Reopen logs on SIGHUP

This makes it possible to manage chasquid logs using logrotate.

Amended-by: Alberto Bertogli <albertito@blitiri.com.ar>
  Added tests, minor style and comment changes.

chasquid.go

@@ -14,8 +14,10 @@ import (
 	"math/rand"
 	"net"
 	"os"
+	"os/signal"
 	"path/filepath"
 	"strconv"
+	"syscall"
 	"time"
 
 	"blitiri.com.ar/go/chasquid/internal/config"
@@ -90,6 +92,8 @@ func main() {
 
 	initMailLog(conf.MailLogPath)
 
+	go signalHandler()
+
 	if conf.MonitoringAddress != "" {
 		launchMonitoringServer(conf.MonitoringAddress)
 	}
@@ -224,6 +228,32 @@ func initMailLog(path string) {
 	}
 }
 
+func signalHandler() {
+	var err error
+
+	signals := make(chan os.Signal, 1)
+	signal.Notify(signals, syscall.SIGHUP)
+
+	for {
+		switch sig := <-signals; sig {
+		case syscall.SIGHUP:
+			// SIGHUP triggers a reopen of the log files. This is used for log
+			// rotation.
+			err = log.Default.Reopen()
+			if err != nil {
+				log.Fatalf("Error reopening log: %v", err)
+			}
+
+			err = maillog.Default.Reopen()
+			if err != nil {
+				log.Fatalf("Error reopening maillog: %v", err)
+			}
+		default:
+			log.Errorf("Unexpected signal %v", sig)
+		}
+	}
+}
+
 // Helper to load a single domain configuration into the server.
 func loadDomain(name, dir string, s *smtpsrv.Server) {
 	log.Infof("  %s", name)