add a helper actor to the link reader to make it play nicer with backpressure

src/yggdrasil/link.go

@@ -16,6 +16,8 @@ import (
 	"github.com/yggdrasil-network/yggdrasil-go/src/address"
 	"github.com/yggdrasil-network/yggdrasil-go/src/crypto"
 	"github.com/yggdrasil-network/yggdrasil-go/src/util"
+
+	"github.com/Arceliar/phony"
 )
 
 type link struct {
@@ -388,14 +390,17 @@ func (intf *linkInterface) handler() error {
 		}
 	}()
 	// Run reader loop
-	for {
+	var helper phony.Actor
+	done := make(chan struct{})
+	var helperFunc func()
+	helperFunc = func() {
+		// The helper reads in a loop and sends to the peer
+		// It loops by sending itself a message, which can be delayed by backpressure
+		// So if the peer is busy, backpressure will pause reading until the peer catches up
 		msg, err := intf.msgIO.readMsg()
 		if len(msg) > 0 {
 			// TODO rewrite this if the link becomes an actor
-			// FIXME this could theoretically send traffic faster than the peer can handle
-			//  The alternative is to SyncExec, but that causes traffic to block while *other* links work
-			//  Need to figure out why and find a workaround
-			intf.peer.handlePacketFrom(nil, msg)
+			intf.peer.handlePacketFrom(&helper, msg)
 		}
 		if err != nil {
 			if err != io.EOF {
@@ -404,13 +409,19 @@ func (intf *linkInterface) handler() error {
 				default:
 				}
 			}
-			break
+			close(done)
+			return
 		}
 		select {
 		case signalAlive <- len(msg) > 0:
 		default:
 		}
+		// Now try to read again
+		helper.EnqueueFrom(nil, helperFunc)
 	}
+	// Start the read loop
+	helper.EnqueueFrom(nil, helperFunc)
+	<-done // Wait for the helper to exit
 	////////////////////////////////////////////////////////////////////////////////
 	// Remember to set `err` to something useful before returning
 	select {