Goroutine only works when fmt.Println is executed

Issue

For some reason, when I remove the fmt.Printlns then the code is blocking.
I’ve got no idea why it happens. All I want to do is to implement a simple concurrency limiter…

I’ve never experienced such a weird thing. It’s like that fmt flushes the variables or something and makes it work.

Also, when I use a regular function instead of a goroutine then it works too.

Here’s the following code –

package main

import "fmt"

type ConcurrencyLimit struct {
    active int
    Limit  int
}

func (c *ConcurrencyLimit) Block() {
    for {
        fmt.Println(c.active, c.Limit)
        // If should block
        if c.active == c.Limit {
            continue
        }
        c.active++
        break
    }
}

func (c *ConcurrencyLimit) Decrease() int {
    fmt.Println("decrease")
    if c.active > 0 {
        c.active--
    }
    return c.active
}

func main() {
    c := ConcurrencyLimit{Limit: 1}
    c.Block()
    go func() {
        c.Decrease()
    }()
    c.Block()
}

Clarification: Even though I’ve accepted @kaedys ‘s answer(here) a solution was answered by @Kaveh Shahbazian (here)

Solution

You’re not giving c.Decrease() a chance to run. c.Block() runs an infinite for loop, but it never blocks in that for loop, just calling continue over and over on every iteration. The main thread spins at 100% usage endlessly.

However, when you add an fmt.Print() call, that makes a syscall, which allows the other goroutine to run.

This post has details on how exactly goroutines yield or are pre-empted. Note, however, that it’s slightly out of date, as entering a function now has a random chance to yield that thread to another goroutine, to prevent similar style flooding of threads.

Answered By – Kaedys

Answer Checked By – Jay B. (GoLangFix Admin)

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