Reimplemnt the logs proxy without hijacking

**What**
- Create an alternative proxy implementation using CloseNotifier and
Flusher

Signed-off-by: Lucas Roesler <roesler.lucas@gmail.com>

gateway/handlers/logs.go

@@ -17,6 +17,74 @@ const upstreamLogsEndpoint = "/system/logs"
 
 // NewLogHandlerFunc creates and http HandlerFunc from the supplied log Requestor.
 func NewLogHandlerFunc(logProvider url.URL) http.HandlerFunc {
+	writeRequestURI := false
+	if _, exists := os.LookupEnv("write_request_uri"); exists {
+		writeRequestURI = exists
+	}
+
+	upstreamLogProviderBase := strings.TrimSuffix(logProvider.String(), "/")
+
+	return func(w http.ResponseWriter, r *http.Request) {
+		ctx := r.Context()
+		if r.Body != nil {
+			defer r.Body.Close()
+		}
+
+		logRequest := buildUpstreamRequest(r, upstreamLogProviderBase, upstreamLogsEndpoint)
+		if logRequest.Body != nil {
+			defer logRequest.Body.Close()
+		}
+
+		cn, ok := w.(http.CloseNotifier)
+		if !ok {
+			log.Println("LogHandler: response is not a CloseNotifier, required for streaming response")
+			http.NotFound(w, r)
+			return
+		}
+
+		wf, ok := w.(writerFlusher)
+		if !ok {
+			log.Println("LogHandler: response is not a Flusher, required for streaming response")
+			http.NotFound(w, r)
+			return
+		}
+
+		if writeRequestURI {
+			log.Printf("LogProxy: proxying request to %s %s\n", logRequest.Host, logRequest.URL.String())
+		}
+
+		ctx, cancel := context.WithCancel(ctx)
+		logRequest = logRequest.WithContext(ctx)
+		defer cancel()
+
+		logResp, err := http.DefaultTransport.RoundTrip(logRequest)
+		if err != nil {
+			log.Printf("LogProxy: forwarding request failed: %s\n", err.Error())
+			http.Error(w, "log request failed", http.StatusInternalServerError)
+			return
+		}
+		defer logResp.Body.Close()
+
+		// watch for connection closures and stream data
+		// connections and contexts should have cancel methods deferred already
+		select {
+		case err := <-copyNotify(&unbufferedWriter{wf}, logResp.Body):
+			if err != nil {
+				log.Printf("LogProxy: error while copy: %s", err.Error())
+				return
+			}
+		case <-cn.CloseNotify():
+			log.Printf("LogProxy: client connection closed")
+			return
+		}
+
+		return
+	}
+}
+
+// NewHijackLogHandlerFunc creates and http HandlerFunc from the supplied log Requestor.
+// This remains in the package for reference purposes, providers should instead use NewLogHandlerFunc
+func NewHijackLogHandlerFunc(logProvider url.URL) http.HandlerFunc {
 
 	writeRequestURI := false
 	if _, exists := os.LookupEnv("write_request_uri"); exists {
@@ -50,8 +118,11 @@ func NewLogHandlerFunc(logProvider url.URL) http.HandlerFunc {
 		}
 
 		defer clientConn.Close()
-		clientConn.SetWriteDeadline(time.Time{}) // allow arbitrary time between log writes
-		buf.Flush()                              // will write the headers and the initial 200 response
+		// A zero value for t means Write will not time out, allowing us to stream the logs while
+		// following even if there is a large gap in logs
+		clientConn.SetWriteDeadline(time.Time{})
+		// flush the headers and the initial 200 status code to the client
+		buf.Flush()
 
 		if writeRequestURI {
 			log.Printf("LogProxy: proxying request to %s %s\n", logRequest.Host, logRequest.URL.String())
@@ -68,6 +139,7 @@ func NewLogHandlerFunc(logProvider url.URL) http.HandlerFunc {
 			buf.Flush()
 			return
 		}
+		// Body is always closeable if err is nil
 		defer logResp.Body.Close()
 
 		// write response headers directly to the buffer per RFC 2616
@@ -97,6 +169,26 @@ func NewLogHandlerFunc(logProvider url.URL) http.HandlerFunc {
 	}
 }
 
+type writerFlusher interface {
+	io.Writer
+	http.Flusher
+}
+
+// unbufferedWriter is an io Writer that immediately flushes the after every call to Write.
+// This can wrap any http.ResponseWriter that also implements Flusher.  This ensures that log
+// lines are immediately sent to the client
+type unbufferedWriter struct {
+	dst writerFlusher
+}
+
+// Write writes to the dst writer and then immediately flushes the writer
+func (u *unbufferedWriter) Write(p []byte) (n int, err error) {
+	n, err = u.dst.Write(p)
+	u.dst.Flush()
+
+	return n, err
+}
+
 func copyNotify(destination io.Writer, source io.Reader) <-chan error {
 	done := make(chan error, 1)
 	go func() {

gateway/handlers/logs_test.go

@@ -48,12 +48,12 @@ func Test_logsProxyDoesNotLeakGoroutinesWhenProviderClosesConnection(t *testing.
 
 	resp, err := http.Get(testSrv.URL + "?name=funcFoo")
 	if err != nil {
-		t.Fatalf("unexpected error sneding log request: %s", err)
+		t.Fatalf("unexpected error sending log request: %s", err)
 	}
 
 	body, err := ioutil.ReadAll(resp.Body)
 	if err != nil {
-		t.Fatalf("unexpected error reading teh response body: %s", err)
+		t.Fatalf("unexpected error reading the response body: %s", err)
 	}
 
 	if string(body) != string(expectedMsg) {