ThinkerQAQ

NOTE

2.2 sync.WaitGroup

Waiting for a group of goroutines to finish with Add, Done, and Wait, including the counter lifecycle and common Add/Wait misuse.

GoCreated Updated 1 min readhistorical

This is a historical learning note and may contain outdated or incomplete understanding.

1. What Is sync.WaitGroup?

A WaitGroup lets one goroutine wait until a group of tasks has completed.

var wg sync.WaitGroup

wg.Add(2)

go func() {
    defer wg.Done()
    work1()
}()

go func() {
    defer wg.Done()
    work2()
}()

wg.Wait()

2. Mental Model

A WaitGroup maintains a counter:

  • Add(n) changes the counter by n;
  • Done() is equivalent to Add(-1);
  • Wait() blocks until the counter reaches zero.

If the counter becomes negative, the program panics.

3. Correct Lifecycle

When starting a new group from a zero counter, positive Add calls should happen before the corresponding Wait can begin. The classic pattern is therefore:

wg.Add(1)
go func() {
    defer wg.Done()
    // work
}()

Putting Add(1) inside the new goroutine creates a race with Wait: the waiter may observe zero before the goroutine increments the counter.

A WaitGroup may be reused after a previous Wait has returned, but a WaitGroup must not be copied after first use.

4. WaitGroup vs. Context

WaitGroup coordinates completion; it does not cancel work or propagate errors. Combine it with context or a higher-level task-group abstraction when you also need cancellation/error propagation.