runtime: add cheap atomic condition variable

testdata/coroutines.go

@@ -1,6 +1,7 @@
 package main
 
 import (
+	"runtime"
 	"sync"
 	"time"
 )
@@ -71,6 +72,8 @@ func main() {
 	startSimpleFunc(emptyFunc)
 
 	time.Sleep(2 * time.Millisecond)
+
+	testCond()
 }
 
 func acquire(m *sync.Mutex) {
@@ -127,3 +130,44 @@ type simpleFunc func()
 
 func emptyFunc() {
 }
+
+func testCond() {
+	var cond runtime.Cond
+	go func() {
+		// Wait for the caller to wait on the cond.
+		time.Sleep(time.Millisecond)
+
+		// Notify the caller.
+		ok := cond.Notify()
+		if !ok {
+			panic("notification not sent")
+		}
+
+		// This notification will be buffered inside the cond.
+		ok = cond.Notify()
+		if !ok {
+			panic("notification not queued")
+		}
+
+		// This notification should fail, since there is already one buffered.
+		ok = cond.Notify()
+		if ok {
+			panic("notification double-sent")
+		}
+	}()
+
+	// Verify that the cond has no pending notifications.
+	ok := cond.Poll()
+	if ok {
+		panic("unexpected early notification")
+	}
+
+	// Wait for the goroutine spawned earlier to send a notification.
+	cond.Wait()
+
+	// The goroutine should have also queued a notification in the cond.
+	ok = cond.Poll()
+	if !ok {
+		panic("missing queued notification")
+	}
+}