xnet: Teach Tracer to enable/disable delivery of trace events at runtime.

Some NEO/go tests need to start a cluster under tracing and then to
continue running test workload with tracing disabled.

xnet/trace.go

@@ -23,6 +23,7 @@ package xnet
 import (
 	"context"
 	"net"
+	"sync/atomic"
 )
 
 // NetTrace wraps underlying networker with IO tracing layer.
@@ -42,7 +43,7 @@ import (
 //
 // WARNING NetTrace functionality is currently very draft.
 func NetTrace(inner Networker, tracerx TraceReceiver) *Tracer {
-	return &Tracer{inner, tracerx}
+	return &Tracer{inner, tracerx, 1}
 }
 
 // TraceReceiver is the interface that needs to be implemented by network trace receivers.
@@ -86,6 +87,21 @@ type TraceTx struct {
 type Tracer struct {
 	inner Networker
 	rx    TraceReceiver
+	on    int32 // atomic (tracing can be enabled/disabled at runtime)
+}
+
+// TraceOn tells the tracer to (re)enable delivery of trace events.
+func (t *Tracer) TraceOn() {
+	atomic.StoreInt32(&t.on, 1)
+}
+
+// TraceOff tells tracer to disable delivery of trace events.
+func (t *Tracer) TraceOff() {
+	atomic.StoreInt32(&t.on, 0)
+}
+
+func (t *Tracer) enabled() bool {
+	return (atomic.LoadInt32(&t.on) != 0)
 }
 
 // Network implements Networker.
@@ -106,12 +122,16 @@ func (t *Tracer) Close() error {
 
 // Dial implements Networker.
 func (t *Tracer) Dial(ctx context.Context, addr string) (net.Conn, error) {
+	if t.enabled() {
 		t.rx.TraceNetDial(&TraceDial{Dialer: t.inner.Name(), Addr: addr})
+	}
 	c, err := t.inner.Dial(ctx, addr)
 	if err != nil {
 		return nil, err
 	}
+	if t.enabled() {
 		t.rx.TraceNetConnect(&TraceConnect{Src: c.LocalAddr(), Dst: c.RemoteAddr(), Dialed: addr})
+	}
 	return &traceConn{t, c}, nil
 }
 
@@ -122,7 +142,9 @@ func (t *Tracer) Listen(ctx context.Context, laddr string) (Listener, error) {
 	if err != nil {
 		return nil, err
 	}
+	if t.enabled() {
 		t.rx.TraceNetListen(&TraceListen{Laddr: l.Addr()})
+	}
 	return &netTraceListener{t, l}, nil
 }
 
@@ -150,7 +172,9 @@ func (tc *traceConn) Write(b []byte) (int, error) {
 	// XXX +TraceNetTxPre ?
 	n, err := tc.Conn.Write(b)
 	if err == nil {
+		if tc.t.enabled() {
 			tc.t.rx.TraceNetTx(&TraceTx{Src: tc.LocalAddr(), Dst: tc.RemoteAddr(), Pkt: b})
 		}
+	}
 	return n, err
 }