Signal/Interrupt package

[cfromknecht]

Moves most of the contents in signal.go into it's own package named signal. This is done in preparation of having standalone watchtowers, so each daemon can share this code.

Notable changes:

• handlers are now executed in LIFO order. Before we were registering closures that executed in order, though this allows us to have similar semantics as defer. The handlers in lnd.go have now been separated, and are registered after each Start().
• handlers are now func() error instead of func(). All of the handlers we use are of this type, so it makes it cleaner even though we don't handle the errors during an interrupt.

[cfromknecht]

One question I have is whether we even need AddInterruptHandler, instead of just preferring to defer the Stop() methods. If we also defer the log message stating that shutdown is complete at the beginning of lndMain, everything should already be in lifo order.

[cfromknecht]

I added two commits that impl the defer approach, lmk which you all prefer

[Roasbeef]

Nice! We'll be able to use this in a few other projects we have in mind.

One question: what's the rationale behind removing the interrupt handlers in favor of using regular defers everywhere?

[Roasbeef]

Nice addition! Should've been this way all along.

[cfromknecht]

Rationale for using defer: the current system executes FIFO order, which is fine since only two functions are registered (one of which is composite, and for the most part, executes Stop in LIFO order of resource creation). Instead of reimplementing a LIFO queue, I chose defer since it's built into the language, is consistent with all other teardown paradigms in lnd, and results in less overall code.

[cfromknecht]

can squash the last two commits if rationale is sound

[Roasbeef]

👍

[cfromknecht]

squashed commits and rebased on master

[Roasbeef]

LGTM 💫