engine responds to invalidated dependencies

internal/buildengine/build.go

@@ -17,9 +17,13 @@ import (
 	"github.com/TBD54566975/ftl/internal/schema"
 )
 
+var invalidateDependenciesError = errors.New("dependencies need to be updated")
+
 // Build a module in the given directory given the schema and module config.
 //
 // A lock file is used to ensure that only one build is running at a time.
+//
+// Returns invalidateDependenciesError if the build failed due to a change in dependencies.
 func build(ctx context.Context, plugin languageplugin.LanguagePlugin, projectRootDir string, bctx languageplugin.BuildContext, buildEnv []string, devMode bool) (moduleSchema *schema.Module, deploy []string, err error) {
 	logger := log.FromContext(ctx).Module(bctx.Config.Module).Scope("build")
 	ctx = log.ContextWithLogger(ctx, logger)
@@ -46,6 +50,10 @@ func handleBuildResult(ctx context.Context, c moduleconfig.ModuleConfig, eitherR
 		result = eitherResult.Get()
 	}
 
+	if result.InvalidateDependencies {
+		return nil, nil, invalidateDependenciesError
+	}
+
 	var errs []error
 	for _, e := range result.Errors {
 		if e.Level == builderrors.WARN {

internal/buildengine/engine.go

@@ -633,7 +633,6 @@ func (e *Engine) BuildAndDeploy(ctx context.Context, replicas int32, waitForDepl
 type buildCallback func(ctx context.Context, module Module) error
 
 func (e *Engine) buildWithCallback(ctx context.Context, callback buildCallback, moduleNames ...string) error {
-
 	if len(moduleNames) == 0 {
 		e.moduleMetas.Range(func(name string, meta moduleMeta) bool {
 			moduleNames = append(moduleNames, name)
@@ -820,6 +819,15 @@ func (e *Engine) build(ctx context.Context, moduleName string, builtModules map[
 	}, e.buildEnv, e.devMode)
 	if err != nil {
 		terminal.UpdateModuleState(ctx, moduleName, terminal.BuildStateFailed)
+		if errors.Is(err, invalidateDependenciesError) {
+			go func() {
+				logger := log.FromContext(ctx)
+				err := e.BuildAndDeploy(ctx, 1, true, moduleName)
+				e.reportBuildFailed(err)
+				terminal.UpdateModuleState(ctx, moduleName, terminal.BuildStateFailed)
+				logger.Errorf(err, "Build and deploy failed for module %q", moduleName)
+			}()
+		}
 		return err
 	}
 	// update files to deploy

internal/buildengine/languageplugin/plugin.go

@@ -31,7 +31,7 @@ type BuildResult struct {
 	// Files to deploy, relative to the module config's DeployDir
 	Deploy []string
 
-	// Docs
+	// Whether the module needs to recalculate its dependencies
 	InvalidateDependencies bool
 }