feat: add 'watcher' interface to file sync

core/pkg/sync/builder/syncbuilder.go

@@ -5,7 +5,6 @@ import (
 	"net/http"
 	"os"
 	"regexp"
-	msync "sync"
 	"time"
 
 	"github.com/open-feature/flagd/core/pkg/logger"
@@ -26,6 +25,8 @@ import (
 
 const (
 	syncProviderFile       = "file"
+	syncProviderFsNotify   = "fsnotify"
+	syncProviderFileInfo   = "fileinfo"
 	syncProviderGrpc       = "grpc"
 	syncProviderKubernetes = "kubernetes"
 	syncProviderHTTP       = "http"
@@ -91,8 +92,13 @@ func (sb *SyncBuilder) SyncsFromConfig(sourceConfigs []sync.SourceConfig, logger
 func (sb *SyncBuilder) syncFromConfig(sourceConfig sync.SourceConfig, logger *logger.Logger) (sync.ISync, error) {
 	switch sourceConfig.Provider {
 	case syncProviderFile:
-		logger.Debug(fmt.Sprintf("using filepath sync-provider for: %q", sourceConfig.URI))
 		return sb.newFile(sourceConfig.URI, logger), nil
+	case syncProviderFsNotify:
+		logger.Debug(fmt.Sprintf("using fsnotify sync-provider for: %q", sourceConfig.URI))
+		return sb.newFsNotify(sourceConfig.URI, logger), nil
+	case syncProviderFileInfo:
+		logger.Debug(fmt.Sprintf("using fileinfo sync-provider for: %q", sourceConfig.URI))
+		return sb.newFileInfo(sourceConfig.URI, logger), nil
 	case syncProviderKubernetes:
 		logger.Debug(fmt.Sprintf("using kubernetes sync-provider for: %s", sourceConfig.URI))
 		return sb.newK8s(sourceConfig.URI, logger)
@@ -107,20 +113,45 @@ func (sb *SyncBuilder) syncFromConfig(sourceConfig sync.SourceConfig, logger *lo
 		return sb.newGcs(sourceConfig, logger), nil
 
 	default:
-		return nil, fmt.Errorf("invalid sync provider: %s, must be one of with '%s', '%s', '%s' or '%s'",
-			sourceConfig.Provider, syncProviderFile, syncProviderKubernetes, syncProviderHTTP, syncProviderKubernetes)
+		return nil, fmt.Errorf("invalid sync provider: %s, must be one of with '%s', '%s', '%s', %s', '%s' or '%s'",
+			sourceConfig.Provider, syncProviderFile, syncProviderFsNotify, syncProviderFileInfo, syncProviderKubernetes, syncProviderHTTP, syncProviderKubernetes)
 	}
 }
 
+// newFile returns an fsinfo sync if we are in k8s or fileinfo if not
 func (sb *SyncBuilder) newFile(uri string, logger *logger.Logger) *file.Sync {
-	return &file.Sync{
-		URI: regFile.ReplaceAllString(uri, ""),
-		Logger: logger.WithFields(
+	switch os.Getenv("KUBERNETES_SERVICE_HOST") {
+	case "":
+		// no k8s service host env; use fileinfo
+		return sb.newFileInfo(uri, logger)
+	default:
+		// default to fsnotify
+		return sb.newFsNotify(uri, logger)
+	}
+}
+
+// return a new file.Sync that uses fsnotify under the hood
+func (sb *SyncBuilder) newFsNotify(uri string, logger *logger.Logger) *file.Sync {
+	return file.NewFileSync(
+		regFile.ReplaceAllString(uri, ""),
+		file.FSNOTIFY,
+		logger.WithFields(
 			zap.String("component", "sync"),
-			zap.String("sync", "filepath"),
+			zap.String("sync", syncProviderFsNotify),
 		),
-		Mux: &msync.RWMutex{},
-	}
+	)
+}
+
+// return a new file.Sync that uses os.Stat/fs.FileInfo under the hood
+func (sb *SyncBuilder) newFileInfo(uri string, logger *logger.Logger) *file.Sync {
+	return file.NewFileSync(
+		regFile.ReplaceAllString(uri, ""),
+		file.FILEINFO,
+		logger.WithFields(
+			zap.String("component", "sync"),
+			zap.String("sync", syncProviderFileInfo),
+		),
+	)
 }
 
 func (sb *SyncBuilder) newK8s(uri string, logger *logger.Logger) (*kubernetes.Sync, error) {

