multi: thread bandwidth check amount down to MayAddOutgoingHtlc

Pass htlc amount down to the channel so that we don't need to rely
on minHtlc (and pad it when the channel sets a 0 min htlc). Update
test to just check some sane values since we're no longer relying
on minHtlc amount at all.

htlcswitch/interfaces.go

@@ -102,8 +102,9 @@ type ChannelUpdateHandler interface {
 	EligibleToForward() bool
 
 	// MayAddOutgoingHtlc returns an error if we may not add an outgoing
-	// htlc to the channel.
-	MayAddOutgoingHtlc() error
+	// htlc to the channel, taking the amount of the htlc to add as a
+	// parameter.
+	MayAddOutgoingHtlc(lnwire.MilliSatoshi) error
 
 	// ShutdownIfChannelClean shuts the link down if the channel state is
 	// clean. This can be used with dynamic commitment negotiation or coop

htlcswitch/link.go

@@ -2203,10 +2203,12 @@ func (l *channelLink) Bandwidth() lnwire.MilliSatoshi {
 	return l.channel.AvailableBalance()
 }
 
-// MayAddOutgoingHtlc indicates whether we may add any more outgoing htlcs to
-// this channel.
-func (l *channelLink) MayAddOutgoingHtlc() error {
-	return l.channel.MayAddOutgoingHtlc()
+// MayAddOutgoingHtlc indicates whether we can add an outgoing htlc with the
+// amount provided to the link. This check does not reserve a space, since
+// forwards or other payments may use the available slot, so it should be
+// considered best-effort.
+func (l *channelLink) MayAddOutgoingHtlc(amt lnwire.MilliSatoshi) error {
+	return l.channel.MayAddOutgoingHtlc(amt)
 }
 
 // getDustSum is a wrapper method that calls the underlying channel's dust sum

htlcswitch/mock.go

@@ -784,7 +784,7 @@ func (f *mockChannelLink) Peer() lnpeer.Peer                            { return
 func (f *mockChannelLink) ChannelPoint() *wire.OutPoint                 { return &wire.OutPoint{} }
 func (f *mockChannelLink) Stop()                                        {}
 func (f *mockChannelLink) EligibleToForward() bool                      { return f.eligible }
-func (f *mockChannelLink) MayAddOutgoingHtlc() error                    { return nil }
+func (f *mockChannelLink) MayAddOutgoingHtlc(lnwire.MilliSatoshi) error { return nil }
 func (f *mockChannelLink) ShutdownIfChannelClean() error                { return nil }
 func (f *mockChannelLink) setLiveShortChanID(sid lnwire.ShortChannelID) { f.shortChanID = sid }
 func (f *mockChannelLink) UpdateShortChanID() (lnwire.ShortChannelID, error) {