rootmodule: avoiding loading module twice at the same time

This would never happen anyway since RootModuleManager is the only caller
and adding new modules is already gated to run only once
but it doesn't hurt to have an extra check.

internal/terraform/rootmodule/root_module.go

@@ -150,13 +150,17 @@ func (rm *rootModule) SetLogger(logger *log.Logger) {
 	rm.logger = logger
 }
 
-func (rm *rootModule) StartLoading() {
+func (rm *rootModule) StartLoading() error {
+	if !rm.IsLoadingDone() {
+		return fmt.Errorf("root module is already being loaded")
+	}
 	ctx, cancelFunc := context.WithCancel(context.Background())
 	rm.cancelLoading = cancelFunc
 
 	go func(ctx context.Context) {
 		rm.setLoadErr(rm.load(ctx))
 	}(ctx)
+	return nil
 }
 
 func (rm *rootModule) CancelLoading() {

internal/terraform/rootmodule/root_module_manager.go

@@ -103,7 +103,10 @@ func (rmm *rootModuleManager) AddAndStartLoadingRootModule(ctx context.Context,
 	}
 
 	rmm.logger.Printf("asynchronously loading root module %s", dir)
-	rm.StartLoading()
+	err = rm.StartLoading()
+	if err != nil {
+		return rm, err
+	}
 
 	return rm, nil
 }

internal/terraform/rootmodule/types.go

@@ -60,7 +60,7 @@ func (rms RootModules) Paths() []string {
 type RootModule interface {
 	Path() string
 	LoadError() error
-	StartLoading()
+	StartLoading() error
 	IsLoadingDone() bool
 	IsKnownPluginLockFile(path string) bool
 	IsKnownModuleManifestFile(path string) bool