fix: return error if ftl serve is already running (#1040)

![Screenshot 2024-03-07 at 8 59
04 AM](https://github.com/TBD54566975/ftl/assets/51647/1eacd3ef-bdd0-4e20-a225-bf6d03557943)

cmd/ftl/cmd_dev.go

@@ -2,6 +2,7 @@ package main
 
 import (
 	"context"
+	"errors"
 	"time"
 
 	"golang.org/x/sync/errgroup"
@@ -20,25 +21,28 @@ type devCmd struct {
 
 func (d *devCmd) Run(ctx context.Context) error {
 	client := rpc.ClientFromContext[ftlv1connect.ControllerServiceClient](ctx)
-	engine, err := buildengine.New(ctx, client, d.Dirs...)
-	if err != nil {
-		return err
-	}
 
 	g, ctx := errgroup.WithContext(ctx)
 
 	if !d.NoServe {
+		if d.ServeCmd.isRunning(ctx, client) {
+			return errors.New("FTL is already running")
+		}
 		g.Go(func() error {
 			return d.ServeCmd.Run(ctx)
 		})
 	}
 
-	err = d.ServeCmd.pollControllerOnine(ctx, client)
+	err := d.ServeCmd.pollControllerOnine(ctx, client)
 	if err != nil {
 		return err
 	}
 
 	g.Go(func() error {
+		engine, err := buildengine.New(ctx, client, d.Dirs...)
+		if err != nil {
+			return err
+		}
 		return engine.Dev(ctx, d.Watch)
 	})
 

cmd/ftl/cmd_serve.go

@@ -70,6 +70,10 @@ func (s *serveCmd) Run(ctx context.Context) error {
 		return killBackgroundProcess(logger)
 	}
 
+	if s.isRunning(ctx, client) {
+		return errors.New("FTL is already running")
+	}
+
 	logger.Infof("Starting FTL with %d controller(s) and %d runner(s)", s.Controllers, s.Runners)
 
 	dsn, err := s.setupDB(ctx)
@@ -179,16 +183,16 @@ func killBackgroundProcess(logger *log.Logger) error {
 		return nil
 	}
 
+	if err := os.Remove(pidFilePath); err != nil {
+		logger.Errorf(err, "Failed to remove pid file: %v", err)
+	}
+
 	if err := syscall.Kill(pid, syscall.SIGTERM); err != nil {
 		if !errors.Is(err, syscall.ESRCH) {
 			return err
 		}
 	}
 
-	if err := os.Remove(pidFilePath); err != nil {
-		logger.Errorf(err, "Failed to remove pid file: %v", err)
-	}
-
 	logger.Infof("`ftl serve` stopped (pid: %d)", pid)
 	return nil
 }
@@ -377,3 +381,8 @@ func (s *serveCmd) pollControllerOnine(ctx context.Context, client ftlv1connect.
 		}
 	}
 }
+
+func (s *serveCmd) isRunning(ctx context.Context, client ftlv1connect.ControllerServiceClient) bool {
+	_, err := client.Ping(ctx, connect.NewRequest(&ftlv1.PingRequest{}))
+	return err == nil
+}