Why Does My Go WaitGroup Cause a 'All Goroutines Are Asleep - Deadlock!' Error?

WaitGroup Deadlock: Understanding "All Goroutines Are Asleep - Deadlock!"

When attempting to coordinate goroutines using a WaitGroup, it's possible to encounter the error "fatal error: all goroutines are asleep - deadlock!". This occurs when the goroutines responsible for decrementing the WaitGroup's counter inadvertently end up sleeping.

The Problem:

The following Go code illustrates how this error can arise:

package main

import (
    "fmt"
    "sync"
)

func main() {
    var wg sync.WaitGroup
    for i := 0; i < 10; i++ {
        wg.Add(1)
        go func() {
            defer wg.Done()
            fmt.Println("Hello", i)
        }()
    }
    wg.Wait()
}

Upon execution, this code should print "Hello" for each goroutine. However, instead, the program exits with the dreaded "deadlock" error.

The Reason:

The issue lies in the way the WaitGroup is passed to the goroutines. Go makes a copy of the value when it's passed to the anonymous function. So, each goroutine operates on a separate copy of the WaitGroup, none of which have their counter decremented, resulting in a perpetual sleep of the goroutines.

The Solution:

To resolve the deadlock, a pointer to the WaitGroup must be passed instead of the WaitGroup itself. Thus, all goroutines will reference the same WaitGroup, enabling them to decrement its counter correctly:

package main

import (
    "fmt"
    "sync"
)

func main() {
    wg := &sync.WaitGroup{}
    for i := 0; i < 10; i++ {
        wg.Add(1)
        go func() {
            defer wg.Done()
            fmt.Println("Hello", i)
        }()
    }
    wg.Wait()
}

By passing a pointer, the goroutines share the same WaitGroup object and can successfully coordinate their execution. This version of the code will correctly print "Hello" for each goroutine without triggering the deadlock.

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