fix: add a build.lock file to gate the buildengine

.gitignore

@@ -24,3 +24,4 @@ buildengine/.gitignore
 go.work*
 junit*.xml
 /readme-tests
+_ftl/

buildengine/engine.go

@@ -6,6 +6,7 @@ import (
 	"crypto/sha256"
 	"errors"
 	"fmt"
+	"os"
 	"path/filepath"
 	"runtime"
 	"strings"
@@ -22,6 +23,7 @@ import (
 	"github.com/TBD54566975/ftl/backend/protos/xyz/block/ftl/v1/ftlv1connect"
 	"github.com/TBD54566975/ftl/backend/schema"
 	"github.com/TBD54566975/ftl/common/projectconfig"
+	"github.com/TBD54566975/ftl/internal"
 	"github.com/TBD54566975/ftl/internal/exec"
 	"github.com/TBD54566975/ftl/internal/log"
 	"github.com/TBD54566975/ftl/internal/rpc"
@@ -83,6 +85,13 @@ func WithListener(listener Listener) Option {
 //
 // "dirs" are directories to scan for local modules.
 func New(ctx context.Context, client ftlv1connect.ControllerServiceClient, moduleDirs []string, externalDirs []string, options ...Option) (*Engine, error) {
+	logger := log.FromContext(ctx)
+	lockFile, err := createLockFile()
+	if err != nil {
+		return nil, err
+	}
+	logger.Infof("Created build.lock file at %s", lockFile)
+
 	ctx = rpc.ContextWithClient(ctx, client)
 	e := &Engine{
 		client:           client,
@@ -161,12 +170,37 @@ func (e *Engine) startSchemaSync(ctx context.Context) func(ctx context.Context,
 	}
 }
 
-// Close stops the Engine's schema sync.
+// Close stops the Engine's schema sync and deletes the build lock file.
 func (e *Engine) Close() error {
+	if err := deleteLockFile(); err != nil {
+		fmt.Println("deleteLockFile: %w", err)
+	}
 	e.cancel()
 	return nil
 }
 
+func createLockFile() (string, error) {
+	lockFile := filepath.Join(internal.GitRoot(""), "_ftl", "build.lock")
+	if err := os.MkdirAll(filepath.Dir(lockFile), 0750); err != nil {
+		return "", fmt.Errorf("failed to create directory for build.lock file: %w", err)
+	}
+	if _, err := os.Stat(lockFile); err == nil || !errors.Is(err, os.ErrNotExist) {
+		return "", fmt.Errorf("build.lock file already exists at: %s", lockFile)
+	}
+	if err := os.WriteFile(lockFile, []byte(""), 0640); err != nil {
+		return "", fmt.Errorf("failed to write build.lock file: %w", err)
+	}
+	return lockFile, nil
+}
+
+func deleteLockFile() error {
+	lockFile := filepath.Join(internal.GitRoot(""), "_ftl", "build.lock")
+	if err := os.Remove(lockFile); err != nil {
+		return fmt.Errorf("failed to remove build.lock file: %w", err)
+	}
+	return nil
+}
+
 // Graph returns the dependency graph for the given modules.
 //
 // If no modules are provided, the entire graph is returned. An error is returned if

cmd/ftl/cmd_build.go

@@ -29,5 +29,6 @@ func (b *buildCmd) Run(ctx context.Context, projConfig projectconfig.Config) err
 	if err != nil {
 		return err
 	}
+	defer engine.Close()
 	return engine.Build(ctx)
 }

cmd/ftl/cmd_deploy.go

@@ -21,5 +21,6 @@ func (d *deployCmd) Run(ctx context.Context) error {
 	if err != nil {
 		return err
 	}
+	defer engine.Close()
 	return engine.Deploy(ctx, d.Replicas, !d.NoWait)
 }

cmd/ftl/cmd_dev.go

@@ -3,6 +3,10 @@ package main
 import (
 	"context"
 	"errors"
+	"fmt"
+	"os"
+	"os/signal"
+	"syscall"
 	"time"
 
 	"golang.org/x/sync/errgroup"
@@ -58,6 +62,9 @@ func (d *devCmd) Run(ctx context.Context, projConfig projectconfig.Config) error
 		g.Go(func() error { return d.ServeCmd.Run(ctx) })
 	}
 
+	sigs := make(chan os.Signal, 1)
+	signal.Notify(sigs, syscall.SIGINT, syscall.SIGTERM)
+
 	g.Go(func() error {
 		err := waitForControllerOnline(ctx, d.ServeCmd.StartupTimeout, client)
 		if err != nil {
@@ -78,6 +85,15 @@ func (d *devCmd) Run(ctx context.Context, projConfig projectconfig.Config) error
 		if err != nil {
 			return err
 		}
+
+		// Close the engine on an interrupt signal.
+		go func() {
+			<-sigs
+			err = engine.Close()
+			fmt.Println("Interrupted, closing engine.")
+		}()
+
+		defer engine.Close()
 		return engine.Dev(ctx, d.Watch, projConfig.Commands)
 	})