Reopen log files after receiving a SIGHUP signal (#13112)

* Reopen log files after receiving a SIGHUP signal

Implements a system signals listener in log.FileLogWriter.

After receiving a SIGHUP signal from the system, the listener will close the current log file and then open it again.

The listener will finishes after receiving a SIGINT, SIGKILL or SIGTERM signal.

Closes grafana/grafana#2497

* Move the SIGHUP handling to the main file

pkg/cmd/grafana-server/main.go

@@ -96,13 +96,17 @@ func main() {
 
 func listenToSystemSignals(server *GrafanaServerImpl) {
 	signalChan := make(chan os.Signal, 1)
-	ignoreChan := make(chan os.Signal, 1)
+	sighupChan := make(chan os.Signal, 1)
 
-	signal.Notify(ignoreChan, syscall.SIGHUP)
+	signal.Notify(sighupChan, syscall.SIGHUP)
 	signal.Notify(signalChan, os.Interrupt, os.Kill, syscall.SIGTERM)
 
-	select {
-	case sig := <-signalChan:
-		server.Shutdown(fmt.Sprintf("System signal: %s", sig))
+	for {
+		select {
+		case _ = <-sighupChan:
+			log.Reload()
+		case sig := <-signalChan:
+			server.Shutdown(fmt.Sprintf("System signal: %s", sig))
+		}
 	}
 }