Let Web server shut down gracefully.

That allows us to cache bridge lines of connections that were still open
while we received the signal to shut down.

main.go

 package main
 
 import (
+	"context"
 	"flag"
 	"io"
 	"log"
@@ -114,23 +115,36 @@ func main() {
 	if err := cache.ReadFromDisk(cacheFile); err != nil {
 		log.Printf("Could not read cache because: %s", err)
 	}
-	// When catching SIGINT or SIGTERM, write our cache to disk before exiting.
-	signalChan := make(chan os.Signal, 1)
-	signal.Notify(signalChan, syscall.SIGINT)
-	signal.Notify(signalChan, syscall.SIGTERM)
+
+	var srv http.Server
+	srv.Addr = addr
+	srv.Handler = NewRouter()
+	log.Printf("Starting service on port %s.", addr)
 	go func() {
-		<-signalChan
-		if err := cache.WriteToDisk(cacheFile); err != nil {
-			log.Printf("Could not write cache because: %s", err)
+		if certFilename != "" && keyFilename != "" {
+			srv.ListenAndServeTLS(certFilename, keyFilename)
+		} else {
+			srv.ListenAndServe()
 		}
-		os.Exit(1)
 	}()
 
-	router := NewRouter()
-	log.Printf("Starting service on port %s.", addr)
-	if certFilename != "" && keyFilename != "" {
-		log.Fatal(http.ListenAndServeTLS(addr, certFilename, keyFilename, router))
-	} else {
-		log.Fatal(http.ListenAndServe(addr, router))
+	signalChan := make(chan os.Signal, 1)
+	signal.Notify(signalChan, syscall.SIGINT)
+	signal.Notify(signalChan, syscall.SIGTERM)
+	log.Printf("Waiting for signal to shut down.")
+	<-signalChan
+
+	log.Printf("Received signal to shut down.")
+	// Give our Web server a maximum of a minute to shut down, and finish
+	// handling open connections.
+	t := time.Now().Add(time.Minute)
+	ctx, cancel := context.WithDeadline(context.Background(), t)
+	defer cancel()
+	if err := srv.Shutdown(ctx); err != nil {
+		log.Printf("Failed to shut down Web server: %s", err)
+	}
+
+	if err := cache.WriteToDisk(cacheFile); err != nil {
+		log.Printf("Failed to write cache to disk: %s", err)
 	}
 }