network: don't try too many connections

Consider mainnet, it has an AttemptConnPeers of 20, so may already have 3
peers and request 20 more, then have 4th connected and attemtp 20 more again,
this leads to a huge number of connections easily.

pkg/network/discovery.go

@@ -49,6 +49,7 @@ type DefaultDiscovery struct {
 	goodAddrs        map[string]capability.Capabilities
 	unconnectedAddrs map[string]int
 	attempted        map[string]bool
+	outstanding      int32
 	optimalFanOut    int32
 	networkSize      int32
 	requestCh        chan int
@@ -119,6 +120,8 @@ func (d *DefaultDiscovery) pushToPoolOrDrop(addr string) {
 
 // RequestRemote tries to establish a connection with n nodes.
 func (d *DefaultDiscovery) RequestRemote(requested int) {
+	outstanding := int(atomic.LoadInt32(&d.outstanding))
+	requested -= outstanding
 	for ; requested > 0; requested-- {
 		var nextAddr string
 		d.lock.Lock()
@@ -146,6 +149,7 @@ func (d *DefaultDiscovery) RequestRemote(requested int) {
 		}
 		d.attempted[nextAddr] = true
 		d.lock.Unlock()
+		atomic.AddInt32(&d.outstanding, 1)
 		go d.tryAddress(nextAddr)
 	}
 }
@@ -291,6 +295,7 @@ func (d *DefaultDiscovery) updateNetSize() {
 
 func (d *DefaultDiscovery) tryAddress(addr string) {
 	p, err := d.transport.Dial(addr, d.dialTimeout)
+	atomic.AddInt32(&d.outstanding, -1)
 	d.lock.Lock()
 	delete(d.attempted, addr)
 	if err == nil {