martian: cancel request context when connection is closed

[mmatczuk]

For Martian TCP listener implementation support request context cancelation when downstream connection dies.

[Choraden]

Determining when a connection dies is not a simple task.

For example http.Server starts background read after it consumed all the request body. If the read fails the context will be cancelled.

func (cr *connReader) backgroundRead() {
    n, err := cr.conn.rwc.Read(cr.byteBuf[:])
    cr.lock()
    if n == 1 {
        cr.hasByte = true
    }
    if ne, ok := err.(net.Error); ok && cr.aborted && ne.Timeout() {
        // Ignore this error. It's the expected error from
        // another goroutine calling abortPendingRead.
    } else if err != nil {
        cr.handleReadError(err)
    }
    cr.aborted = false
    cr.inRead = false
    cr.unlock()
    cr.cond.Broadcast()
}

// requestBodyRemains reports whether future calls to Read
// on rc might yield more data.
func requestBodyRemains(rc io.ReadCloser) bool {
    if rc == NoBody {
        return false
    }
    switch v := rc.(type) {
    case *expectContinueReader:
        return requestBodyRemains(v.readCloser)
    case *body:
        return v.bodyRemains()
    default:
        panic("unexpected type " + fmt.Sprintf("%T", rc))
    }
}

[Choraden]

The way it could be implemented is very similar to the http.Server approach. We need:

1. http request body wrapper that would notify about the body being consumed in a round tripper
2. conn wrapper that would have the functionality of a backgroundRead()