Logging

[peterjan]

This PR adds structured logging to the autopilot. I thought of using logrus but apparently that's in maintenance mode. Seems like zap is the new go-to for structured logging and it's super fast.

The idea is to use named loggers that add a "component" context to the log entries. Structured logging is definitely a bonus since we'll be able to easily parse relevant info for a single host or subsystem. I played around with the config a little bit but kept it simple for now. There's also a zaptest package that allows creating a test logger to which you can pass testing.T, which is pretty nice because you get logs only in case of failure or when you run it with -v. Removed that though for the time being.

[lukechampine]

this is a siad/skyd pattern that I am trying to avoid replicating. 😅

The rule of thumb is: Don't call constructors within a constructor. Doing so is convenient for the caller (since they can pass one dir argument instead of a bunch of interfaces), but it has three big downsides:

• It prevents you from using interfaces. In this case, we are stuck with a logger that writes to a file, even during testing.
• It forces the constructed object to be responsible for cleaning up its dependencies. (Should an autopilot really be responsible for closing a logger?)
• It prevents you from sharing a dependency. What if we want all of our subsystems to share a logger?

There are exceptions, of course. But to me, this is a pretty clear example of where we should be passing a Logger interface instead of calling a constructor within a constructor.

I think the constructor-within-a-constructor pattern derives from a more general design philosophy of building systems out of "actors," by which I mean "black boxes that have a lot of state and make a lot of decisions." I'm moving away from that philosophy, and towards systems built from "dumb" components, which have less state and make fewer decisions. Essentially, the goal is to centralize the complexity of the system by moving it out of the individual components.

[peterjan]

That makes sense 👍 - couldn't pass an interface because the guys at Uber decided to remove the interface and make it a struct. Should be fine though, it actually cleaned up the cleanup function, thanks!