Adds --flush-interval to set the flush interval of proxy handler

[JoshVanL]

This PR adds the flag --flush-interval. This sets the proxy handler to flush request bodies at the given duration. The default is 50ms. If zero, no flushing is done. If negative, it will flush immediately. The proxy handler will ignore the value is a streaming request is recognized and will flush immediately.

/assign @simonswine fixes #135

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[mkinsley]

This looks great, thanks a bunch. 50 msec seems like a reasonable default setting. I'm a bit curious about the comment:

The proxy handler will ignore the value is a streaming request is recognized and will flush immediately.

From what I've observed (the root of the issue), the proxy wasn't ever flushing partial buffers on streaming connections. The streaming response from GET pods watch=true was where things were hanging.

[JoshVanL]

@mkinsley so I'd need to dig into the std library, but seems that in that case, it just wasn't detecting that as a streaming connection. For example, exec is streaming correctly. It looks like it's just not "smart" enough. I could be missing something though, and misunderstanding

[mkinsley]

I took a look: https://golang.org/src/net/http/httputil/reverseproxy.go#L367 . The stdlib is only keying off Content-Type to determine if the response is streaming.

If I'm understanding the HTTP RFC (and assuming HTTP/2) , the absence of a Content-Length header implies streaming. In HTTP 1.1 we could look for transfer-encoding: chunked.

// flushInterval returns the p.FlushInterval value, conditionally
// overriding its value for a specific request/response.
func (p *ReverseProxy) flushInterval(req *http.Request, res *http.Response) time.Duration {
    resCT := res.Header.Get("Content-Type")

    // For Server-Sent Events responses, flush immediately.
    // The MIME type is defined in https://www.w3.org/TR/eventsource/#text-event-stream
    if resCT == "text/event-stream" {
        return -1 // negative means immediately
    }

    // TODO: more specific cases? e.g. res.ContentLength == -1?
    return p.FlushInterval
}

[munnerz]

/cc

[JoshVanL]

/unassign @simonswine /assign @munnerz

[munnerz]

/lgtm