When to Use sync.Cond vs. Simple Locking: A Problem with sync.Cond and an Alternative Solution

Linda Hamilton
Release: 2024-11-12 00:16:03
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When to Use sync.Cond vs. Simple Locking: A Problem with sync.Cond and an Alternative Solution

Troubleshooting Incorrect sync.Cond Usage

Initial Problem Outline:

An attempt to use sync.Cond results in a race condition, causing an immediate panic due to a deadlock. The developer suspects an issue between locking the Mutex and invoking the condition's Wait method.

Clarification of the Intended Use Case:

Apart from the race condition, the primary goal is to create a synchronization mechanism where multiple goroutines wait for HTTP headers to become available from a long-running downloading goroutine.

Resolution:

  • Reevaluate the Need for sync.Cond: In this case, a simple sync.Mutex would suffice since there is only one writer (the downloader) and multiple readers (goroutines accessing headers).
  • Example with Multiple Readers and One Writer:

    var sharedRsc = make(map[string]interface{})
    func main() {
      var wg sync.WaitGroup
      wg.Add(2)
      m := sync.Mutex{}
      c := sync.NewCond(&m)
      go func() {
          c.L.Lock()
          for len(sharedRsc) == 0 {
              c.Wait()
          }
          fmt.Println(sharedRsc["rsc1"])
          c.L.Unlock()
          wg.Done()
      }()
      go func() {
          c.L.Lock()
          for len(sharedRsc) == 0 {
              c.Wait()
          }
          fmt.Println(sharedRsc["rsc2"])
          c.L.Unlock()
          wg.Done()
      }()
      c.L.Lock()
      sharedRsc["rsc1"] = "foo"
      sharedRsc["rsc2"] = "bar"
      c.Broadcast()
      c.L.Unlock()
      wg.Wait()
    }
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Alternative Solution:

If using channels is feasible in this situation, it is still the preferred method for passing data around.

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