Unexpected net/http context cancellation

[jesse....@gmail.com]

According to https://golang.org/pkg/net/http/#Request.Context:

For incoming server requests, the context is canceled when the client's connection closes, the request is canceled (with HTTP/2), or when the ServeHTTP method returns.

My expectation, then, is for the following code to block:

package main

import (
        "fmt"
        "io/ioutil"
        "net/http"
)

func main() {
        s := &http.Server{
                Addr: ":8080",
                Handler: http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
                        ioutil.ReadAll(r.Body)

                        <-r.Context().Done()

                        w.Write([]byte(fmt.Sprintf(
                                "request context done: %+v\n",
                                r.Context().Err(),
                        )))
                }),
        }
        s.ListenAndServe()
}

Instead, it always returns immediately, reporting that the context was canceled:

> printf "POST / HTTP/1.1\r\nHost: localhost\r\nContent-Length: 4\r\n\r\nbody" | nc localhost 8080
HTTP/1.1 200 OK
Date: Mon, 15 Jan 2018 16:01:46 GMT
Content-Length: 39
Content-Type: text/plain; charset=utf-8


request context done: context canceled

Curiously, if I don't ioutil.ReadAll(r.Body), the request blocks as expected.

Am I missing something? My hope was to be able to use the default context to detect client disconnection.

[Axel Wagner]

I assume netcat closes the connection once it gets an EOF from stdin. Thus the server notices the client disconnecting and cancels the context, as advertised.

Try running nc without piping the input in and typing it out on the console.

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[jesse....@gmail.com]

Thank you -- that makes sense. I was initially thrown off by the fact that the connection remains open for netcat to get the full HTTP response.

For posterity, another way to get the desired behaviour is to invoke netcat as follows, noting that it won't exit by itself anymore:

> cat ~/tmp/payload - | nc localhost 8080

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