Wait in a Loop
We should use Cond.Wait() in a loop like so:
c.L.Lock()
for !condition() {
c.Wait()
}
// make use of condition
c.L.Unlock()From the documentation:
Waitatomically unlocksc.Land suspends execution of the calling goroutine. After later resuming execution,Waitlocksc.Lbefore returning.
The unlocking and suspending are atomic on entrance, but wake-up (resuming) and locking are not atomic on exit
The code for the Wait:
func (c *Cond) Wait() {
c.checker.check()
t := runtime_notifyListAdd(&c.notify)
c.L.Unlock()
runtime_notifyListWait(&c.notify, t)
c.L.Lock()
}As we can see, Wait unlocks c.L, and then waits for notification and then locks c.L again. So the condition may change in some other goroutine between c.L.Unlock() and c.L.Lock(), in particular after runtime_notifyListWait() but before c.L.Lock()
Thus, we must check the condition in the loop to be able to wait again. Because after Wait() returns, we own the lock c.L and can be sure that the condition check in a loop is not racy
Signal Scheduling
sync.Cond does not affect goroutine scheduling priority, and Signal() only makes the waiting goroutine available to be scheduled, but does not force it
For example, suppose we have one goroutine waiting on Wait() and other goroutines blocking on Lock() on the same c.L. Another goroutine currently holding the c.L calls Signal() and then Unlock() on c.L
Then after a Signal() (and Unlock()), every other waiting goroutine then contends (fairly) for the lock. Meaning, we cannot assume that the goroutine waiting on Wait() will be the first to execute