runtime: add netpollBreak

The new netpollBreak function can be used to interrupt a blocking netpoll.
This function is not currently used; it will be used by later CLs.

Updates #27707

Change-Id: I5cb936609ba13c3c127ea1368a49194fc58c9f4d
Reviewed-on: https://go-review.googlesource.com/c/go/+/171824
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>

src/runtime/proc_test.go

@@ -981,3 +981,42 @@ func TestPreemptionAfterSyscall(t *testing.T) {
 func TestGetgThreadSwitch(t *testing.T) {
 	runtime.RunGetgThreadSwitchTest()
 }
+
+// TestNetpollBreak tests that netpollBreak can break a netpoll.
+// This test is not particularly safe since the call to netpoll
+// will pick up any stray files that are ready, but it should work
+// OK as long it is not run in parallel.
+func TestNetpollBreak(t *testing.T) {
+	if runtime.GOMAXPROCS(0) == 1 {
+		t.Skip("skipping: GOMAXPROCS=1")
+	}
+
+	// Make sure that netpoll is initialized.
+	time.Sleep(1)
+
+	start := time.Now()
+	c := make(chan bool, 2)
+	go func() {
+		c <- true
+		runtime.Netpoll(10 * time.Second.Nanoseconds())
+		c <- true
+	}()
+	<-c
+	// Loop because the break might get eaten by the scheduler.
+	// Break twice to break both the netpoll we started and the
+	// scheduler netpoll.
+loop:
+	for {
+		runtime.Usleep(100)
+		runtime.NetpollBreak()
+		runtime.NetpollBreak()
+		select {
+		case <-c:
+			break loop
+		default:
+		}
+	}
+	if dur := time.Since(start); dur > 5*time.Second {
+		t.Errorf("netpollBreak did not interrupt netpoll: slept for: %v", dur)
+	}
+}