How to wait for all goroutines to finish without using time.Sleep?

This code selects all xml files in the same folder, as the invoked executable and asynchronously applies processing to each result in the callback method (in the example below, just the name of the file is printed out).

How do I avoid using the sleep method to keep the main method from exiting? I have problems wrapping my head around channels (I assume that's what it takes, to synchronize the results) so any help is appreciated!

package main

import (
	"fmt"
	"io/ioutil"
	"path"
	"path/filepath"
	"os"
	"runtime"
	"time"
)

func eachFile(extension string, callback func(file string)) {
	exeDir := filepath.Dir(os.Args[0])
	files, _ := ioutil.ReadDir(exeDir)
	for _, f := range files {
			fileName := f.Name()
			if extension == path.Ext(fileName) {
				go callback(fileName)
			}
	}
}


func main() {
	maxProcs := runtime.NumCPU()
	runtime.GOMAXPROCS(maxProcs)

	eachFile(".xml", func(fileName string) {
				// Custom logic goes in here
				fmt.Println(fileName)
			})

	// This is what i want to get rid of
	time.Sleep(100 * time.Millisecond)
}

You can use sync.WaitGroup. Quoting the linked example:

package main

import (
        "net/http"
        "sync"
)

func main() {
        var wg sync.WaitGroup
        var urls = []string{
                "http://www.golang.org/",
                "http://www.google.com/",
                "http://www.somestupidname.com/",
        }
        for _, url := range urls {
                // Increment the WaitGroup counter.
                wg.Add(1)
                // Launch a goroutine to fetch the URL.
                go func(url string) {
                        // Decrement the counter when the goroutine completes.
                        defer wg.Done()
                        // Fetch the URL.
                        http.Get(url)
                }(url)
        }
        // Wait for all HTTP fetches to complete.
        wg.Wait()
}

WaitGroups are definitely the canonical way to do this. Just for the sake of completeness, though, here's the solution that was commonly used before WaitGroups were introduced. The basic idea is to use a channel to say "I'm done," and have the main goroutine wait until each spawned routine has reported its completion.

func main() {
    c := make(chan struct{}) // We don't need any data to be passed, so use an empty struct
    for i := 0; i < 100; i++ {
        go func() {
            doSomething()
            c <- struct{}{} // signal that the routine has completed
        }()
    }
    
    // Since we spawned 100 routines, receive 100 messages.
    for i := 0; i < 100; i++ {
        <- c
    }
}

sync.WaitGroup can help you here.

package main

import (
	"fmt"
	"sync"
	"time"
)


func wait(seconds int, wg * sync.WaitGroup) {
	defer wg.Done()
	
	time.Sleep(time.Duration(seconds) * time.Second)
	fmt.Println("Slept ", seconds, " seconds ..")
}


func main() {
 	var wg sync.WaitGroup
	
	for i := 0; i <= 5; i++ {
		wg.Add(1)	
		go wait(i, &wg)
	}
	wg.Wait()
}

Although sync.waitGroup (wg) is the canonical way forward, it does require you do at least some of your wg.Add calls before you wg.Wait for all to complete. This may not be feasible for simple things like a web crawler, where you don't know the number of recursive calls beforehand and it takes a while to retrieve the data that drives the wg.Add calls. After all, you need to load and parse the first page before you know the size of the first batch of child pages.

I wrote a solution using channels, avoiding waitGroup in my solution the the Tour of Go - web crawler exercise. Each time one or more go-routines are started, you send the number to the children channel. Each time a go routine is about to complete, you send a 1 to the done channel. When the sum of children equals the sum of done, we are done.

My only remaining concern is the hard-coded size of the the results channel, but that is a (current) Go limitation.

// recursionController is a data structure with three channels to control our Crawl recursion.
// Tried to use sync.waitGroup in a previous version, but I was unhappy with the mandatory sleep.
// The idea is to have three channels, counting the outstanding calls (children), completed calls 
// (done) and results (results).  Once outstanding calls == completed calls we are done (if you are
// sufficiently careful to signal any new children before closing your current one, as you may be the last one).
//
type recursionController struct {
	results  chan string
	children chan int
	done     chan int
}

// instead of instantiating one instance, as we did above, use a more idiomatic Go solution
func NewRecursionController() recursionController {
	// we buffer results to 1000, so we cannot crawl more pages than that.  
	return recursionController{make(chan string, 1000), make(chan int), make(chan int)}
}

// recursionController.Add: convenience function to add children to controller (similar to waitGroup)
func (rc recursionController) Add(children int) {
	rc.children <- children
}

// recursionController.Done: convenience function to remove a child from controller (similar to waitGroup)
func (rc recursionController) Done() {
	rc.done <- 1
}

// recursionController.Wait will wait until all children are done
func (rc recursionController) Wait() {
	fmt.Println("Controller waiting...")
	var children, done int
	for {
		select {
		case childrenDelta := <-rc.children:
			children += childrenDelta
			// fmt.Printf("children found %v total %v\n", childrenDelta, children)
		case <-rc.done:
			done += 1
			// fmt.Println("done found", done)
		default:
			if done > 0 && children == done {
				fmt.Printf("Controller exiting, done = %v, children =  %v\n", done, children)
				close(rc.results)
				return
			}
		}
	}
}

Full source code for the solution

Here is a solution that employs WaitGroup.

First, define 2 utility methods:

package util

import (
	"sync"
)

var allNodesWaitGroup sync.WaitGroup

func GoNode(f func()) {
	allNodesWaitGroup.Add(1)
	go func() {
		defer allNodesWaitGroup.Done()
		f()
	}()
}

func WaitForAllNodes() {
	allNodesWaitGroup.Wait()
}

Then, replace the invocation of callback:

go callback(fileName)

With a call to your utility function:

util.GoNode(func() { callback(fileName) })

Last step, add this line at the end of your main, instead of your sleep. This will make sure the main thread is waiting for all routines to finish before the program can stop.

func main() {
  // ...
  util.WaitForAllNodes()
}