multi: link quit can interrupt commitment signing

In this commit, we make sig job handling when singing a next commitment
non-blocking by allowing the shutdown of a channel link to prevent
further waiting on sig jobs by the channel state machine. This addresses
possible cases where the aux signer may be shut down via a separate quit
signal, so the state machine could block indefinitely on receiving an
update on a sig job.

htlcswitch/link_isolated_test.go

@@ -1,6 +1,7 @@
 package htlcswitch
 
 import (
+	"context"
 	"crypto/sha256"
 	"testing"
 	"time"
@@ -94,7 +95,9 @@ func (l *linkTestContext) receiveHtlcAliceToBob() {
 func (l *linkTestContext) sendCommitSigBobToAlice(expHtlcs int) {
 	l.t.Helper()
 
-	sigs, err := l.bobChannel.SignNextCommitment()
+	testQuit, testQuitFunc := context.WithCancel(context.Background())
+	defer testQuitFunc()
+	sigs, err := l.bobChannel.SignNextCommitment(testQuit)
 	if err != nil {
 		l.t.Fatalf("error signing commitment: %v", err)
 	}