Prune the environment when installing an app

[kyri-petrou]

Hi team 👋

Context

There is a bug that's been around for some time (probably since the 0.x days) that I never paid too much attention to before because I didn't understand much about ZIO.

Currently, when we bootstrap / wire an app, we provide the current ZEnvironment in the NettyRuntime. This environment, however, contains pretty much all the services that were used to wire the app. Let's take the following app as an example:

object HelloWorld extends ZIOAppDefault {
  final class Service {
    def printEnv: UIO[Unit] = ZIO.environment.debug.unit
  }

  val homeRoute = Method.GET / Root -> handler(ZIO.serviceWithZIO[Service](_.printEnv.as(Response.text("foo"))))

  override val run =
    Server.serve(Routes(homeRoute)).provide(Server.default, ZLayer.succeed(new Service))
}

In this case, sending a request to the server will print the following in the console. As you can see, the environment contains 6 services in this case, where in theory it should only be containing our Service.

ZEnvironment(EventLoopGroup -> io.netty.channel.nio.NioEventLoopGroup@81f8e, ChannelFactory[=ServerChannel] -> zio.http.netty.ChannelFactories$$anon$1@41f33ad, Server::Config -> Config(None,0.0.0.0/0.0.0.0:8080,false,true,No,None,Disabled(102400),4096,8192,true,PT10S,WebSocketConfig(None,10000,-1,true,NormalClosure,true,SocketDecoder(65536,true,false,false,true,true)),None), NettyConfig -> NettyConfig(SIMPLE,AUTO,0,PT2S,PT15S), HelloWorld::Service -> example.HelloWorld$Service@55615130, Server -> ServerLive(NettyDriver(Config(None,0.0.0.0/0.0.0.0:8080,false,true,No,None,Disabled(102400),4096,8192,true,PT10S,WebSocketConfig(None,10000,-1,true,NormalClosure,true,SocketDecoder(65536,true,false,false,true,true)),None)),8080))

Why is this a problem?

The way that ZEnvironment works, is that lookups are O(n) complex (n = size of environment) when a service is accessed for the first time it goes into a private cache and then subsequent access for that service is O(1) complex. Since the environment is immutable, anything that adds something to it, e.g., ZIO.scoped creates a new instance to it, so accessing that service becomes O(n) again. So having an environment as lean as possible will yield better performance

Main change

The changes in this PR ensure that the environment is pruned to contained only the services that are required to run the app. The downside to this is that we require an implicit EnvironmentTag now in the install and serve methods. I think that in most cases this should be okay, since most users would be using install or serve when R is statically known.

Alternative approach (no breaking changes)

An alternative that doesn't involve breaking changes would be to revert the implicit EnvironmentTag changes, and keep only the change made to AppRef. This way, we leave it up to the user to properly clean the environment prior to calling serve or install. The downside to this is that it's unlikely that users will do this