goroutine race condition solution

Issue

I’m trying to understand how can I fix this race condition for below code.

sayHello := func() {
    fmt.Println("Hello from goroutine")
}

go sayHello()
time.Sleep(1)

fmt.Println("Hello, playground")

Expectation: I just want to know whats the best solution, should I use WaitGroup or is there any better solution ?

So I came up with below solution :

var wg sync.WaitGroup
//defer wg.Wait()
sayHello := func() {
    defer wg.Done()
    fmt.Println("Hello from goroutine")
}
wg.Add(1)

go sayHello()
wg.Wait()

fmt.Println("Hello, playground")

But its blocking the main goroutine until code is executed !

As well, if I use defer wg.Wait() the output is different ! https://play.golang.org/p/_xkLb7HvNF8

Race condition I meant where go sayHello() never even gets executed, cause the main func will finish executing before the goroutine even started. Hence it creates a race condition if I try to put a time.Sleep

Solution

There is no a race condition in your code.

First question

But its blocking the main goroutine until code is executed !

You are using right after the sayHello call:

wg.Wait()

This blocks your code and waits until the goroutine will be executed. So, go sayHello() will print "Hello from goroutine" always before "Hello, playground".

See documentation here:

Wait blocks until the WaitGroup counter is zero.

Second question

As well, if I use defer wg.Wait() the output is different !

Yes, in this case wg.Wait() will be executed before exit the main function. This means sayHello() will print "Hello from goroutine" before or after "Hello, playground" – this depends on Go scheduler

See more about defer here

A defer statement pushes a function call onto a list. The list of saved calls is executed after the surrounding function returns. Defer is commonly used to simplify functions that perform various clean-up actions.

Update:

Using WaitGroup is recommended in comparison to another solution using channels. You should use wg.Wait() at the right place to achieve the expected output (still not provided).

Answered By – Nik

Answer Checked By – Willingham (GoLangFix Volunteer)

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