Provide a CancelOnGracefulShutdownService

[fabianfett]

If an adopter already has a reference type that implements a run() method it is currently impossible to support ServiceLifecycle, even though the implementation works easily:

extension MyExistingService: Service {}

The reason for this is, that ServiceLifecycle uses gracefulShutdown to signal shutdown instead of Task cancellation. That is the correct call. However it is impossible to add gracefulShutdown support to object that already expose a run() method. That's why im proposing that ServiceLifecycle offers a CancelOnGracefulShutdownService<Underlying: Service>.

[adam-fowler]

Conforming MyExistingService to Service doesn't seem correct as it doesn't support graceful shutdown.
Perhaps we need another protocol CancellableService for a Service that is only cancellable. Then your service could look like

protocol CancellableService {
    func run() async throws
}

struct CancelOnGracefulShutdownService<S: CancellableService> {
    @usableFromInline
    let service: S

    @inlinable
    func run() async throws {
        try await CancelOnGracefulShutdown {
            try await self.service.run()
        }
    }
}

[supersonicbyte]

hey folks! Can I try to tackle this one?

[FranzBusch]

I am not yet sure we need this. In one of the latest releases we introduced support to escalate graceful shutdown to cancellation. This should help with services that don't support graceful shutdown right now.

I would like to wait for more user reports here if we really need such a new protocol