Is it Safe to Check a Shared Variable After WaitGroup.Wait() Returns?

WaitGroup.Wait() and Memory Barriers

In a multi-threaded environment where shared variables are accessed, it is essential to enforce synchronization to prevent unexpected outcomes. One such mechanism in Go is the "sync.WaitGroup" package, which facilitates the management of concurrently running goroutines.

The question at hand revolves around the relationship between "WaitGroup.Wait()" and memory barriers within a specific code snippet. In this snippet, multiple goroutines are launched to check a specific condition for a set of items. After all goroutines have completed, the "WaitGroup.Wait()" function is invoked to block the calling goroutine until the wait count reaches zero.

The question arises: is it safe to check the condition of the shared variable "condition" after "WaitGroup.Wait()" returns?

Memory Barriers Dissected

A memory barrier is a hardware instruction that enforces a specific ordering of memory accesses across different threads. It ensures that the effects of memory writes performed before the barrier are visible to subsequent memory reads performed after the barrier.

In the Go language, memory barriers are not explicitly exposed to the programmer. Instead, synchronization primitives like "WaitGroup" and "sync.Mutex" implicitly enforce memory barriers when necessary.

WaitGroup.Wait() and Happens-Before Relationship

The documentation for "WaitGroup.Wait()" states that it blocks until the wait count reaches zero, without explicitly establishing a happens-before relationship. However, internal implementation details reveal that "WaitGroup.Wait()" does indeed establish a happens-before relationship. This relationship means that all memory writes performed before "WaitGroup.Wait()" are guaranteed to be visible to memory reads performed after "WaitGroup.Wait()".

Safety of Condition Check

Based on the happens-before relationship established by "WaitGroup.Wait()", it is safe to check the condition of the shared variable "condition" after "WaitGroup.Wait()" returns. This guarantee ensures that all goroutines have completed their execution, ensuring that the value of "condition" has been modified by at least one goroutine if the condition was met for any of the items.

Race Condition Caveat

It is important to note that the safety of checking "condition" after "WaitGroup.Wait()" only holds if the number of items being processed is greater than one. If the number of items is one, a race condition can occur, where no goroutine modifies "condition" before "WaitGroup.Wait()" is called. Therefore, it is advisable to avoid this scenario by ensuring that the number of items is always greater than one.

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