`sync.Cond` is a condition variable. It lets goroutines wait until a certain condition becomes true, and be signaled when it changes.

• Implement producer-consumer patterns.
• Build your own blocking queues or resource pools.
• Coordinate state changes beyond simple locking.

• You need to wait for “state” changes, not just mutual exclusion.
• A channel-based solution is not convenient for your design.

package main
 
import "sync"
 
type Queue struct {
    mu   sync.Mutex
    cond *sync.Cond
    data []int
}
 
func NewQueue() *Queue {
    q := &Queue{}
    q.cond = sync.NewCond(&q.mu)
    return q
}
 
func (q *Queue) Push(v int) {
    q.mu.Lock()
    q.data = append(q.data, v)
    q.mu.Unlock()
    q.cond.Signal() // wake one waiter
}
 
func (q *Queue) Pop() int {
    q.mu.Lock()
    defer q.mu.Unlock()
 
    for len(q.data) == 0 {
        q.cond.Wait() // unlocks mu, waits, then re-locks mu
    }
 
    v := q.data[0]
    q.data = q.data[1:]
    return v
}

• Always check the condition in a `for` loop (spurious wakeups can happen).
• `Wait()` must be called with the lock held.
• Prefer channels for many common cases; use `Cond` when you need advanced coordination.

• sync.Mutex
• Channel

• condition variable /kənˈdɪʃən ˈveriəbəl/: biến điều kiện
• signal /ˈsɪɡnəl/: đánh thức/báo hiệu
• broadcast /ˈbrɔːdkæst/: đánh thức tất cả (Cond.Broadcast)
• producer-consumer /prəˈduːsər kənˈsuːmər/: mô hình sản xuất-tiêu thụ
• spurious wakeup /ˈspjʊriəs ˈweɪkʌp/: thức dậy “ảo” (không có tín hiệu thật)
• coordinate /koʊˈɔːrdɪneɪt/: điều phối