Refactor: move service launchers to test code

[josecelano]

Originated in: #268 (comment)

The API, UDP, and HTTP tracker use a similar strategy to run the service instances. There are two levels of logic:

• The ServiceController
• The ServiceLauncher

ServiceController

The ServiceController is responsible for the following:

• Storing the configuration needed to start the service (contained from the config.toml) file.
• Starting and stopping the service
• Knowing the current state of the service: running or stopped.

It does not store the configuration changes. If you stop the service, the next time you start it it will use the initial configuration when the ServiceController was instantiated.

ServiceLauncher

The ServiceLauncher is responsible for the following:

• Starting the service instance.
• Starting the service instance with a graceful shutdown. It can receive a shutdown signal.

Current implementations

Tracker API:

• torrust_tracker::servers::apis::server::ApiServer<S> (ServiceController)
• torrust_tracker::servers::apis::server and torrust_tracker::servers::apis::server::start_tls functions (ServiceLauncher)

HTTP tracker:

• torrust_tracker::servers::http::server::HttpServer (ServiceController)
• torrust_tracker::servers::http::v1::launcherstart and torrust_tracker::servers::http::v1::launcherstart_tls functions (ServiceLauncher)

UDP tracker:

• torrust_tracker::servers::udp::server::UdpServer (ServiceController)
• torrust_tracker::servers::udp::server::Udp (ServiceLauncher)

Proposed changes

Some structs are only used for testing for the time being. We should move them to test modules until we needed in production code. For example, if we add a feature to check the state of a given service (we could show all the services and the ports they are using in an admin panel).

Structs only used for testing:

• torrust_tracker::servers::apis::server::ApiServer<S> (ServiceController)
• torrust_tracker::servers::http::server::HttpServer (ServiceController)
• torrust_tracker::servers::udp::server::UdpServer (ServiceController)

Other dependencies are used only for testing too.

Future improvements

This is out of the scope of this issue, but maybe we could generalize a ServiceController for services that need a socket port, which is, I think, what they have in common.