emissary/internal/agent/controller/persistence/controller.go

package persistence

import (
	"context"
	"encoding/json"
	"fmt"
	"io/ioutil"
	"os"
	"path/filepath"

	"forge.cadoles.com/Cadoles/emissary/internal/agent"
	"github.com/pkg/errors"
	"gitlab.com/wpetit/goweb/logger"
)

type Controller struct {
	previousState *agent.State
	filename      string
}

// Name implements node.Controller.
func (c *Controller) Name() string {
	return "persistence-controller"
}

// Reconcile implements node.Controller.
func (c *Controller) Reconcile(ctx context.Context, state *agent.State) (*agent.State, error) {
	defer func() {
		c.previousState = state
	}()

	switch {
	// If state did not change, return
	case state == c.previousState:
		return state, nil

	// If first cycle, load state from file system
	case c.previousState == nil:
		state, err := c.loadState(ctx)
		if err != nil {
			if errors.Is(err, os.ErrNotExist) {
				return state, nil
			}

			return nil, errors.WithStack(err)
		}

		return state, nil

	// If state has changed, save it
	case state != c.previousState:
		logger.Info(ctx, "saving state", logger.F("stateFile", c.filename))

		if err := c.writeState(ctx, state); err != nil {
			return nil, errors.WithStack(err)
		}

	}

	return state, nil
}

func (c *Controller) loadState(ctx context.Context) (*agent.State, error) {
	data, err := ioutil.ReadFile(c.filename)
	if err != nil {
		return nil, errors.WithStack(err)
	}

	state := agent.NewState()

	if err := json.Unmarshal(data, state); err != nil {
		return nil, errors.WithStack(err)
	}

	return state, nil
}

func (c *Controller) writeState(ctx context.Context, state *agent.State) error {
	dir, file := filepath.Split(c.filename)
	if dir == "" {
		dir = "."
	}

	f, err := ioutil.TempFile(dir, file)
	if err != nil {
		return errors.Errorf("cannot create temp file: %v", err)
	}

	defer func() {
		if err == nil {
			return
		}

		if err := os.Remove(f.Name()); err != nil {
			logger.Error(ctx, "could not remove temporary file", logger.E(errors.WithStack(err)))
		}
	}()

	defer func() {
		if err := f.Close(); err != nil {
			if errors.Is(err, os.ErrClosed) {
				return
			}

			logger.Error(ctx, "could not close temporary file", logger.E(errors.WithStack(err)))
		}
	}()

	data, err := json.Marshal(state)
	if err != nil {
		return errors.WithStack(err)
	}

	name := f.Name()
	if err := ioutil.WriteFile(name, data, os.ModePerm); err != nil {
		return errors.Errorf("cannot write data to temporary file %q: %v", name, err)
	}

	if err := f.Sync(); err != nil {
		return errors.Errorf("can't flush temporary file %q: %v", name, err)
	}

	if err := f.Close(); err != nil {
		return errors.Errorf("can't close temporary file %q: %v", name, err)
	}

	// get the file mode from the original file and use that for the replacement
	// file, too.
	destInfo, err := os.Stat(c.filename)

	switch {
	case os.IsNotExist(err):
		// Do nothing

	case err != nil:
		return errors.WithStack(err)

	default:
		sourceInfo, err := os.Stat(name)
		if err != nil {
			return errors.WithStack(err)
		}

		if sourceInfo.Mode() != destInfo.Mode() {
			if err := os.Chmod(name, destInfo.Mode()); err != nil {
				return fmt.Errorf("can't set filemode on temporary file %q: %v", name, err)
			}
		}
	}

	if err := os.Rename(name, c.filename); err != nil {
		return fmt.Errorf("cannot replace %q with temporary file %q: %v", c.filename, name, err)
	}

	return nil
}

func NewController(filename string) *Controller {
	return &Controller{
		filename: filename,
	}
}

var _ agent.Controller = &Controller{}