Go - runtime error: invalid memory address or nil pointer dereference

I am trying to use Go to make a proxy server which changes certain values in the body of a request to an API however when the request is sent the following panic occurs and the request fails:

2015/05/03 14:17:52 http: panic serving 192.168.1.139:42818: runtime error: invalid memory address or nil pointer dereference
goroutine 72 [running]:
net/http.func·011()
    /usr/lib/go/src/pkg/net/http/server.go:1100 +0xb1
runtime.panic(0x8258ee0, 0x83b3733)
    /usr/lib/go/src/pkg/runtime/panic.c:248 +0x176
main.viewResponse(0x0, 0x1861b470, 0x1860ed01)
    /home/bradley/turbocharger/proxy.go:40 +0xa2
github.com/elazarl/goproxy.FuncRespHandler.Handle(0x82e6480, 0x0, 0x1861b470, 0x1)
    /home/bradley/gopath/src/github.com/elazarl/goproxy/actions.go:35 +0x39
github.com/elazarl/goproxy.func·016(0x0, 0x1861b470, 0x827a768)
    /home/bradley/gopath/src/github.com/elazarl/goproxy/dispatcher.go:279 +0x18f
github.com/elazarl/goproxy.FuncRespHandler.Handle(0x1868c9b0, 0x0, 0x1861b470, 0x4)
    /home/bradley/gopath/src/github.com/elazarl/goproxy/actions.go:35 +0x39
github.com/elazarl/goproxy.(*ProxyHttpServer).filterResponse(0x18682640, 0x0, 0x1861b470, 0x0)
    /home/bradley/gopath/src/github.com/elazarl/goproxy/proxy.go:69 +0x95
github.com/elazarl/goproxy.(*ProxyHttpServer).ServeHTTP(0x18682640, 0xb74dd780, 0x18601260, 0x18600bd0)
    /home/bradley/gopath/src/github.com/elazarl/goproxy/proxy.go:115 +0x39b
net/http.serverHandler.ServeHTTP(0x18681dc0, 0xb74dd780, 0x18601260, 0x18600bd0)
    /usr/lib/go/src/pkg/net/http/server.go:1673 +0x155
net/http.(*conn).serve(0x18628a00)
    /usr/lib/go/src/pkg/net/http/server.go:1174 +0x8c6
created by net/http.(*Server).Serve
    /usr/lib/go/src/pkg/net/http/server.go:1721 +0x2be

Here is the script:

func viewResponse(response *http.Response, ctx *goproxy.ProxyCtx) *http.Response {
    log.Printf("Response: %s", response.Status)

    return response
}

Answers


response is nil.

From the doc: "In case of error, resp will be nil, and ctx.RoundTrip.Error will contain the error." https://github.com/elazarl/goproxy/blob/master/actions.go#L22-L28

So the correct way to implement your response handler is something like this:

func viewResponse(response *http.Response, ctx *goproxy.ProxyCtx) *http.Response {
    if response == nil {
        log.Printf("Error: %s", ctx.RoundTrip.Error)
    } else {
        log.Printf("Response: %s", response.Status)
    }
    return response
}

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