===== sync.Cond =====

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

==== What is it used for? ====
  * Implement producer-consumer patterns.
  * Build your own blocking queues or resource pools.
  * Coordinate state changes beyond simple locking.

==== When to use ====
  * You need to wait for "state" changes, not just mutual exclusion.
  * A channel-based solution is not convenient for your design.

==== Example (producer/consumer) ====
<code go>
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
}
</code>

==== Notes / pitfalls ====
  * 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.

==== Related ====
  * [[go:concurrency:mutex|sync.Mutex]]
  * [[go:concurrency:channel|Channel]]

==== Hard words (English) ====
  * **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