feat: watch blueprint.yaml for update and auto resend

Add a file watcher that re-pushes the blueprint to the server whenever
the blueprint file is modified while Newt is running, matching the
reactive behavior of the Docker event monitor.


Former-commit-id: 025497138d

common.go

@@ -16,6 +16,7 @@ import (
 	"github.com/fosrl/newt/logger"
 	"github.com/fosrl/newt/proxy"
 	"github.com/fosrl/newt/websocket"
+	"github.com/fsnotify/fsnotify"
 	"golang.org/x/net/icmp"
 	"golang.org/x/net/ipv4"
 	"golang.zx2c4.com/wireguard/tun/netstack"
@@ -518,3 +519,61 @@ func sendBlueprint(client *websocket.Client) error {
 
 	return nil
 }
+
+func watchBlueprintFile(ctx context.Context, filePath string, send func() error) {
+	watcher, err := fsnotify.NewWatcher()
+	if err != nil {
+		logger.Error("blueprint watcher: failed to create: %v", err)
+		return
+	}
+	defer watcher.Close()
+
+	if err := watcher.Add(filePath); err != nil {
+		logger.Error("blueprint watcher: failed to watch %s: %v", filePath, err)
+		return
+	}
+
+	logger.Info("Watching blueprint file for changes: %s", filePath)
+
+	var debounce *time.Timer
+	scheduleSend := func() {
+		if debounce != nil {
+			debounce.Stop()
+		}
+		debounce = time.AfterFunc(500*time.Millisecond, func() {
+			logger.Info("Blueprint file changed, resending...")
+			if err := send(); err != nil {
+				logger.Error("blueprint watcher: resend failed: %v", err)
+			}
+		})
+	}
+
+	for {
+		select {
+		case <-ctx.Done():
+			if debounce != nil {
+				debounce.Stop()
+			}
+			return
+		case event, ok := <-watcher.Events:
+			if !ok {
+				return
+			}
+			switch {
+			case event.Has(fsnotify.Write) || event.Has(fsnotify.Create):
+				if event.Has(fsnotify.Create) {
+					_ = watcher.Add(filePath)
+				}
+				scheduleSend()
+			case event.Has(fsnotify.Remove) || event.Has(fsnotify.Rename):
+				_ = watcher.Add(filePath)
+				scheduleSend()
+			}
+		case err, ok := <-watcher.Errors:
+			if !ok {
+				return
+			}
+			logger.Error("blueprint watcher: %v", err)
+		}
+	}
+}