os: fix file_ext function (#14566)

vlib/os/os.v

@@ -173,8 +173,20 @@ pub fn is_dir_empty(path string) bool {
 
 // file_ext will return the part after the last occurence of `.` in `path`.
 // The `.` is included.
+// Examples:
+// ```v
+// assert os.file_ext('file.v') == '.v'
+// assert os.file_ext('.ignore_me') == ''
+// assert os.file_ext('.') == ''
+// ```
 pub fn file_ext(path string) string {
-	pos := path.last_index('.') or { return '' }
+	if path.len < 3 {
+		return empty_str
+	}
+	pos := path.last_index(dot_str) or { return empty_str }
+	if pos + 1 >= path.len || pos == 0 {
+		return empty_str
+	}
 	return path[pos..]
 }
 

vlib/os/os_test.v

@@ -585,9 +585,19 @@ fn test_is_executable_writable_readable() ? {
 	os.rm(file_name) or { panic(err) }
 }
 
-fn test_ext() {
+fn test_file_ext() {
 	assert os.file_ext('file.v') == '.v'
+	assert os.file_ext('file.js.v') == '.v'
+	assert os.file_ext('file.ext1.ext2.ext3') == '.ext3'
+	assert os.file_ext('.ignore_me.v') == '.v'
 	assert os.file_ext('file') == ''
+	assert os.file_ext('.git') == ''
+	assert os.file_ext('file.') == ''
+	assert os.file_ext('.') == ''
+	assert os.file_ext('..') == ''
+	assert os.file_ext('file...') == ''
+	assert os.file_ext('.file.') == ''
+	assert os.file_ext('..file..') == ''
 }
 
 fn test_join() {