routing: launch fetchFundingTx in goroutine so router can exit

[Crypt-iQ]

This commit introduces a wrapper function fetchFundingTxWrapper which calls fetchFundingTx in a goroutine. This is to avoid an issue with pruned nodes where the router is attempting to stop, but the prunedBlockDispatcher is waiting to connect to peers that can serve the block. This can cause the shutdown process to hang until we connect to a peer that can send us the block. Marking as draft for now until I can test this. I opted for this approach rather than passing in a quit channel to GetBlock.

See #7928 (comment) for more info

[ProofOfKeags]

I'd love to see this generalized. The pattern here is that you have some sync function that you'd like to make async and multiplexed over an alternative quit signal.

func RunAsyncWithCancel(func()(A, error), <-chan struct{}, error) (A, error) { ... }

then call with

RunAsyncWithCancel(func() { return r.fetchFundingTx(&channelID) }, r.quit, ErrRouterShuttingDown)

You can place this in the new fn package.

[ProofOfKeags]

Confirmed that this typechecks and compiles:

func RunAsyncWithCancel[A any](
	f func() (A, error),
	quitSig <-chan struct{},
	quitErr error,
) (*A, error) {
	resChan := make(chan A, 1)
	errChan := make(chan error, 1)

	go func() {
		res, err := f()
		if err != nil {
			errChan <- err
		} else {
			resChan <- res
		}
	}()

	select {
	case res := <- resChan:
		return &res, nil
	case err := <- errChan:
		return nil, err
	case <-quitSig:
		return nil, quitErr
	}
}

[Crypt-iQ]

I like the idea, but I think it's better left to another PR. I like to keep my PRs very minimal. In another PR, we could replace call-sites in one sweep (I think the itests use this pattern a lot)

[ProofOfKeags]

Do we have a project management mechanism for scheduling these sorts of things? If not, it will never get prioritized and we won't be able to reap the benefits of having improved abstractions. If so, what do we need to do to get it into the queue? Or do I just need to do it when I have a spare moment?

[Crypt-iQ]

You can make a PR and I'll review it

[ProofOfKeags]

Minor change I'd suggest but isn't required. I'd like to start extracting common patterns out and making them broadly available so we don't have to keep writing them over and over again. I think this is a good candidate here.

[ellemouton]

LGTM ✅
I like the change suggested by @ProofOfKeags too 👍

[lightninglabs-deploy]

@Crypt-iQ, remember to re-request review from reviewers when ready