Go Deadlock: Why Does 'throw: all goroutines are asleep' Happen?

Deadlock in Go: "throw: all goroutines are asleep"

In a Go program, a deadlock occurs when two or more goroutines wait for each other to finish, resulting in a frozen state where no progress can be made. This issue is often reported as "throw: all goroutines are asleep - deadlock!"

Let's analyze a simplified Go program to understand why this deadlock occurs:

package main

import (
    "fmt"
)

func total(ch chan int) {
    res := 0
    for iter := range ch {
        res += iter
    }
    ch <- res
}

func main() {
    ch := make(chan int)
    go total(ch)
    ch <- 1
    ch <- 2
    ch <- 3
    fmt.Println("Total is ", <-ch)
}

In this program, the total function calculates the sum of the numbers sent via the ch channel and sends the result back on the same channel. The deadlock arises because the following conditions are met:

• The total function blocks inside the for loop waiting for more input via ch (with the range keyword).
• The main function sends three values to ch but does not close the channel.
• Since the total function is still waiting for more input, it does not send the result back on ch.
• The main function blocks at the fmt.Println line waiting for the result from ch.

This creates a deadlock situation where both goroutines (the one running total and the one in main) are waiting for the other to act, leading to the "throw: all goroutines are asleep" error.

To resolve this deadlock, we can close the ch channel in the main function after sending the last value:

ch <- 3
close(ch)

Closing the channel signals to the total goroutine that there is no more input, allowing it to complete its calculation and send the result back on ch.

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