support reading systemctl config automatically on baremetal setups (#15066)

this allows for customers to use `mc admin service restart`
directly even when performing RPM, DEB upgrades. Upon such 'restart'
after upgrade MinIO will re-read the /etc/default/minio for any
newer environment variables.

As long as `MINIO_CONFIG_ENV_FILE=/etc/default/minio` is set, this
is honored.

cmd/common-main.go

@@ -526,9 +526,6 @@ func parsEnvEntry(envEntry string) (envKV, error) {
 func minioEnvironFromFile(envConfigFile string) ([]envKV, error) {
 	f, err := os.Open(envConfigFile)
 	if err != nil {
-		if os.IsNotExist(err) { // ignore if file doesn't exist.
-			return nil, nil
-		}
 		return nil, err
 	}
 	defer f.Close()
@@ -624,7 +621,7 @@ func loadEnvVarsFromFiles() {
 
 	if env.IsSet(config.EnvConfigEnvFile) {
 		ekvs, err := minioEnvironFromFile(env.Get(config.EnvConfigEnvFile, ""))
-		if err != nil {
+		if err != nil && !os.IsNotExist(err) {
 			logger.Fatal(err, "Unable to read the config environment file")
 		}
 		for _, ekv := range ekvs {

cmd/server-main.go

@@ -138,6 +138,13 @@ func serverCmdArgs(ctx *cli.Context) []string {
 		logger.FatalIf(err, "Unable to validate passed arguments in %s:%s",
 			config.EnvArgs, os.Getenv(config.EnvArgs))
 	}
+	if v == "" {
+		v, _, _, err = env.LookupEnv(config.EnvVolumes)
+		if err != nil {
+			logger.FatalIf(err, "Unable to validate passed arguments in %s:%s",
+				config.EnvVolumes, os.Getenv(config.EnvVolumes))
+		}
+	}
 	if v == "" {
 		// Fall back to older environment value MINIO_ENDPOINTS
 		v, _, _, err = env.LookupEnv(config.EnvEndpoints)
@@ -422,12 +429,12 @@ func serverMain(ctx *cli.Context) {
 	erasureSelfTest()
 	compressSelfTest()
 
-	// Handle all server command args.
-	serverHandleCmdArgs(ctx)
-
 	// Handle all server environment vars.
 	serverHandleEnvVars()
 
+	// Handle all server command args.
+	serverHandleCmdArgs(ctx)
+
 	// Set node name, only set for distributed setup.
 	globalConsoleSys.SetNodeName(globalLocalNodeName)