Reduce errors on filebeat syslog stop (#8347) (#8377)

Fix a couple of errors seen when syslog input is stopped.

In case the input couldn't be started (e.g. port was already in use),
there was a nil pointer reference error when trying to stop it.

In any case, on stop, an error about use of closed connection was logged
lots of times.

(cherry picked from commit 9b27040)

CHANGELOG.asciidoc

@@ -33,6 +33,8 @@ https://github.com/elastic/beats/compare/v6.4.1...6.4[Check the HEAD diff]
 
 *Filebeat*
 
+- Fix some errors happening when stopping syslog input. {pull}8347[8347]
+
 *Heartbeat*
 
 *Metricbeat*

filebeat/input/syslog/input.go

@@ -174,6 +174,7 @@ func (p *Input) Run() {
 		err := p.server.Start()
 		if err != nil {
 			p.log.Error("Error starting the server", "error", err)
+			return
 		}
 		p.started = true
 	}
@@ -185,6 +186,10 @@ func (p *Input) Stop() {
 	p.Lock()
 	defer p.Unlock()
 
+	if !p.started {
+		return
+	}
+
 	p.log.Info("Stopping Syslog input")
 	p.server.Stop()
 	p.started = false

filebeat/inputsource/udp/server.go

@@ -96,7 +96,15 @@ func (u *Server) run() {
 				continue
 			}
 
-			u.log.Errorw("Error reading from the socket", "error", err)
+			// Closed network error string will never change in Go 1.X
+			// https://github.com/golang/go/issues/4373
+			opErr, ok := err.(*net.OpError)
+			if ok && strings.Contains(opErr.Err.Error(), "use of closed network connection") {
+				u.log.Info("Connection has been closed")
+				return
+			}
+
+			u.log.Errorf("Error reading from the socket %s", err)
 
 			// On Windows send the current buffer and mark it as truncated.
 			// The buffer will have content but length will return 0, addr will be nil.
@@ -115,8 +123,8 @@ func (u *Server) run() {
 // Stop stops the current udp server.
 func (u *Server) Stop() {
 	u.log.Info("Stopping UDP server")
-	u.Listener.Close()
 	close(u.done)
+	u.Listener.Close()
 	u.wg.Wait()
 	u.log.Info("UDP server stopped")
 }