os: add fast path to mkdir_all, when the given folder already exists (#19869)

vlib/os/os.v

@@ -695,6 +695,12 @@ pub struct MkdirParams {
 
 // mkdir_all will create a valid full path of all directories given in `path`.
 pub fn mkdir_all(opath string, params MkdirParams) ! {
+	if exists(opath) {
+		if is_dir(opath) {
+			return
+		}
+		return error('path `${opath}` already exists, and is not a folder')
+	}
 	other_separator := if path_separator == '/' { '\\' } else { '/' }
 	path := opath.replace(other_separator, path_separator)
 	mut p := if path.starts_with(path_separator) { path_separator } else { '' }

vlib/os/os_test.c.v

@@ -1016,3 +1016,17 @@ fn test_mv_across_partitions() {
 fn test_page_size() {
 	assert os.page_size() >= 4096 // this is normal and assumed.
 }
+
+fn test_mkdir_at_file_dst() {
+	f3 := os.join_path('myfolder', 'f1', 'f2', 'f3')
+	os.mkdir_all(f3)!
+	assert os.is_dir(f3)
+	path := os.join_path(f3, 'no_ext_doc')
+	os.write_file(path, '')!
+	assert os.exists(path) && os.is_file(path)
+	os.mkdir_all(path) or {
+		assert err.msg() == 'path `${path}` already exists, and is not a folder', err.msg()
+		return
+	}
+	assert false
+}