reuseport: env var to turn it off

reuseport is a hack. It is necessary for us to do certain kinds of
tcp nat traversal. Ideally, reuseport would be available in go:

  https://github.com/golang/go/issues/9661

But until that issue is fixed, we're stuck with this. In some cases,
reuseport is strictly a detriment: nodes are not NATed. This commit
introduces an ENV var IPFS_REUSEPORT that can be set to false to
avoid using reuseport entirely:

  IPFS_REUSEPORT=false ipfs daemon

This approach addresses our current need. It could become a config
var if necessary. If reuseport continues to give problems, we should
look into improving it.

p2p/net/conn/dial.go

@@ -118,7 +118,7 @@ func (d *Dialer) rawConnDial(ctx context.Context, raddr ma.Multiaddr, remote pee
 	// make a copy of the manet.Dialer, we may need to change its timeout.
 	madialer := d.Dialer
 
-	if laddr != nil && reuseport.Available() {
+	if laddr != nil && reuseportIsAvailable() {
 		// we're perhaps going to dial twice. half the timeout, so we can afford to.
 		// otherwise our context would expire right after the first dial.
 		madialer.Dialer.Timeout = (madialer.Dialer.Timeout / 2)

p2p/net/conn/listen.go

@@ -169,7 +169,7 @@ func manetListen(addr ma.Multiaddr) (manet.Listener, error) {
 		return nil, err
 	}
 
-	if reuseport.Available() {
+	if reuseportIsAvailable() {
 		nl, err := reuseport.Listen(network, naddr)
 		if err == nil {
 			// hey, it worked!

p2p/net/conn/reuseport.go

@@ -0,0 +1,35 @@
+package conn
+
+import (
+	"os"
+	"strings"
+
+	reuseport "github.com/ipfs/go-ipfs/Godeps/_workspace/src/github.com/jbenet/go-reuseport"
+)
+
+// envReuseport is the env variable name used to turn off reuse port.
+// It default to true.
+const envReuseport = "IPFS_REUSEPORT"
+
+// envReuseportVal stores the value of envReuseport. defaults to true.
+var envReuseportVal = true
+
+func init() {
+	v := strings.ToLower(os.Getenv(envReuseport))
+	if v == "false" || v == "f" || v == "0" {
+		envReuseportVal = false
+		log.Infof("REUSEPORT disabled (IPFS_REUSEPORT=%s)", v)
+	}
+}
+
+// reuseportIsAvailable returns whether reuseport is available to be used. This
+// is here because we want to be able to turn reuseport on and off selectively.
+// For now we use an ENV variable, as this handles our pressing need:
+//
+//   IPFS_REUSEPORT=false ipfs daemon
+//
+// If this becomes a sought after feature, we could add this to the config.
+// In the end, reuseport is a stop-gap.
+func reuseportIsAvailable() bool {
+	return envReuseportVal && reuseport.Available()
+}