autopilot/agent: distribute available funds among channels

This commit fixes a regression in how we allocate funds to attempted
channels. We would earlier stay within the channel size limits, but we
wouldn't account for funds consumed by other channels being opened in
parallel.

We fix this by introducing a loop which greadily tries to distribute the
funds among the channels to open, and reduces the number of channels to
open in case not enough funds are available to satisfy the channel size
limits.

autopilot/agent.go

@@ -567,10 +567,15 @@ func (a *Agent) openChans(availableFunds btcutil.Amount, numChans uint32,
 
 	// As channel size we'll use the maximum channel size available.
 	chanSize := a.cfg.Constraints.MaxChanSize()
-	if availableFunds-chanSize < 0 {
+	if availableFunds < chanSize {
 		chanSize = availableFunds
 	}
 
+	if chanSize < a.cfg.Constraints.MinChanSize() {
+		return fmt.Errorf("not enough funds available to open a " +
+			"single channel")
+	}
+
 	// Use the heuristic to calculate a score for each node in the
 	// graph.
 	scores, err := a.cfg.Heuristic.NodeScores(
@@ -601,6 +606,17 @@ func (a *Agent) openChans(availableFunds btcutil.Amount, numChans uint32,
 			continue
 		}
 
+		// Track the available funds we have left.
+		if availableFunds < chanSize {
+			chanSize = availableFunds
+		}
+		availableFunds -= chanSize
+
+		// If we run out of funds, we can break early.
+		if chanSize < a.cfg.Constraints.MinChanSize() {
+			break
+		}
+
 		chanCandidates[nID] = &AttachmentDirective{
 			NodeID:  nID,
 			ChanAmt: chanSize,
@@ -725,8 +741,7 @@ func (a *Agent) executeDirective(directive AttachmentDirective) {
 	// fewer slots were available, and other successful attempts finished
 	// first.
 	a.pendingMtx.Lock()
-	if uint16(len(a.pendingOpens)) >=
+	if uint16(len(a.pendingOpens)) >= a.cfg.Constraints.MaxPendingOpens() {
-		a.cfg.Constraints.MaxPendingOpens() {
 		// Since we've reached our max number of pending opens, we'll
 		// disconnect this peer and exit. However, if we were
 		// previously connected to them, then we'll make sure to
@@ -799,6 +814,9 @@ func (a *Agent) executeDirective(directive AttachmentDirective) {
 
 	// Since the channel open was successful and is currently pending,
 	// we'll trigger the autopilot agent to query for more peers.
+	// TODO(halseth): this triggers a new loop before all the new channels
+	// are added to the pending channels map. Should add before executing
+	// directive in goroutine?
 	a.OnChannelPendingOpen()
 }