chore(cluster) head request to check wrpc endpoint

[chronolaw]

Summary

We do not use /version-handshake endpoint, just send a simple HEAD request to /v1/wrpc,
If it works then we use wrpc, else we fall back to websocket + json.

If this PR is merged, I will remove the code and test case of version_negotiation.
Now I just keep it for compatibility.

Full changelog

• Add a new function head_request_version in version_negotiation/init.lua
• Change the behavior of functioin request_version_negotiation
• Check the result in init_worker

[javierguerragiraldez]

this has the same issue that forced us to move version negotiation as a wRPC service and not as a HTTP request: the websocket and http client libraries won't use the same connection, so there's no guarantee both will go to the same server.

PR #8775 implements the inline negotiation and does the equivalent of this PR: "try the wRPC connection and fall back to old protocol if it can't connect"

[dndx]

@javierguerragiraldez What is the advantage of "keeping the connection on the same backend server". It seems to be very fragile and I don't know why would anyone want to deploy their CP cluster this way.

[javierguerragiraldez]

@javierguerragiraldez What is the advantage of "keeping the connection on the same backend server".

Otherwise it requires shared state even before establishing the connection, as the CP has to carry the negotiation result from one event to another. If there's no guarantee that it's the same CP node, it has to be in a database.

When it's part of the wRPC itself, on the other hand, it seamlessly activates the right services (and version) for the connection, without any convoluted state management.

[dndx]

This documentation is not 100% accurate, it describes the goal, but not what this function does.

Suggested change

	--- detect '/v1/wrpc' endpoint
	--- if there is no '/v1/wrpc', fallback to websocket + json
	--- Return the highest supported Hybrid mode protocol version.

Also, does this needs to be inside init.lua? The CP shouldn't need this functionality.

[chronolaw]

I consider moving it to utils.lua, but it may cause conflict with PR #8861.
So I want to do it in #8861, Do you think it is OK?

[dndx]

Could it be moved to wrpc_data_plane.lua?

[chronolaw]

I think it is not suitable for wrpc_data_plane.lua, because at this point we have not decided which module will be required.

[dndx]

Discussed with @chronolaw offline, this should probably be removed as the name is misleading.

[chronolaw]

done