cmd/stdiscosrv: Generate keys if missing on startup (fixes #3511)

cmd/stdiscosrv/main.go

@@ -14,6 +14,7 @@ import (
 	"time"
 
 	"github.com/syncthing/syncthing/lib/protocol"
+	"github.com/syncthing/syncthing/lib/tlsutil"
 	"github.com/thejerf/suture"
 )
 
@@ -86,7 +87,11 @@ func main() {
 	if !useHTTP {
 		cert, err = tls.LoadX509KeyPair(certFile, keyFile)
 		if err != nil {
-			log.Fatalln("Failed to load X509 key pair:", err)
+			log.Println("Failed to load keypair. Generating one, this might take a while...")
+			cert, err = tlsutil.NewCertificate(certFile, keyFile, "stdiscosrv", 3072)
+			if err != nil {
+				log.Fatalln("Failed to generate X509 key pair:", err)
+			}
 		}
 
 		devID := protocol.NewDeviceID(cert.Certificate[0])