runtime: fix network timers related crash

Fixes #6103.

R=golang-dev, alex.brainman
CC=golang-dev
https://golang.org/cl/12686045

src/pkg/net/timeout_test.go

@@ -703,3 +703,36 @@ func TestProlongTimeout(t *testing.T) {
 		c.Write(buf[:])
 	}
 }
+
+func TestDeadlineRace(t *testing.T) {
+	switch runtime.GOOS {
+	case "plan9":
+		t.Skipf("skipping test on %q", runtime.GOOS)
+	}
+
+	defer runtime.GOMAXPROCS(runtime.GOMAXPROCS(4))
+	ln := newLocalListener(t)
+	defer ln.Close()
+	c, err := Dial("tcp", ln.Addr().String())
+	if err != nil {
+		t.Fatalf("Dial: %v", err)
+	}
+	defer c.Close()
+	done := make(chan bool)
+	go func() {
+		t := time.NewTicker(2 * time.Microsecond).C
+		for {
+			if err := c.SetDeadline(time.Now().Add(2 * time.Microsecond)); err != nil {
+				break
+			}
+			<-t
+		}
+		done <- true
+	}()
+	var buf [1]byte
+	for i := 0; i < 1024; i++ {
+		c.Read(buf[:]) // ignore possible timeout errors
+	}
+	c.Close()
+	<-done
+}

src/pkg/runtime/netpoll.goc

@@ -113,10 +113,13 @@ func runtime_pollWait(pd *PollDesc, mode int) (err int) {
 	runtime·lock(pd);
 	err = checkerr(pd, mode);
 	if(err == 0) {
-		if(!netpollblock(pd, mode)) {
+		while(!netpollblock(pd, mode)) {
 			err = checkerr(pd, mode);
-			if(err == 0)
+			if(err != 0)
-				runtime·throw("runtime_pollWait: unblocked by ioready");
+				break;
+			// Can happen if timeout has fired and unblocked us,
+			// but before we had a chance to run, timeout has been reset.
+			// Pretend it has not happened and retry.
 		}
 	}
 	runtime·unlock(pd);