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//go:build !scheduler.none
package runtime
import (
"internal/task"
"sync/atomic"
"unsafe"
)
// notifiedPlaceholder is a placeholder task which is used to indicate that the condition variable has been notified.
var notifiedPlaceholder task.Task
// Cond is a simplified condition variable, useful for notifying goroutines of interrupts.
type Cond struct {
t *task.Task
}
// Notify sends a notification.
// If the condition variable already has a pending notification, this returns false.
func (c *Cond) Notify() bool {
for {
t := (*task.Task)(atomic.LoadPointer((*unsafe.Pointer)(unsafe.Pointer(&c.t))))
switch t {
case nil:
// Nothing is waiting yet.
// Apply the notification placeholder.
if atomic.CompareAndSwapPointer((*unsafe.Pointer)(unsafe.Pointer(&c.t)), unsafe.Pointer(t), unsafe.Pointer(¬ifiedPlaceholder)) {
return true
}
case ¬ifiedPlaceholder:
// The condition variable has already been notified.
return false
default:
// Unblock the waiting task.
if atomic.CompareAndSwapPointer((*unsafe.Pointer)(unsafe.Pointer(&c.t)), unsafe.Pointer(t), nil) {
scheduleTask(t)
return true
}
}
}
}
// Poll checks for a notification.
// If a notification is found, it is cleared and this returns true.
func (c *Cond) Poll() bool {
for {
t := (*task.Task)(atomic.LoadPointer((*unsafe.Pointer)(unsafe.Pointer(&c.t))))
switch t {
case nil:
// No notifications are present.
return false
case ¬ifiedPlaceholder:
// A notification arrived and there is no waiting goroutine.
// Clear the notification and return.
if atomic.CompareAndSwapPointer((*unsafe.Pointer)(unsafe.Pointer(&c.t)), unsafe.Pointer(t), nil) {
return true
}
default:
// A task is blocked on the condition variable, which means it has not been notified.
return false
}
}
}
// Wait for a notification.
// If the condition variable was previously notified, this returns immediately.
func (c *Cond) Wait() {
cur := task.Current()
for {
t := (*task.Task)(atomic.LoadPointer((*unsafe.Pointer)(unsafe.Pointer(&c.t))))
switch t {
case nil:
// Condition variable has not been notified.
// Block the current task on the condition variable.
if atomic.CompareAndSwapPointer((*unsafe.Pointer)(unsafe.Pointer(&c.t)), nil, unsafe.Pointer(cur)) {
task.Pause()
return
}
case ¬ifiedPlaceholder:
// A notification arrived and there is no waiting goroutine.
// Clear the notification and return.
if atomic.CompareAndSwapPointer((*unsafe.Pointer)(unsafe.Pointer(&c.t)), unsafe.Pointer(t), nil) {
return
}
default:
panic("interrupt.Cond: condition variable in use by another goroutine")
}
}
}
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