How to Avoid Race Conditions When Using sync.Cond: A Guide to Safe Synchronization in Go

Debugging sync.Cond Usage: Understanding Race Conditions and Synchronization Constructs

Understanding the Problem:

The question arises from a scenario where a race condition occurs between locking a mutex and invoking the sync.Cond.Wait method. This happens when the condition is checked before the lock is acquired, leading to an incorrect assumption that the condition is satisfied.

Correct Use of sync.Cond:

To avoid the race condition, it's crucial to always acquire the lock before checking the condition and calling Wait. This ensures that the goroutine has exclusive access to the shared data before evaluating the condition.

m.Lock()
for !condition {
  c.Wait()
}
// ...
m.Unlock()

Choosing the Right Synchronization Construct:

In cases where you need to wait for the availability of multiple resources from a single source, sync.Cond is a suitable choice. However, if data is only communicated between a single reader and writer, a sync.Mutex might suffice.

Alternative Implementation:

In your specific scenario, where you need to wait for download completion and retrieve HTTP headers, an alternative implementation using channels could work well:

// Channel to signal download completion
downloadComplete := make(chan bool)

// Download goroutine
go func() {
  // Download file and store headers
  ...
  close(downloadComplete)
}()

// Other goroutines
go func() {
  // Wait for download completion
  <-downloadComplete
  // Access HTTP headers
  ...
}()

Conclusion:

By correctly using sync.Cond, acquiring the lock before checking the condition, and choosing the appropriate synchronization construct for your needs, you can effectively avoid race conditions and ensure proper coordination between goroutines.

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