Basic implementation of working server to server message.

[sfence]

• Goal of the PR
  Add ability for server-to-server communication in connection with Add transfer_player function to request client move to new server #14129.
• How does the PR work?
  Send string from one to another server. It is on Lua to serialize/deserialize string and do something around it.
• Does it resolve any reported issue?
• Yes, it should fix part of Send players from one server to an other server. #5813.
• If not a bug fix, why is this PR needed? What usecases does it solve?
  With this, a network of servers should be created. So some networks like one server for space and another server for each planet, can be created and players can be transferred between servers by Add transfer_player function to request client move to new server #14129 and their inventories and other states can be synced by server-to-server communication.

To do

This PR is a Work in Progress.

• Get feedback.
• Apply feedback.
• Add better authorization.

How to test

1. Compile Minetest with this branch.

2. Create two worlds with the game devtest.

3. To server0 world directory save file network_server_0.txt as network.conf.

4. To server1 world directory save file network_server_1.txt as network.conf.

5. Host server for game server0 on port 30000.

6. Host server for game server1 on port 30000.

7. Call command /send_msg_to_server server0 HELLO_SERVER! on server1 or
   command /send_msg_to_server server1 HELLO_SERVER! on server0.

8. The first message is lost at the moment, so send the same message again to see it.

9. Received message should be printed as a message for all players on the second server.

simplescreenrecorder-2024-01-07_14.31.45.mp4

[sfan5]

Where did the "Concept approved" come from?

[sfence]

Where did the "Concept approved" come from?

#5883 ?

[sfan5]

I assume you meant to link #5813.
The submitter proposes a server networking solution and there is some discussion around it but I interpret this #5813 (comment) to refer to an implementation of minetest.transfer_player only.
Maybe @rubenwardy can clarify.