server: add an exponential back off to initialPeerBootstrap

In this commit we add exponential back off to the `initialPeerBootstrap`
method. Before this change, if the DNS seed was down, we would hammer it
in an attempt to get our initial set of peers. This makes this section a
bit less aggressive, but saves log spam and also will hit the DNS
servers less frequently.

server.go

@@ -1723,8 +1723,16 @@ const bootstrapBackOffCeiling = time.Minute * 5
 func (s *server) initialPeerBootstrap(ignore map[autopilot.NodeID]struct{},
 	numTargetPeers uint32, bootstrappers []discovery.NetworkPeerBootstrapper) {
 
+	// We'll start off by waiting 2 seconds between failed attempts, then
+	// double each time we fail until we hit the bootstrapBackOffCeiling.
+	var delaySignal <-chan time.Time
+	delayTime := time.Second * 2
 
-	for {
+	// As want to be more aggressive, we'll use a lower back off celling
+	// then the main peer bootstrap logic.
+	backOffCeiling := bootstrapBackOffCeiling / 5
+
+	for attempts := 0; ; attempts++ {
 		// Check if the server has been requested to shut down in order
 		// to prevent blocking.
 		if s.Stopped() {
@@ -1741,8 +1749,31 @@ func (s *server) initialPeerBootstrap(ignore map[autopilot.NodeID]struct{},
 			return
 		}
 
-		// Otherwise, we'll request for the remaining number of peers in
-		// order to reach our target.
+		if attempts > 0 {
+			srvrLog.Debugf("Waiting %v before trying to locate "+
+				"bootstrap peers (attempt #%v)", delayTime,
+				attempts)
+
+			// We've completed at least one iterating and haven't
+			// finished, so we'll start to insert a delay period
+			// between each attempt.
+			delaySignal = time.After(delayTime)
+			select {
+			case <-delaySignal:
+			case <-s.quit:
+				return
+			}
+
+			// After our delay, we'll double the time we wait up to
+			// the max back off period.
+			delayTime *= 2
+			if delayTime > backOffCeiling {
+				delayTime = backOffCeiling
+			}
+		}
+
+		// Otherwise, we'll request for the remaining number of peers
+		// in order to reach our target.
 		peersNeeded := numTargetPeers - numActivePeers
 		bootstrapAddrs, err := discovery.MultiSourceBootstrap(
 			ignore, peersNeeded, bootstrappers...,