stages/files: add previousReport to result report

This change notices if a report already exists and accordingly
nest the report in previousReport.
related to #1214

internal/exec/stages/files/filesystemEntries.go

@@ -142,18 +142,48 @@ func (s *stage) createResultFile() error {
 	s.Logger.PushPrefix("createResultFile")
 	defer s.Logger.PopPrefix()
 
+	var prevReport interface{}
+	prevPath, err := s.JoinPath(distro.ResultFilePath())
+	if err != nil {
+		return fmt.Errorf("building previous result file path: %w", err)
+	}
+	prevData, err := ioutil.ReadFile(prevPath)
+	if err != nil && !os.IsNotExist(err) {
+		return fmt.Errorf("reading previous report: %w", err)
+	}
+	if err == nil {
+		// To check if it's a live system
+		cmd := exec.Command("is-lve-image")
+		err := cmd.Run()
+		if err != nil {
+			s.Logger.Notice("failed to run is-live-image hook")
+		}
+		if exitErr, ok := err.(*exec.ExitError); ok {
+			if exitErr.ExitCode() != 0 {
+				s.Logger.Warning("couldn't run is-live-image: %w", string(exitErr.Stderr))
+			}
+		}
+		s.Logger.Warning("Unexpected behavior may occur. Ignition is not designed to run more than once per system")
+		err = json.Unmarshal(prevData, &prevReport)
+		if err != nil {
+			return fmt.Errorf("couldn't unmarshal previous report output: %v", err)
+		}
+	}
+
 	bootIDBytes, err := ioutil.ReadFile(distro.BootIDPath())
 	if err != nil {
 		return fmt.Errorf("reading boot ID: %w", err)
 	}
 
 	result := struct {
-		ProvisioningBootID string `json:"provisioningBootID"`
-		ProvisioningDate   string `json:"provisioningDate"`
-		UserConfigProvided bool   `json:"userConfigProvided"`
+		ProvisioningBootID string      `json:"provisioningBootID"`
+		ProvisioningDate   string      `json:"provisioningDate"`
+		UserConfigProvided bool        `json:"userConfigProvided"`
+		PreviousReport     interface{} `json:"previousReport,omitempty"`
 	}{
 		ProvisioningBootID: strings.TrimSpace(string(bootIDBytes)),
 		ProvisioningDate:   time.Now().Format(time.RFC3339),
+		PreviousReport:     prevReport,
 	}
 	for _, config := range s.State.FetchedConfigs {
 		if config.Kind == "user" {