golang error: fatal error: all goroutines are asleep deadlock

Question

I want to develop a simple email sender in Go, but I have encountered some problems, This is my actual code:

package main


import (
    "flag"
    "sync"
    "fmt"
)

var logLevel = 0

func sendEmail(try combo){
        fmt.Printf("test send %s %s %s\n", try.to, try.from, try.subject)
}

// where we actually do the work
func work(toSend chan combo, wg *sync.WaitGroup) {
    for send := range toSend {
        sendEmail(send)
    }

    // let the main thread know we're done
    wg.Done()
}

// the basic unit that we pass around
type combo struct {
    to       string
    from     string
    subject  string
    header   string
    body     string
    success bool
}


func main() {

    //defaults variables
    emailsList, smtpList        := "", ""  
    typeConnect, ConnFileName  := "", ""
    delimStart, delimEnd       := "_STARTSUB_", "_ENDSUB_"
    threads, bcc := 5,  1
    skip := 0
    logFile := ""

    // Args parse
    flag.StringVar(&emailsList, "e", "", "load email list (required)") 
    flag.StringVar(&smtpList, "s", "", "load smtp list - (required)")
    flag.IntVar(&bcc, "b", 1, "number of emails sent per connection")
    flag.IntVar(&threads,"t", 2, "run `N` attempts in parallel threads")
    flag.StringVar(&typeConnect, "c", "","direct - send emails directly through smtp\n"+"\tsocks - send emails through smtp via socks5 [requires -f argument]\n"+"\thosts - send emails through smtp via server's ip addresses [requires -f argument]\n")
    flag.StringVar(&ConnFileName, "f", "", "if sending via socks the list should contain socks5 proxies in the following formats\n"+"\tip:port\n"+"\tip:port:user:pass\n")
    flag.StringVar(&delimStart, "q", "_STARTSUB_", "start delimiter for subject.")
    flag.StringVar(&delimEnd, "w", "_ENDSUB_", "end delimiter for subject.")
    flag.IntVar(&skip, "l", 0, "skip first `n` lines of input")
    flag.StringVar(&logFile, "debug", "", "write debug output to `file`")
    flag.IntVar(&logLevel, "d", 0, "set debug `level`")

    flag.Parse()

    var wg sync.WaitGroup // keep track of the workers
    toSend := make(chan combo) // to the workers

    // initialize n workers
    wg.Add(int(threads))
    for i := 0; i < int(threads); i++ {
        go work(toSend, &wg)
    }

    for email := range StreamLines("EMAILS", emailsList, skip) {

        from := "info@testfrom.com"
        subject := "test subject"
        header := "test header"
        body   := "test boady"

        try := combo{email,from, subject, header, body, false}
        toSend <- try
    }

    wg.Wait()
    close(toSend)

    fmt.Println("Send emails Done!")
}

I'm try to use channels for pass email to workers in golang, and return this error:

F:\dev\GoLang\gitlab\EasySend>go run main.go usage.go utils.go  -e emails.txt
test send test@gmail.com info@testfrom.com test subject
test send test@gmail.com info@testfrom.com test subject
test send test@gmail.com info@testfrom.com test subject
test send test@gmail.com info@testfrom.com test subject
test send test@gmail.com info@testfrom.com test subject
test send test@gmail.com info@testfrom.com test subject
test send test@gmail.com info@testfrom.com test subject
fatal error: all goroutines are asleep - deadlock!

goroutine 1 [semacquire]:
sync.runtime_Semacquire(0xc04204c0fc)
        C:/Go/src/runtime/sema.go:56 +0x40
sync.(*WaitGroup).Wait(0xc04204c0f0)
        C:/Go/src/sync/waitgroup.go:131 +0x79
main.main()
        F:/dev/GoLang/gitlab/EasySend/main.go:90 +0x7af

goroutine 18 [chan receive]:
main.work(0xc04203e0c0, 0xc04204c0f0)
        F:/dev/GoLang/gitlab/EasySend/main.go:19 +0x110
created by main.main
        F:/dev/GoLang/gitlab/EasySend/main.go:76 +0x5db

goroutine 19 [chan receive]:
main.work(0xc04203e0c0, 0xc04204c0f0)
        F:/dev/GoLang/gitlab/EasySend/main.go:19 +0x110
created by main.main
        F:/dev/GoLang/gitlab/EasySend/main.go:76 +0x5db
exit status 2

I want to know where I'm wrong? Way return this error

"fatal error: all goroutines are asleep - deadlock!"

Answer 1

The program deadlocks because main is waiting for the goroutines to complete and the goroutines are waiting for work on the channel. To fix the problem, swap the order of these lines in main

wg.Wait()
close(toSend)

to

close(toSend)
wg.Wait()

When the channel is closed, the for loop on the channel in the workers exit and the workers call wg.Done().