why goroutine block main func in this http server?

Question

I want to setup a http server with httprouter listening on two ports 8888 and 8080 just like the code below.

package main

import (
    "fmt"
    "github.com/julienschmidt/httprouter"
    "log"
    "net/http"
)

func Index(w http.ResponseWriter, r *http.Request, _ httprouter.Params) {
    fmt.Fprint(w, "Welcome!\n")
}

func main() {
    router := httprouter.New()
    router.GET("/", Index)

    fmt.Println("listen on 8080")
    // this is where blocked
    go log.Fatal(http.ListenAndServe(":8080", router))
    fmt.Println("listen on 8888")
    log.Fatal(http.ListenAndServe(":8888", router))
}

But it doesn't work properly,my server only listen on 8080 .If I make some change:

go func() { log.Fatal(http.ListenAndServe(":8080", router)) }()

It works finely both on 8080 and 8888 .So why? It's about closure or something else?

Answer 1

The function value and parameters are evaluated as usual in the calling goroutine

— Go language spec, "Go statements" .

You're creating a goroutine for the call to log.Fatal , but the arguments to log.Fatal are evaluated beforehand, in the main goroutine. And Fatal 's argument is the return value of http.ListenAndServe . So the new goroutine doesn't start until after ListenAndServe returns.

Answer 2

Since http.ListenAndServe() is blocking, and in your scenario, there are two of them, then try to put one of them in a goroutine. The idea is to separate the execution of those two web server initialization statements into separate goroutine.

func main() {
    router := httprouter.New()
    router.GET("/", Index)

    go func() {
        fmt.Println("listen on 8080")
        log.Fatal(http.ListenAndServe(":8080", router))
    }()

    fmt.Println("listen on 8888")
    log.Fatal(http.ListenAndServe(":8888", router))
}