vfs: read directory and check for a file before mkdir

Before this change when doing Mkdir the VFS layer could add the new
item to an unread directory which caused confusion.

It could also do mkdir on a file when run on a bucket based remote
which would temporarily overwrite the file with a directory.

Fixes #2993

vfs/dir.go

@@ -458,8 +458,23 @@ func (d *Dir) Mkdir(name string) (*Dir, error) {
 		return nil, EROFS
 	}
 	path := path.Join(d.path, name)
+	node, err := d.stat(name)
+	switch err {
+	case ENOENT:
+		// not found, carry on
+	case nil:
+		// found so check what it is
+		if node.IsDir() {
+			return node.(*Dir), err
+		}
+		return nil, EEXIST
+	default:
+		// a different error - report
+		fs.Errorf(d, "Dir.Mkdir failed to read directory: %v", err)
+		return nil, err
+	}
 	// fs.Debugf(path, "Dir.Mkdir")
-	err := d.f.Mkdir(path)
+	err = d.f.Mkdir(path)
 	if err != nil {
 		fs.Errorf(d, "Dir.Mkdir failed to create directory: %v", err)
 		return nil, err
@@ -600,7 +615,7 @@ func (d *Dir) Rename(oldName, newName string, destDir *Dir) error {
 		}
 	default:
 		err = errors.Errorf("unknown type %T", oldNode)
-		fs.Errorf(d.path, "Dir.ReadDirAll error: %v", err)
+		fs.Errorf(d.path, "Dir.Rename error: %v", err)
 		return err
 	}