [ ] 1. Design choices, user documentation and any additional doc are added (or updated) in READMEs.
• [ ] 2. Any new behaviour is tested and small units of code that can be are unit tested.
• [ ] 3. Code comments are added on tests to explain what they do.
• [ ] 4. Errors are systematically wrapped with a meaningful message using xerrors.Errorf and the %v verb.
• [ ] 5. Hard limit of 80 chars is always respected.
• [ ] 6. Changes are backward compatible.
• [ ] 7. Indentation level does not exceed 5, although 4 is already suspicious.
• [ ] 8. Functions, files, and packages are kept to a manageable size and decomposed into smaller units if needed.
• [ ] 9. There are no magic values.

[ineiti]

How is this different from https://github.com/dedis/cothority/blob/11cfd645218140693d889d7cb0d8951a82809444/status/service/status.go#L60 ?

[gnarula]

I should've grepped or asked you before :) Closing this one and I'll rather propose changes in 'status' if required.

[ineiti]

Yes, and it's difficult to grep through it if you don't know. I was also not sure whether this has actually been implemented or not ;) But one line in #engineers with a mention is enough and I'll try to chime in. Or open an issue and mention @ineiti should also work.

Btw: one thing I'm sure the status-thingie doesn't do is to check opening a connection from both ends. E.g.: if you start from a node that is in your local network, it will be able to start connections with all other nodes and they will be able to reply. But if the other nodes will try to contact yours, it will fail.