Does WaitGroup.Wait() in Go Guarantee Memory Ordering?

WaitGroup.Wait() and Memory Barrier

In Go, WaitGroup.Wait() blocks until the WaitGroup counter reaches zero. Does this imply the existence of a memory barrier?

Explanation

Yes, WaitGroup.Wait() introduces a happens-before relationship, which is a type of memory barrier. This relationship ensures the following ordering:

1. All updates made by goroutines that complete before WaitGroup.Wait() are visible to the main goroutine after WaitGroup.Wait() returns.
2. The main goroutine's check of condition after WaitGroup.Wait() occurs only after all goroutines have completed their checks.

Reasoning

This guarantee is essential to avoid data races. For example, if the main goroutine could check condition before all goroutines had finished, it could potentially read an outdated value. WaitGroup.Wait() ensures that this doesn't happen.

Case with One Item

Even with only one item in the items slice, there is still a happens-before relationship. This is because the runtime maintains a global variable that tracks the number of active goroutines, and WaitGroup.Wait() waits until this count drops to zero.

Conclusion

WaitGroup.Wait() provides a reliable mechanism for synchronizing goroutines and establishing a happens-before relationship. This ensures that updates made by goroutines before WaitGroup.Wait() are visible to the main goroutine after WaitGroup.Wait() returns.

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