runtime: add cheap atomic condition variable

src/runtime/cond_nosched.go

@@ -0,0 +1,38 @@
+// +build scheduler.none
+
+package runtime
+
+import "runtime/interrupt"
+
+// Cond is a simplified condition variable, useful for notifying goroutines of interrupts.
+type Cond struct {
+	notified bool
+}
+
+// Notify sends a notification.
+// If the condition variable already has a pending notification, this returns false.
+func (c *Cond) Notify() bool {
+	i := interrupt.Disable()
+	prev := c.notified
+	c.notified = true
+	interrupt.Restore(i)
+	return !prev
+}
+
+// Poll checks for a notification.
+// If a notification is found, it is cleared and this returns true.
+func (c *Cond) Poll() bool {
+	i := interrupt.Disable()
+	notified := c.notified
+	c.notified = false
+	interrupt.Restore(i)
+	return notified
+}
+
+// Wait for a notification.
+// If the condition variable was previously notified, this returns immediately.
+func (c *Cond) Wait() {
+	for !c.Poll() {
+		waitForEvents()
+	}
+}