找不到Go Lang HTTP连接

Question

I'm a newbie to Go, writing a script to test whether my web-server's are up and mai if not. Everything's working except one niggling issue which is driving me crazy, tried quite a few solutions posted elsewhere.

The issue is when the web server is totally down and does not reply, Go is crashing:

panic: runtime error: invalid memory address or nil pointer dereference
[signal 0xc0000005 code=0x1 addr=0x0 pc=0x40140e]

goroutine 16 [running]:
runtime.panic(0x68b1c0, 0x83bfe2)
        c:/go/src/pkg/runtime/panic.c:279 +0x11f
main.httpGET(0x6f0530, 0x15)
httpcheck.go
:57 +0x32e
main.main()
httpcheck.go
:43 +0x3b

goroutine 19 [finalizer wait]:
runtime.park(0x415bb0, 0x840c38, 0x83eda9)
        c:/go/src/pkg/runtime/proc.c:1369 +0xac
runtime.parkunlock(0x840c38, 0x83eda9)
        c:/go/src/pkg/runtime/proc.c:1385 +0x42
runfinq()
        c:/go/src/pkg/runtime/mgc0.c:2644 +0xdd
runtime.goexit()
        c:/go/src/pkg/runtime/proc.c:1445
exit status 2

Here's what i have till the crash....

import (
    "fmt"
    "io/ioutil"
    "log"
    "net/http"
    "github.com/sendgrid/sendgrid-go"
    "os"
    "time"
    "net"
)

var finishedWaiting bool
var timeout = time.Duration(2) * time.Second
var httpClient = http.DefaultClient

func main() {
    const url = "http://doesnotexist.com"
    httpGET(url)
    log.Println("main is running")

}

func httpGET(url string) {
    log.Println("httpGet is runnning")

    resp, err := httpClient.Get(url)

    if err != nil && !doesFileExist("down") {

    }

    if resp.StatusCode != 200 && !doesFileExist("down") {
        isDown()

    } else if resp.StatusCode == 200 && doesFileExist("down") {
        log.Println("200 response")
        sendmail("up")
        removeDownFile()

    }
}

func sendmail(status string) {
    sg := sendgrid.NewSendGridClient("user", "pass")
    message := sendgrid.NewMail()
    message.AddTo("[e-mail]")
    message.AddToName("WebBot")

    if status == "down" {
        message.SetSubject("Websites Down")
        message.SetText("Sites may be down")
    } else {
        message.SetSubject("Website Up")
        message.SetText("Sites are back Up")
    }
    message.SetFrom("[e-mail]")
    if r := sg.Send(message); r == nil {
        fmt.Println("Email sent!")
    } else {
        fmt.Println(r)
    }
}

//checks if the file name is present
func doesFileExist(name string) bool {
    if _, err := os.Stat(name); err != nil {
        if os.IsNotExist(err) {
            return false
        }
    }
    return true
}

// write down file
func writeDownFile() {
    contents, _ := ioutil.ReadFile("")
    ioutil.WriteFile("down", contents, 0x777)

}

//remove down file if server is back up
func removeDownFile() {
    os.Remove("down")

}

func sleepForTen() {
    log.Println("sleeping for 10.....")
    time.Sleep(10000 * time.Millisecond)
    finishedWaiting = true
    main()

}

func isDown() {
    if finishedWaiting != true {
        sleepForTen()

    } else {
        writeDownFile()
        log.Println("200 response")
        sendmail("down")
    }
}

A nudge in the right direction would be appreciated, to help me gracefully determine if the connection cant be established at all.

Thanks.

Answer 1

If an error occurs in the get you just continue with the program. No wonder resp will be nil later.

That's ofc only a guess, since we can only guess the line numbers.