addrmgr: Always use a 50% chance between tried and new entries.

This change was suggested as Countermeasure 2 in
Eclipse Attacks on Bitcoin's Peer-to-Peer Network, Ethan
Heilman, Alison Kendler, Aviv Zohar, Sharon Goldberg. ePrint Archive
Report 2015/263. March 2015.

This mimics Bitcoin Core commit c6a63ceeb4956933588995bcf01dc3095aaeb1fc

addrmgr/addrmanager.go

@@ -12,7 +12,6 @@ import (
 	"encoding/json"
 	"fmt"
 	"io"
-	"math"
 	"math/rand"
 	"net"
 	"os"
@@ -741,7 +740,7 @@ func NetAddressKey(na *wire.NetAddress) string {
 // random one from the possible addresses with preference given to ones that
 // have not been used recently and should not pick 'close' addresses
 // consecutively.
-func (a *AddrManager) GetAddress(class string, newBias int) *KnownAddress {
+func (a *AddrManager) GetAddress(class string) *KnownAddress {
 	// Protect concurrent access.
 	a.mtx.Lock()
 	defer a.mtx.Unlock()
@@ -750,20 +749,8 @@ func (a *AddrManager) GetAddress(class string, newBias int) *KnownAddress {
 		return nil
 	}
 
-	if newBias > 100 {
-		newBias = 100
-	}
-	if newBias < 0 {
-		newBias = 0
-	}
-
-	// Bias between new and tried addresses.
-	triedCorrelation := math.Sqrt(float64(a.nTried)) *
-		(100.0 - float64(newBias))
-	newCorrelation := math.Sqrt(float64(a.nNew)) * float64(newBias)
-
-	if ((newCorrelation + triedCorrelation) * a.rand.Float64()) <
-		triedCorrelation {
+	// Use a 50% chance for choosing between tried and new table entries.
+	if a.nTried > 0 && (a.nNew == 0 || a.rand.Intn(2) == 0) {
 		// Tried entry.
 		large := 1 << 30
 		factor := 1.0

addrmgr/addrmanager_test.go

@@ -221,7 +221,7 @@ func TestAttempt(t *testing.T) {
 	if err != nil {
 		t.Fatalf("Adding address failed: %v", err)
 	}
-	ka := n.GetAddress("any", 100)
+	ka := n.GetAddress("any")
 
 	if !ka.LastAttempt().IsZero() {
 		t.Errorf("Address should not have attempts, but does")
@@ -243,7 +243,7 @@ func TestConnected(t *testing.T) {
 	if err != nil {
 		t.Fatalf("Adding address failed: %v", err)
 	}
-	ka := n.GetAddress("any", 100)
+	ka := n.GetAddress("any")
 	na := ka.NetAddress()
 	na.Timestamp = time.Now().Add(time.Hour * -1) // make it an hour ago
 
@@ -334,7 +334,7 @@ func TestGetAddress(t *testing.T) {
 	n := addrmgr.New("testgetaddress", lookupFunc)
 
 	// Get an address from an empty set (should error)
-	if rv := n.GetAddress("any", 10); rv != nil {
+	if rv := n.GetAddress("any"); rv != nil {
 		t.Errorf("GetAddress failed: got: %v want: %v\n", rv, nil)
 	}
 
@@ -343,7 +343,7 @@ func TestGetAddress(t *testing.T) {
 	if err != nil {
 		t.Fatalf("Adding address failed: %v", err)
 	}
-	ka := n.GetAddress("any", 120) // 100 bias is max, but shouldn't error
+	ka := n.GetAddress("any")
 	if ka == nil {
 		t.Fatalf("Did not get an address where there is one in the pool")
 	}
@@ -353,7 +353,7 @@ func TestGetAddress(t *testing.T) {
 
 	// Mark this as a good address and get it
 	n.Good(ka.NetAddress())
-	ka = n.GetAddress("any", -10) // 0 bias is min, but shouldn't error
+	ka = n.GetAddress("any")
 	if ka == nil {
 		t.Fatalf("Did not get an address where there is one in the pool")
 	}

server.go

@@ -695,15 +695,11 @@ out:
 		tries := 0
 		for state.NeedMoreOutbound() &&
 			atomic.LoadInt32(&s.shutdown) == 0 {
-			// We bias like bitcoind does, 10 for no outgoing
-			// up to 90 (8) for the selection of new vs tried
-			//addresses.
-
 			nPeers := state.OutboundCount()
 			if nPeers > 8 {
 				nPeers = 8
 			}
-			addr := s.addrManager.GetAddress("any", 10+nPeers*10)
+			addr := s.addrManager.GetAddress("any")
 			if addr == nil {
 				break
 			}