Make new output an error if the directory isn't empty (#1810)

This allows us to get rid of the `mkdir <dir> && cd <dir>` instructions in all our tutorials before `pulumi new`, because anyone who runs `pulumi new` in a non-empty directory will be forced to create a new directory in order to proceed.

cmd/new.go

@@ -16,6 +16,7 @@ package cmd
 
 import (
 	"fmt"
+	"io/ioutil"
 	"os"
 	"os/exec"
 	"path/filepath"
@@ -92,6 +93,13 @@ func newNewCmd() *cobra.Command {
 				}
 			}
 
+			// Return an error if the directory isn't empty.
+			if !force {
+				if err = errorIfNotEmptyDirectory(cwd); err != nil {
+					return err
+				}
+			}
+
 			// If we're going to be creating a stack, get the current backend, which
 			// will kick off the login flow (if not already logged-in).
 			var b backend.Backend
@@ -337,6 +345,21 @@ func newNewCmd() *cobra.Command {
 	return cmd
 }
 
+// errorIfNotEmptyDirectory returns an error if path is not empty.
+func errorIfNotEmptyDirectory(path string) error {
+	infos, err := ioutil.ReadDir(path)
+	if err != nil {
+		return err
+	}
+
+	if len(infos) > 0 {
+		return errors.Errorf("%s is not empty; "+
+			"rerun in an empty directory, pass the path to an empty directory to --dir, or use --force", path)
+	}
+
+	return nil
+}
+
 // getDevStackName returns the stack name suffixed with -dev.
 func getDevStackName(name string) string {
 	const suffix = "-dev"