Handle context canceled errors + better http proxy error handling (#644)

proxy/http_handler.go

@@ -1,12 +1,19 @@
 package proxy
 
 import (
+	"context"
+	"io"
+	"log"
+	"net"
 	"net/http"
 	"net/http/httputil"
 	"net/url"
 	"time"
 )
 
+// StatusClientClosedRequest non-standard HTTP status code for client disconnection
+const StatusClientClosedRequest = 499
+
 func newHTTPProxy(target *url.URL, tr http.RoundTripper, flush time.Duration) http.Handler {
 	return &httputil.ReverseProxy{
 		// this is a simplified director function based on the
@@ -25,5 +32,35 @@ func newHTTPProxy(target *url.URL, tr http.RoundTripper, flush time.Duration) ht
 		},
 		FlushInterval: flush,
 		Transport:     tr,
+		ErrorHandler:  httpProxyErrorHandler,
 	}
 }
+
+func httpProxyErrorHandler(w http.ResponseWriter, r *http.Request, err error) {
+	// According to https://golang.org/src/net/http/httputil/reverseproxy.go#L74, Go will return a 502 (Bad Gateway) StatusCode by default if no ErroHandler is provided
+	// If a "context canceled" error is returned by the http.Request handler this means the client closed the connection before getting a response
+	// So we are changing the StatusCode on these situations to the non-standard 499 (Client Closed Request)
+
+	statusCode := http.StatusInternalServerError
+
+	if e, ok := err.(net.Error); ok {
+		if e.Timeout() {
+			statusCode = http.StatusGatewayTimeout
+		} else {
+			statusCode = http.StatusBadGateway
+		}
+	} else if err == io.EOF {
+		statusCode = http.StatusBadGateway
+	} else if err == context.Canceled {
+		statusCode = StatusClientClosedRequest
+	}
+
+	w.WriteHeader(statusCode)
+	// Theres nothing we can do if the client closes the connection and logging the "context canceled" errors will just add noise to the error log
+	// Note: The access_log will still log the 499 response status codes
+	if statusCode != StatusClientClosedRequest {
+		log.Print("[ERROR] ", err)
+	}
+
+	return
+}