core/pkg/sync/file/fileinfo_watcher.go

@@ -0,0 +1,201 @@
+package file
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"io/fs"
+	"os"
+	"sync"
+	"time"
+
+	"github.com/fsnotify/fsnotify"
+	"github.com/open-feature/flagd/core/pkg/logger"
+)
+
+// Implements file.Watcher using a timer and os.FileInfo
+type fileInfoWatcher struct {
+	// Event Chan
+	evChan chan fsnotify.Event
+	// Errors Chan
+	erChan chan error
+	// logger
+	logger *logger.Logger
+	// Func to wrap os.Stat (injection point for test helpers)
+	statFunc func(string) (fs.FileInfo, error)
+	// thread-safe interface to underlying files we are watching
+	mu      sync.RWMutex
+	watches map[string]fs.FileInfo // filename -> info
+}
+
+// NewFsNotifyWatcher returns a new fsNotifyWatcher
+func NewFileInfoWatcher(ctx context.Context, logger *logger.Logger) Watcher {
+	fiw := &fileInfoWatcher{
+		evChan:   make(chan fsnotify.Event, 32),
+		erChan:   make(chan error, 32),
+		statFunc: getFileInfo,
+		logger:   logger,
+	}
+	fiw.run(ctx, (1 * time.Second))
+	return fiw
+}
+
+// fileInfoWatcher explicitly implements file.Watcher
+var _ Watcher = &fileInfoWatcher{}
+
+// Close calls close on the underlying fsnotify.Watcher
+func (f *fileInfoWatcher) Close() error {
+	// close all channels and exit
+	close(f.evChan)
+	close(f.erChan)
+	return nil
+}
+
+// Add calls Add on the underlying fsnotify.Watcher
+func (f *fileInfoWatcher) Add(name string) error {
+	f.mu.Lock()
+	defer f.mu.Unlock()
+
+	// exit early if name already exists
+	if _, ok := f.watches[name]; ok {
+		return nil
+	}
+
+	info, err := f.statFunc(name)
+	if err != nil {
+		return err
+	}
+
+	f.watches[name] = info
+
+	return nil
+}
+
+// Remove calls Remove on the underlying fsnotify.Watcher
+func (f *fileInfoWatcher) Remove(name string) error {
+	f.mu.Lock()
+	defer f.mu.Unlock()
+
+	// no need to exit early, deleting non-existent key is a no-op
+	delete(f.watches, name)
+
+	return nil
+}
+
+// Watchlist calls watchlist on the underlying fsnotify.Watcher
+func (f *fileInfoWatcher) WatchList() []string {
+	f.mu.RLock()
+	defer f.mu.RUnlock()
+	out := []string{}
+	for name := range f.watches {
+		n := name
+		out = append(out, n)
+	}
+	return out
+}
+
+// Events returns the underlying watcher's Events chan
+func (f *fileInfoWatcher) Events() chan fsnotify.Event {
+	return f.evChan
+}
+
+// Errors returns the underlying watcher's Errors chan
+func (f *fileInfoWatcher) Errors() chan error {
+	return f.erChan
+}
+
+// run is a blocking function that starts the filewatcher's timer thread
+func (f *fileInfoWatcher) run(ctx context.Context, s time.Duration) {
+	// timer thread
+	go func() {
+		// execute update on the configured interval of time
+		ticker := time.NewTicker(s)
+		defer ticker.Stop()
+
+		for {
+			select {
+			case <-ctx.Done():
+				return
+			case <-ticker.C:
+				if err := f.update(); err != nil {
+					f.erChan <- err
+					return
+				}
+			}
+		}
+	}()
+}
+
+func (f *fileInfoWatcher) update() error {
+	f.mu.Lock()
+	defer f.mu.Unlock()
+
+	for path, info := range f.watches {
+		newInfo, err := f.statFunc(path)
+		if err != nil {
+			// if the file isn't there, it must have been removed
+			// fire off a remove event and remove it from the watches
+			if errors.Is(err, os.ErrNotExist) {
+				f.evChan <- fsnotify.Event{
+					Name: path,
+					Op:   fsnotify.Remove,
+				}
+				delete(f.watches, path)
+				continue
+			}
+			return err
+		}
+
+		// if the new stat doesn't match the old stat, figure out what changed
+		if info != newInfo {
+			event := f.generateEvent(path, newInfo)
+			if event != nil {
+				f.evChan <- *event
+			}
+			f.watches[path] = newInfo
+		}
+	}
+	return nil
+}
+
+// generateEvent figures out what changed and generates an fsnotify.Event for it. (if we care)
+// file removal are handled above in the update() method
+func (f *fileInfoWatcher) generateEvent(path string, newInfo fs.FileInfo) *fsnotify.Event {
+	info := f.watches[path]
+	switch {
+	// new mod time is more recent than old mod time, generate a write event
+	case newInfo.ModTime().After(info.ModTime()):
+		return &fsnotify.Event{
+			Name: path,
+			Op:   fsnotify.Write,
+		}
+		// the file modes changed, generate a chmod event
+	case info.Mode() != newInfo.Mode():
+		return &fsnotify.Event{
+			Name: path,
+			Op:   fsnotify.Chmod,
+		}
+	// nothing changed that we care about
+	default:
+		return nil
+	}
+}
+
+// getFileInfo returns the fs.FileInfo for the given path
+func getFileInfo(path string) (fs.FileInfo, error) {
+	f, err := os.Open(path)
+	if err != nil {
+		return nil, fmt.Errorf("error from os.Open(%s): %w", path, err)
+	}
+
+	info, err := f.Stat()
+	if err != nil {
+		return info, fmt.Errorf("error from fs.Stat(%s): %w", path, err)
+	}
+
+	if err := f.Close(); err != nil {
+		return info, fmt.Errorf("err from fs.Close(%s): %w", path, err)
+	}
+
+	return info, nil
+}