WaitGroup.Wait() and Memory Barriers: Clarifying the Guarantees

In Go, the WaitGroup type is a synchronization primitive used to track the completion of a set of goroutines. The question arises: when wg.Wait() is called to wait for all goroutines to finish, does it imply a memory barrier? We will delve into this question and explore the official documentation and related discussions.

The WaitGroup specification and documentation state that WaitGroup.Wait blocks until the counter reaches zero, indicating that all goroutines have completed. However, it does not explicitly mention a memory barrier.

Discussions on the Go forum hinted at the existence of a happens-before relationship between wg.Wait() and wg.Done(). A happens-before relationship ensures that all operations performed before the first event (wg.Wait() in this case) are guaranteed to have completed before any operations performed after the second event (wg.Done()).

In the given example code, the goroutines check whether items meet a condition, and if so, set the condition variable to true. Without a memory barrier, the condition variable may not be updated immediately, leading to potential race conditions.

However, it has been confirmed by Ian Lance Taylor that there is indeed a happens-before relationship between wg.Wait() and wg.Done(). This means that any updates to the condition variable made before calling wg.Done() are guaranteed to be visible to the main goroutine after wg.Wait() returns.

While this clarifies the safe usage of the condition variable, it is important to note that the code is still vulnerable to race conditions if multiple items are being processed. This is because multiple goroutines could potentially set the condition variable to true simultaneously, resulting in an incorrect value.

Therefore, while wg.Wait() does provide a happens-before relationship, it is crucial to use additional synchronization mechanisms, such as mutexes, when multiple goroutines are accessing shared data to prevent data races.