Go channel：先写结果，再 close(done)

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          package main
        
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          import (
        
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              "fmt"
        
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              "sync"
        
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          )
        
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          func main() {
        
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              result := 0
        
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              done := make(chan struct{})
        
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              var waiters sync.WaitGroup
        
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              waiters.Add(2)
        
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              for i := 1; i <= 2; i++ {
        
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                  go func(id int) {
        
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                      defer waiters.Done()
        
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                      <-done
        
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                      fmt.Println("waiter", id, "reads", result)
        
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                  }(i)
        
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              }
        
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              result = 42
        
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              close(done)
        
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              waiters.Wait()
        
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          }
        

这是可运行的完整示例；省略的只是解释，不是代码。