Add optional arguments in Pebble layer schema

The goal is to introduce the concept of optional and
overrideable arguments for a Pebble service, but we
must ensure that the layering properties of Pebble
are preserved and respected. The proposed syntax re-uses
therefore the existing service “command” attribute:

    services:
	myservice:
	    command: myservice --db=/var/db [ --quiet ]

The markers [ and ] are suggested because these are
typical annotations for optional parameters and are
rarely seen naked because they are interpreted by shells.

Author: rebornplusplus

internal/overlord/servstate/handlers.go

@@ -325,11 +325,9 @@ func logError(err error) {
 // command. It assumes the caller has ensures the service is in a valid state,
 // and it sets s.cmd and other relevant fields.
 func (s *serviceData) startInternal() error {
-	args, err := shlex.Split(s.config.Command)
+	args, err := s.config.GetCommand()
 	if err != nil {
-		// Shouldn't happen as it should have failed on parsing, but
-		// it does not hurt to double check and report.
-		return fmt.Errorf("cannot parse service command: %s", err)
+		return err
 	}
 	s.cmd = exec.Command(args[0], args[1:]...)
 	s.cmd.SysProcAttr = &syscall.SysProcAttr{Setpgid: true}
@@ -386,7 +384,7 @@ func (s *serviceData) startInternal() error {
 	s.cmd.Stderr = logWriter
 
 	// Start the process!
-	logger.Noticef("Service %q starting: %s", serviceName, s.config.Command)
+	logger.Noticef("Service %q starting: %q", serviceName, args)
 	err = reaper.StartCommand(s.cmd)
 	if err != nil {
 		if outputIterator != nil {

internal/plan/plan.go

@@ -65,6 +65,7 @@ type Service struct {
 	Startup     ServiceStartup `yaml:"startup,omitempty"`
 	Override    Override       `yaml:"override,omitempty"`
 	Command     string         `yaml:"command,omitempty"`
+	cmdArgs     []string
 
 	// Service dependencies
 	After    []string `yaml:"after,omitempty"`
@@ -90,6 +91,7 @@ type Service struct {
 // Copy returns a deep copy of the service.
 func (s *Service) Copy() *Service {
 	copied := *s
+	copied.cmdArgs = append([]string(nil), s.cmdArgs...)
 	copied.After = append([]string(nil), s.After...)
 	copied.Before = append([]string(nil), s.Before...)
 	copied.Requires = append([]string(nil), s.Requires...)
@@ -129,6 +131,7 @@ func (s *Service) Merge(other *Service) {
 	}
 	if other.Command != "" {
 		s.Command = other.Command
+		s.cmdArgs = append([]string(nil), other.cmdArgs...)
 	}
 	if other.UserID != nil {
 		userID := *other.UserID
@@ -184,6 +187,53 @@ func (s *Service) Equal(other *Service) bool {
 	return reflect.DeepEqual(s, other)
 }
 
+// Returns the command as a stream of strings.
+// Filters "[", "]" (if present, denoting optional arguments).
+func (s *Service) GetCommand() ([]string, error) {
+	args, err := shlex.Split(s.Command)
+	if err != nil {
+		return nil, fmt.Errorf("cannot parse service %q command: %s", s.Name, err)
+	}
+	fargs := make([]string, 0)
+	for _, arg := range args {
+		if arg == "[" || arg == "]" {
+			if len(s.cmdArgs) > 0 {
+				break
+			} else {
+				continue
+			}
+		}
+		fargs = append(fargs, arg)
+	}
+	for _, arg := range s.cmdArgs {
+		fargs = append(fargs, arg)
+	}
+	return fargs, nil
+}
+
+func (s *Service) checkCommand() error {
+	args, err := shlex.Split(s.Command)
+	if err != nil {
+		return err
+	}
+	leftCnt := 0
+	rightCnt := 0
+	for _, arg := range args {
+		if arg == "[" {
+			leftCnt++
+		}
+		if arg == "]" {
+			rightCnt++
+		}
+	}
+	if leftCnt > 0 || rightCnt > 0 {
+		if leftCnt != 1 || rightCnt != 1 || args[len(args)-1] != "]" {
+			return fmt.Errorf("bad syntax regarding optional/overridable arguments")
+		}
+	}
+	return nil
+}
+
 type ServiceStartup string
 
 const (
@@ -483,7 +533,7 @@ func CombineLayers(layers ...*Layer) (*Layer, error) {
 				Message: fmt.Sprintf(`plan must define "command" for service %q`, name),
 			}
 		}
-		_, err := shlex.Split(service.Command)
+		err := service.checkCommand()
 		if err != nil {
 			return nil, &FormatError{
 				Message: fmt.Sprintf("plan service %q command invalid: %v", name, err),