Hi!
It's true that the context canceled message is not very descriptive, but I don't think we need much more. In the ServeHTTP method of ReverseProxy, it's quite clear that the context will only be canceled when the http.CloseNotifier detects that the underlying connection has gone away.

I think, but I'm not sure, that it would be very easy to add a log specifying that the client has gone away:

case <-notifyChan:
        p.logf("connection closed by client") // or "client went away" or something like that
        cancel()

But I still believe it's not necessary.

What do you think?

@matipan That may indeed be the case. I do not know all the ways the http.ResponseWriter may be prematurely closed. There have been several other similar issues related to error reporting in the reverse proxy; #18838 is one example.

@matipan that's definitely not clear why context is canceled. I have chain of 3 proxy servers, and middle server was returning "context canceled". It's not really obvious message.
"connection unexpectedly closed by client" would be much much better.

@bradfitz

Is there any way to silence the message? I'm seeing the pretty frequently and starting to realize that it's probably not harmful -- albeit pretty annoying to see in the logs daily.

I believe https://golang.org/src/net/http/httputil/reverseproxy.go#L234 will report that error since the latest Go minor's versions, one can influence the error logger, trying to match on these context errors.

Hi @HaraldNordgren did you find a way to silence this error? I am trying to silence it, as my logs are all polluted with this error.

Update: i think i found it. Can do something like this:

proxy := &httputil.ReverseProxy{
	Director: director,
	ErrorHandler: func(http.ResponseWriter, *http.Request, error) {
		// handle error
	},
}

@sergodeeva - we do something like the following, which logs for errors that are not cancels and always returns http.StatusBadGateway. This replaces the ReverseProxy#defaultErrorHandler implementation with one that conditionally logs. This version doesn't make use of the ReverseProxy#ErrorLog, but you can certainly alter how logging is performed.

// noContextCancelationErrors suppresses logging of "context canceled" errors while still
// logging other types of errors.
func noContextCancelationErrors(rw http.ResponseWriter, req *http.Request, err error) {
	if err != context.Canceled {
		log.Printf("http: proxy error: %v", err)
	}
	rw.WriteHeader(http.StatusBadGateway)
}

proxy := &httputil.ReverseProxy{
	// ...
	ErrorHandler: noContextCancelationErrors,
}

One simple non-breaking way to indicate the reason would be to use context.WithCancelCause(...). Did you consider this option?
#64465