X Conn.Recv: receive without select

- BenchmarkLinkNetPipeRTT-4         500000              3768 ns/op             225 B/op          5 allocs/op
+ BenchmarkLinkNetPipeRTT-4         500000              3555 ns/op             225 B/op          5 allocs/op

go/neo/connection.go

@@ -94,8 +94,10 @@ type Conn struct {
 	connId    uint32
 
 	rxq	   chan *PktBuf	 // received packets for this Conn go here
-	rxqActive  atomic32	 // 1 while serveRecv is doing `rxq <- ...`
+	rxqWrite   atomic32	 //  1 while serveRecv is doing `rxq <- ...`
+	rxqRead    atomic32      // +1 while Conn.Recv is doing `... <- rxq`
 	rxdownFlag atomic32	 //  1 when RX is marked no longer operational
+	// XXX ^^^ split to different cache lines?
 
 	rxerrOnce sync.Once     // rx error is reported only once - then it is link down or closed XXX !light?
 //	errMsg	  *Error	// error message for peer if rx is down	XXX try to do without it
@@ -110,7 +112,7 @@ type Conn struct {
 	//
 	// everything below is used during !light mode only.
 
-	rxdown     chan struct{} // ready when RX is marked no longer operational
+//	rxdown     chan struct{} // ready when RX is marked no longer operational
 	rxdownOnce sync.Once	 // ----//----	XXX review
 	rxclosed   atomic32	 // whether CloseRecv was called
 
@@ -208,7 +210,7 @@ var connPool = sync.Pool{New: func() interface{} {
 		rxq:    make(chan *PktBuf, 1),	// NOTE non-blocking - see serveRecv XXX +buf
 		txerr:  make(chan error, 1),	// NOTE non-blocking - see Conn.Send
 		txdown: make(chan struct{}),
-		rxdown: make(chan struct{}),
+//		rxdown: make(chan struct{}),
 	}
 }}
 
@@ -232,14 +234,15 @@ func (c *Conn) reinit() {
 	c.link = nil
 	c.connId = 0
 	// .rxq		- set initially; does not change
-	c.rxqActive.Set(0)	// XXX store relaxed?
+	c.rxqWrite.Set(0)	// XXX store relaxed?
+	c.rxqRead.Set(0)	// ----//----
 	c.rxdownFlag.Set(0)	// ----//----
 
 	c.rxerrOnce = sync.Once{}	// XXX ok?
 
 
 	// XXX vvv not strictly needed for light mode?
-	ensureOpen(&c.rxdown)
+//	ensureOpen(&c.rxdown)
 	c.rxdownOnce = sync.Once{}	// XXX ok?
 	c.rxclosed.Set(0)
 
@@ -405,7 +408,7 @@ func (c *Conn) shutdownTX() {
 // shutdownRX marks .rxq as no loner operational and interrupts Recv.
 func (c *Conn) shutdownRX(errMsg *Error) {
 	c.rxdownOnce.Do(func() {
-		close(c.rxdown)	// wakeup Conn.Recv
+//		close(c.rxdown)	// wakeup Conn.Recv
 		c.downRX(errMsg)
 	})
 }
@@ -429,7 +432,7 @@ func (c *Conn) downRX(errMsg *Error) {
 		//		   rxq until next packet (where it will hit "before it").
 		//
 		// when serveRecv stopped sending we know we are done draining when rxq is empty.
-		if c.rxqActive.Get() == 0 && len(c.rxq) == 0 {
+		if c.rxqWrite.Get() == 0 && len(c.rxq) == 0 {
 			break
 		}
 
@@ -446,6 +449,27 @@ func (c *Conn) downRX(errMsg *Error) {
 	if i != 0 {
 		go c.link.replyNoConn(c.connId, errMsg)
 	}
+
+	// wakeup readers
+	for {
+		// similarly to above:
+		// we set .rxdownFlag=1
+		// now if recvPkt is outside `... <- .rxq` critical section we know that it is either:
+		// - before it	-> it will eventually see .rxdownFlag=1 and won't try to read rxq.
+		// - after it	-> it already read pktfrom rxq and won't touch
+		//                 rxq until next recvPkt (where it will het "before it").
+		if c.rxqRead.Get() == 0 {
+			break
+		}
+
+		select {
+		case c.rxq <- nil:
+			// ok - woken up
+
+		default:
+			// ok - continue spinning
+		}
+	}
 }
 
 
@@ -584,6 +608,39 @@ func (c *Conn) errRecvShutdown() error {
 	}
 }
 
+// recvPkt receives raw packet from connection
+func (c *Conn) recvPkt() (*PktBuf, error) {
+	var pkt *PktBuf
+	var err error
+
+	// semantically equivalent to the following:
+	// (this is hot path and select is not used for performance reason)
+	//
+	// select {
+	// case <-c.rxdown:
+	// 	return nil, c.err("recv", c.errRecvShutdown())
+	//
+	// case pkt := <-c.rxq:
+	// 	return pkt, nil
+	// }
+
+	c.rxqRead.Add(1)
+	rxdown := c.rxdownFlag.Get() != 0
+	if !rxdown {
+		pkt = <-c.rxq
+	}
+
+	// decide about error while under rxqRead - if it was after the Conn can go away to be released
+	if rxdown || pkt == nil {
+		err = c.err("recv", c.errRecvShutdown())
+	}
+
+	c.rxqRead.Add(-1)
+
+	return pkt, err
+}
+
+/*
 // recvPkt receives raw packet from connection
 func (c *Conn) recvPkt() (*PktBuf, error) {
 	select {
@@ -594,6 +651,7 @@ func (c *Conn) recvPkt() (*PktBuf, error) {
 		return pkt, nil
 	}
 }
+*/
 
 // serveRecv handles incoming packets routing them to either appropriate
 // already-established connection or, if node link is accepting incoming
@@ -656,12 +714,12 @@ func (nl *NodeLink) serveRecv() {
 		// NOTE rxq must be buffered with at least 1 element so that
 		// queuing pkt succeeds for incoming connection that is not yet
 		// there in acceptq.
-		conn.rxqActive.Set(1)
+		conn.rxqWrite.Set(1)
 		rxdown := conn.rxdownFlag.Get() != 0
 		if !rxdown {
 			conn.rxq <- pkt
 		}
-		conn.rxqActive.Set(0)
+		conn.rxqWrite.Set(0)
 
 		//fmt.Printf("%p\tconn.rxdown: %v\taccept: %v\n", nl, rxdown, accept)
 
@@ -1454,7 +1512,7 @@ type Request struct {
 //
 // XXX doc
 func (link *NodeLink) Recv1() (Request, error) {
-	conn, err := link.Accept(/*context.TODO()*/)	// XXX remove context?
+	conn, err := link.Accept()
 	if err != nil {
 		return Request{}, err	// XXX or return *Request? (want to avoid alloc)
 	}
@@ -1560,3 +1618,7 @@ func (a *atomic32) Get() int32 {
 func (a *atomic32) Set(v int32) {
 	atomic.StoreInt32(&a.v, v)
 }
+
+func (a *atomic32) Add(δ int32) int32 {
+	return atomic.AddInt32(&a.v, δ)
+}