prevent CPU churn by setting long pcap timeout

pcap timeouts are a mystery. Setting the timeout to -1 used to reduce
latency, but that was before we started setting immediate mode. And it
turns out that with that timeout, pcap chews through a lot of CPU
polling its file descriptor, checking timers, etc. However, the
behaviour of setting *no* timeout is defined as, err, undefined. So
instead we set a very long timeout indeed. And loop if it expires.

Fixes #80

connection_maker.go

@@ -4,7 +4,6 @@ import (
 	"bytes"
 	"fmt"
 	"log"
-	"math"
 	"math/rand"
 	"net"
 	"time"
@@ -59,7 +58,7 @@ func (cm *ConnectionMaker) String() string {
 }
 
 func (cm *ConnectionMaker) queryLoop(queryChan <-chan *ConnectionMakerInteraction) {
-	timer := time.NewTimer(math.MaxInt64)
+	timer := time.NewTimer(MaxDuration)
 	run := func() { timer.Reset(cm.checkStateAndAttemptConnections()) }
 	for {
 		select {
@@ -131,7 +130,7 @@ func (cm *ConnectionMaker) checkStateAndAttemptConnections() time.Duration {
 	})
 
 	now := time.Now() // make sure we catch items just added
-	after := time.Duration(math.MaxInt64)
+	after := MaxDuration
 	for address, target := range cm.targets {
 		if our_connected_targets[address] {
 			delete(cm.targets, address)

consts.go

@@ -2,6 +2,7 @@ package weave
 
 import (
 	"time"
+	"math"
 )
 
 const (
@@ -23,6 +24,7 @@ const (
 	FragTestInterval   = 5 * time.Minute
 	PMTUVerifyAttempts = 8
 	PMTUVerifyTimeout  = 10 * time.Millisecond // gets doubled with every attempt
+	MaxDuration        = time.Duration(math.MaxInt64)
 )
 
 const (

pcap.go

@@ -40,7 +40,7 @@ func newPcapIO(ifName string, promisc bool, snaplen int, bufSz int) (handle *Pca
 	if err = inactive.SetSnapLen(snaplen); err != nil {
 		return
 	}
-	if err = inactive.SetTimeout(-1); err != nil {
+	if err = inactive.SetTimeout(MaxDuration); err != nil {
 		return
 	}
 	if err = inactive.SetImmediateMode(true); err != nil {
@@ -60,7 +60,12 @@ func newPcapIO(ifName string, promisc bool, snaplen int, bufSz int) (handle *Pca
 }
 
 func (pi *PcapIO) ReadPacket() (data []byte, err error) {
-	data, _, err = pi.handle.ZeroCopyReadPacketData()
+	for {
+		data, _, err = pi.handle.ZeroCopyReadPacketData()
+		if err == nil || err != pcap.NextErrorTimeoutExpired {
+			break
+		}
+	}
 	return
 }