Goroutine launch puzzler

this doesn't

package main
import (
  . "fmt"
  . "net/http"
)

const ADDRESS = ":1024"
const SECURE_ADDRESS = ":1025"

func main() {
  message := "hello world"
  HandleFunc("/hello", func(w ResponseWriter, r *Request) {
    w.Header().Set("Content-Type", "text/plain")
    Fprintf(w, message)
  })

  Spawn(
	func() { ListenAndServe(ADDRESS, nil) },
    func() { ListenAndServeTLS(SECURE_ADDRESS, "cert.pem", "key.pem", nil) },
  )
}

func Spawn(f ...func()) {
  done := make(chan bool)

  for _, s := range f {
    go func() {
      s()
      done <- true
    }()
  }

  for l := len(f); l > 0; l-- {
    <- done
  }
}

this works

package main
import (
  . "fmt"
  . "net/http"
)

const ADDRESS = ":1024"
const SECURE_ADDRESS = ":1025"

func main() {
  message := "hello world"
  HandleFunc("/hello", func(w ResponseWriter, r *Request) {
    w.Header().Set("Content-Type", "text/plain")
    Fprintf(w, message)
  })

  Spawn(
	func() { ListenAndServe(ADDRESS, nil) },
    func() { ListenAndServeTLS(SECURE_ADDRESS, "cert.pem", "key.pem", nil) },
  )
}

func Spawn(http, https func()) {
  done := make(chan bool)

  go func() {
    http()
    done <- true
  }()

  go func() {
    https()
    done <- true
  }()

  <- done
  <- done
}

So, with these two programs the aim is to launch two goroutines and block program exit until they're both completed. In one goroutine we're launching an HTTP server, and in the other an HTTPS server.

In the version which works both servers launch and respond to traffic from a web browser.

In the version which doesn't work, the HTTP server completes as soon as the HTTPS server launches. The HTTPS server then responds to traffic from a web browser.

If I reverse the order of the passed functions, it's the HTTPS server which terminates and the HTTP server which works. So in other words, whichever server is launched first will terminate whilst the second server will run normally.

Any ideas on why the two pieces of code are behaving differently when they're apparently doing the same thing?

This took me a little while to figure out. The issue is that the function doesn't have a closure over the function s.

Try changing the body of the for loop in "This doesn't" to something like this.

  for _, s := range f {
    a := func(fun func()) {
      fun()
      done <- true
    }
    go a(s)
  }

Go doesn't capture external variables.

Here is a better explanation that I found https://code.google.com/p/go-wiki/wiki/CommonMistakes

Edit: Changed i back to _