refactor: use exit code 137 to signify a lock was unable to be retrieved when calling ps:retire

Closes #5432

plugins/ps/ps.go

@@ -24,6 +24,28 @@ var (
 	}
 )
 
+// RetireLockFailed wraps error to distinguish between a normal error
+// and an error where the retire lock could not be fetched
+type RetireLockFailed struct {
+	Err *error
+}
+
+// ExitCode returns an exit code to use in case this error bubbles
+// up into an os.Exit() call
+func (err *RetireLockFailed) ExitCode() int {
+	return 137
+}
+
+// Error returns a standard non-existent app error
+func (err *RetireLockFailed) Error() string {
+	if err.Err != nil {
+		e := *err.Err
+		return fmt.Sprintf("Failed to acquire ps:retire lock: %s", e.Error())
+	}
+
+	return fmt.Sprintf("Failed to acquire ps:retire lock")
+}
+
 // Formation contains scaling information for a given process type
 type Formation struct {
 	ProcessType string `json:"process_type"`

plugins/ps/subcommands.go

@@ -111,12 +111,12 @@ func CommandRetire(appName string) error {
 	fileLock := flock.New(lockFile)
 	locked, err := fileLock.TryLock()
 	if err != nil {
-		return fmt.Errorf("Failed to acquire ps:retire lock: %s", err)
+		return &RetireLockFailed{&err}
 	}
 	defer fileLock.Unlock()
 
 	if !locked {
-		return fmt.Errorf("Failed to acquire ps:retire lock")
+		return &RetireLockFailed{}
 	}
 
 	common.LogInfo1("Retiring old containers and images")

plugins/scheduler-docker-local/install

@@ -27,6 +27,7 @@ After=docker.service
 Type=oneshot
 User=$DOKKU_SYSTEM_USER
 ExecStart=$DOKKU_PATH ps:retire
+SuccessExitStatus=137
 
 [Install]
 WantedBy=docker.service