main/config: Prevent to load syncthing along with new config file (fixes

 syncthing#4921)

cmd/syncthing/main.go

@@ -741,6 +741,12 @@ func syncthingMain(runtimeOptions RuntimeOptions) {
 	curParts := strings.Split(Version, "-")
 	if prevParts[0] != curParts[0] {
 		if prevVersion != "" {
+			//Check Syncthing version is wheather downgrading
+			l.Infof("Current Syncthing %s, Loadind %s", prevVersion, Version)
+			if upgrade.CompareVersions(curParts[0], prevParts[0]) == upgrade.Older || upgrade.CompareVersions(curParts[0], prevParts[0]) == upgrade.MajorOlder {
+				l.Infof(`Syncthing Version Conflict.
+						Current Syncthing version: %s, Loading config version: %s: `, prevVersion, Version)
+			}
 			l.Infoln("Detected upgrade from", prevVersion, "to", Version)
 		}
 
@@ -965,6 +971,12 @@ func loadConfigAtStartup() *config.Wrapper {
 	}
 
 	if cfg.RawCopy().OriginalVersion != config.CurrentVersion {
+		//check config version
+		if cfg.RawCopy().OriginalVersion > config.CurrentVersion {
+			l.Fatalf("Current config version: %d, Loading config version: %d (Config Version Conflict)", cfg.RawCopy().OriginalVersion, config.CurrentVersion)
+			//Config version is not compatible with installed version
+			os.Exit(exitError)
+		}
 		err = archiveAndSaveConfig(cfg)
 		if err != nil {
 			l.Fatalln("Config archive:", err)