Manager: add -write-config CLI option to create config file and quit

cmd/flamenco-manager/main.go

@@ -37,7 +37,8 @@ import (
 )
 
 var cliArgs struct {
-	version bool
+	version     bool
+	writeConfig bool
 }
 
 func main() {
@@ -66,6 +67,15 @@ func main() {
 		log.Error().Err(err).Msg("loading configuration")
 	}
 
+	if cliArgs.writeConfig {
+		err := configService.Save()
+		if err != nil {
+			log.Error().Err(err).Msg("could not write configuration file")
+			os.Exit(1)
+		}
+		return
+	}
+
 	// TODO: enable TLS via Let's Encrypt.
 	listen := configService.Get().Listen
 	_, port, _ := net.SplitHostPort(listen)
@@ -235,6 +245,8 @@ func parseCliArgs() {
 	flag.BoolVar(&quiet, "quiet", false, "Only log warning-level and worse.")
 	flag.BoolVar(&debug, "debug", false, "Enable debug-level logging.")
 	flag.BoolVar(&trace, "trace", false, "Enable trace-level logging.")
+	flag.BoolVar(&cliArgs.writeConfig, "write-config", false, "Writes configuration to flamenco-manager.yaml, then exits.")
+
 	flag.Parse()
 
 	var logLevel zerolog.Level

internal/manager/config/service.go

@@ -1,5 +1,7 @@
 package config
 
+import "github.com/rs/zerolog/log"
+
 // SPDX-License-Identifier: GPL-3.0-or-later
 
 // Service provides access to Flamenco Manager configuration.
@@ -29,3 +31,15 @@ func (s *Service) ExpandVariables(valueToExpand string, audience VariableAudienc
 func (s *Service) Get() *Conf {
 	return &s.config
 }
+
+// Save writes the in-memory configuration to the config file.
+func (s *Service) Save() error {
+	err := s.config.Write(configFilename)
+	if err != nil {
+		return err
+	}
+
+	// Do the logging here, as our caller doesn't know `configFilename``.
+	log.Info().Str("filename", configFilename).Msg("configuration file written")
+	return nil
+}