GO language: fatal error: all goroutines are asleep - deadlock

Question

Code below works fine with hard coded JSON data however doesn't work when I read JSON data from a file. I'm getting fatal error: all goroutines are asleep - deadlock error when using sync.WaitGroup.

WORKING EXAMPLE WITH HARD-CODED JSON DATA:

package main

import (
    "bytes"
    "fmt"
    "os/exec"
    "time"
)

func connect(host string) {
    cmd := exec.Command("ssh", host, "uptime")
    var out bytes.Buffer
    cmd.Stdout = &out
    err := cmd.Run()
    if err != nil {
        fmt.Println(err)
    }
    fmt.Printf("%s: %q\n", host, out.String())
    time.Sleep(time.Second * 2)
    fmt.Printf("%s: DONE\n", host)
}

func listener(c chan string) {
    for {
        host := <-c
        go connect(host)
    }
}

func main() {
    hosts := [2]string{"[email protected]", "[email protected]"}
    var c chan string = make(chan string)
    go listener(c)

    for i := 0; i < len(hosts); i++ {
        c <- hosts[i]
    }
    var input string
    fmt.Scanln(&input)
}

OUTPUT:

user@user-VirtualBox:~/go$ go run channel.go
[email protected]: " 09:46:40 up 86 days, 18:16,  0 users,  load average: 5"
[email protected]: " 09:46:40 up 86 days, 17:27,  1 user,  load average: 9"
[email protected]: DONE
[email protected]: DONE

NOT WORKING - EXAMPLE WITH READING JSON DATA FILE:

package main

import (
    "bytes"
    "fmt"
    "os/exec"
    "time"
    "encoding/json"
    "os"
    "sync"
)

func connect(host string) {
    cmd := exec.Command("ssh", host, "uptime")
    var out bytes.Buffer
    cmd.Stdout = &out
    err := cmd.Run()
    if err != nil {
        fmt.Println(err)
    }
    fmt.Printf("%s: %q\n", host, out.String())
    time.Sleep(time.Second * 2)
    fmt.Printf("%s: DONE\n", host)
}

func listener(c chan string) {
    for {
        host := <-c
        go connect(host)
    }
}

type Content struct {
    Username string `json:"username"`
    Ip       string `json:"ip"`
}

func main() {
    var wg sync.WaitGroup

    var source []Content
    var hosts []string
    data := json.NewDecoder(os.Stdin)
    data.Decode(&source)

    for _, value := range source {
        hosts = append(hosts, value.Username + "@" + value.Ip)
    }

    var c chan string = make(chan string)
    go listener(c)

    for i := 0; i < len(hosts); i++ {
        wg.Add(1)
        c <- hosts[i]
        defer wg.Done()
    }

    var input string
    fmt.Scanln(&input)

    wg.Wait()
}

OUTPUT

user@user-VirtualBox:~/go$ go run deploy.go < hosts.txt 
[email protected]: " 09:46:40 up 86 days, 18:16,  0 users,  load average: 5"
[email protected]: " 09:46:40 up 86 days, 17:27,  1 user,  load average: 9"
[email protected] : DONE
[email protected]: DONE
fatal error: all goroutines are asleep - deadlock!

goroutine 1 [semacquire]:
sync.runtime_Semacquire(0xc210000068)
    /usr/lib/go/src/pkg/runtime/sema.goc:199 +0x30
sync.(*WaitGroup).Wait(0xc210047020)
    /usr/lib/go/src/pkg/sync/waitgroup.go:127 +0x14b
main.main()
    /home/user/go/deploy.go:64 +0x45a

goroutine 3 [chan receive]:
main.listener(0xc210038060)
    /home/user/go/deploy.go:28 +0x30
created by main.main
    /home/user/go/deploy.go:53 +0x30b
exit status 2
user@user-VirtualBox:~/go$

HOSTS.TXT

[
   {
      "username":"user1",
      "ip":"111.79.154.111"
   },
   {
      "username":"user2",
      "ip":"111.79.190.222"
   }
]

Answer 1

Go program ends when the main function ends.

From the language specification

Program execution begins by initializing the main package and then invoking the function main. When that function invocation returns, the program exits. It does not wait for other (non-main) goroutines to complete.

Therefore, you need to wait for your goroutines to finish. The common solution for this is to use sync.WaitGroup object.

The simplest possible code to synchronize goroutine:

package main

import "fmt"
import "sync"

var wg sync.WaitGroup // 1

func routine() {
    defer wg.Done() // 3
    fmt.Println("routine finished")
}

func main() {
    wg.Add(1) // 2
    go routine() // *
    wg.Wait() // 4
    fmt.Println("main finished")
}

And for synchronizing multiple goroutines

package main

import "fmt"
import "sync"

var wg sync.WaitGroup // 1

func routine(i int) {
    defer wg.Done() // 3
    fmt.Printf("routine %v finished\n", i)
}

func main() {
    for i := 0; i < 10; i++ {
        wg.Add(1) // 2
        go routine(i) // *
    }
    wg.Wait() // 4
    fmt.Println("main finished")
}

WaitGroup usage in order of execution.

1. Declaration of global variable. Making it global is the easiest way to make it visible to all functions and methods.
2. Increasing the counter. This must be done in main goroutine because there is no guarantee that newly started goroutine will execute before 4 due to memory model guarantees.
3. Decreasing the counter. This must be done at the exit of goroutine. Using deferred call, we make sure that it will be called whenever function ends no matter but no matter how it ends.
4. Waiting for the counter to reach 0. This must be done in main goroutine to prevent program exit.

* The actual parameters are evaluated before starting new gouroutine. Thus it is needed to evaluate them explicitly before wg.Add(1) so the possibly panicking code would not leave increased counter.

Use

param := f(x)
wg.Add(1)
go g(param)

instead of

wg.Add(1)
go g(f(x))

Answer 2

Thanks for the very nice and detailed explanation Grzegorz Żur. One thing that I want to point it out that typically the func that needs to be threaded wont be in main(), so we would have something like this:

package main

import (
    "bufio"
    "fmt"
    "io"
    "io/ioutil"
    "math/rand"
    "os"
    "reflect"
    "regexp"
    "strings"
    "sync"
    "time"
)

var wg sync.WaitGroup    // VERY IMP to declare this globally, other wise one   //would hit "fatal error: all goroutines are asleep - deadlock!"

func doSomething(arg1 arg1Type) {
     // cured cancer
}

func main() {
    r := rand.New(rand.NewSource(time.Now().UnixNano()))
    randTime := r.Intn(10)
    wg.Add(1)    
    go doSomething(randTime)
    wg.Wait()
    fmt.Println("Waiting for all threads to finish")
}

The thing that I want to point it out is that global declaration of wg is very crucial for all threads to finish before main()