How Can Deadlocks Occur in Go Channels and How to Debug Them?

Go Channels and Deadlocks

In Go, channels are used to communicate between goroutines. However, if channels are not handled properly, deadlocks can occur.

Consider the following code:

<code class="go">func main() {
  c1 := make(chan int)
  c2 := make(chan int)

  go func() {
    for i := range c1 {
      println("G1 got", i)
      c2 <- i
    }
  }()

  go func() {
    for i := range c2 {
      println("G2 got", i)
      c1 <- i
    }
  }()

  c1 <- 1

  time.Sleep(1000000000 * 50)
}</code>

This code prints numbers indefinitely until the main function exits. However, if we send another value to one of the channels from the main function, the program blocks:

<code class="go">func main() {
  c1 := make(chan int)
  c2 := make(chan int)

  go func() {
    for i := range c1 {
      println("G1 got", i)
      c2 <- i
    }
  }()

  go func() {
    for i := range c2 {
      println("G2 got", i)
      c1 <- i
    }
  }()

  c1 <- 1

  time.Sleep(1000000000 * 1)

  c1 <- 2

  time.Sleep(1000000000 * 50)
}</code>

This occurs because the second goroutine never receives the value "2" sent to c1. The reason for this deadlock is that the two goroutines are waiting for each other to send and receive values, creating a circular dependency.

Debugging Deadlocks

To debug deadlocks, several approaches can be used:

• Unix-like OS: Use the kill -6 [pid] command to kill the program and print a stack trace for each goroutine.
• gdb: Attach gdb to the process and examine the stack and variables of the active goroutine.

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