federation: Avoid printing context canceled error (#7997)

Golang proactively prints this error
        `http: proxy error: context canceled`

when a request arrived to the current deployment and
redirected to another deployment in a federated setup.

Since this error can confuse users, this commit will
just hide it.

cmd/generic-handlers.go

@@ -746,6 +746,9 @@ func setBucketForwardingHandler(h http.Handler) http.Handler {
 	fwd := handlers.NewForwarder(&handlers.Forwarder{
 		PassHost:     true,
 		RoundTripper: NewCustomHTTPTransport(),
+		Logger: func(err error) {
+			logger.LogIf(context.Background(), err)
+		},
 	})
 	return bucketForwardingHandler{fwd, h}
 }

pkg/handlers/forwarder.go

@@ -1,5 +1,5 @@
 /*
- * MinIO Cloud Storage, (C) 2018 MinIO, Inc.
+ * MinIO Cloud Storage, (C) 2018-2019 MinIO, Inc.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -32,6 +32,7 @@ const defaultFlushInterval = time.Duration(100) * time.Millisecond
 type Forwarder struct {
 	RoundTripper http.RoundTripper
 	PassHost     bool
+	Logger       func(error)
 
 	// internal variables
 	rewriter *headerRewriter
@@ -58,10 +59,20 @@ func (f *Forwarder) ServeHTTP(w http.ResponseWriter, inReq *http.Request) {
 		},
 		Transport:     f.RoundTripper,
 		FlushInterval: defaultFlushInterval,
+		ErrorHandler:  f.customErrHandler,
 	}
 	revproxy.ServeHTTP(w, outReq)
 }
 
+// customErrHandler is originally implemented to avoid having the following error
+//    `http: proxy error: context canceled` printed by Golang
+func (f *Forwarder) customErrHandler(w http.ResponseWriter, r *http.Request, err error) {
+	if f.Logger != nil && err != context.Canceled {
+		f.Logger(err)
+	}
+	w.WriteHeader(http.StatusBadGateway)
+}
+
 func (f *Forwarder) getURLFromRequest(req *http.Request) *url.URL {
 	// If the Request was created by Go via a real HTTP request,  RequestURI will
 	// contain the original query string. If the Request was created in code, RequestURI