Where Should You Place `wg.Add()` in Concurrent Go Programs?

Understanding the Placement of wg.Add()

In concurrent Go programs, the sync.WaitGroup type serves as a synchronization primitive to coordinate the completion of tasks. When working with a WaitGroup, where you place the wg.Add() call is critical for ensuring proper wait behavior.

Consider the following code:

<code class="go">var wg sync.WaitGroup
var v int32 = 0

for i := 0; i < 100; i++ {
    go func() {
        wg.Add(1) // wrong place
        atomic.AddInt32(&v, 1)
        wg.Done()
    }()
}
wg.Wait()
fmt.Println(v)</code>

This code intends to parallelize a series of tasks and increment a shared variable v concurrently. However, the output often results in values less than 100. The issue lies in the placement of wg.Add(1) within the anonymous goroutine.

By placing wg.Add(1) within the goroutine, the main goroutine may resume execution before wg.Done() is called. This could occur because the concurrency model allows both the wg.Add(1) call and wg.Done() call to execute concurrently, potentially leading to premature termination of the wait in the main goroutine.

To resolve this issue, the wg.Add(1) call should be moved outside the goroutine, as shown below:

<code class="go">var wg sync.WaitGroup
var v int32 = 0

for i := 0; i < 100; i++ {
    wg.Add(1)
    go func() {
        atomic.AddInt32(&v, 1)
        wg.Done()
    }()
}
wg.Wait()
fmt.Println(v)</code>

By moving wg.Add(1) outside the goroutine, we ensure that the main goroutine will block on the wg.Wait() call until all goroutines have completed and called wg.Done(). This guarantees that the final value of v is accurate.

As a general practice, it is recommended to always call wg.Add() before launching a goroutine that will call wg.Done(). This ensures proper wait behavior and eliminates any race conditions that may arise.

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