code for concurrency has race condition

[hughlunnon]

I'm learning go here, so might be way off the mark on this :)

the code in this block

package concurrency

type WebsiteChecker func(string) bool
type result struct {
	string
	bool
}

func CheckWebsites(wc WebsiteChecker, urls []string) map[string]bool {
	results := make(map[string]bool)
	resultChannel := make(chan result)

	for _, url := range urls {
		go func() {
			resultChannel <- result{url, wc(url)}
		}()
	}

	for i := 0; i < len(urls); i++ {
		r := <-resultChannel
		results[r.string] = r.bool
	}

	return results
}

doesn't pass the initial (correctness) test - all the values within the channel have the last url and url result within them. I actually get a warning in goland about the code

I don't fully understand that message but I think the value of the url is changing before the goroutine has had a chance to execute. There is more info here.

The below code passes all tests

	for _, url := range urls {
		finalUrl := url
		go func() {
			resultChannel <- result{finalUrl, wc(finalUrl)}
		}()
	}

I haven't raised a PR as my code might not be the "correct" answer, but welcome any feedback

[hughlunnon]

ok I did a bit more digging.

It seems this is fixed in go 1.22 but only if you have the go version specified in go.mod (which it is in your codebase, but wasn't in mine). As a test I added

go 1.24

to my project and it worked, but I suspect I'm not the only person who might miss this requirement? The earlier chapter suggests using go 1.16