os: fix returning parameter in [manualfree]os.real_path(fpath), when fpath==""

vlib/os/os_c.v

@@ -787,7 +787,7 @@ pub fn real_path(fpath string) string {
 		ret := C.GetFullPathName(fpath.to_wide(), max_path_len, fullpath, 0)
 		if ret == 0 {
 			unsafe { free(fullpath) }
-			return fpath
+			return fpath.clone()
 		}
 		res = unsafe { string_from_wide(fullpath) }
 	} $else {
@@ -795,7 +795,7 @@ pub fn real_path(fpath string) string {
 		ret := &char(C.realpath(&char(fpath.str), &char(fullpath)))
 		if ret == 0 {
 			unsafe { free(fullpath) }
-			return fpath
+			return fpath.clone()
 		}
 		res = unsafe { fullpath.vstring() }
 	}

vlib/os/os_test.v

@@ -284,10 +284,51 @@ fn test_cp_all() {
 	assert os.exists(os.join_path('nonexisting', 'ex1.txt'))
 }
 
-fn test_realpath() {
+fn test_realpath_of_empty_string_works() {
 	assert os.real_path('') == ''
 }
 
+fn test_realpath_non_existing() {
+	non_existing_path := 'sdyfuisd_non_existing_file'
+	assert os.real_path(non_existing_path) == non_existing_path
+}
+
+fn test_realpath_existing() {
+	existing_file_name := 'existing_file.txt'
+	existing_file := os.join_path(os.temp_dir(), existing_file_name)
+	os.rm(existing_file) or {}
+	os.write_file(existing_file, 'abc') or {}
+	assert os.exists(existing_file)
+	assert os.real_path(existing_file) == existing_file
+	os.rm(existing_file) or {}
+}
+
+fn test_realpath_removes_dots() {
+	examples_folder := os.join_path(@VEXEROOT, 'vlib', 'v', '..', '..', 'cmd', '.', '..',
+		'examples')
+	real_path_of_examples_folder := os.real_path(examples_folder)
+	assert real_path_of_examples_folder.len < examples_folder.len
+	assert !real_path_of_examples_folder.contains('..')
+}
+
+fn test_realpath_absolutizes_existing_relative_paths() {
+	old_wd := os.getwd()
+	defer {
+		os.chdir(old_wd)
+	}
+	os.chdir(@VEXEROOT)
+	examples_folder := os.join_path('vlib', 'v', '..', '..', 'cmd', '.', '..', 'examples')
+	real_path_of_examples_folder := os.real_path(examples_folder)
+	assert os.is_abs_path(real_path_of_examples_folder)
+}
+
+// TODO: think much more about whether this is desirable
+fn test_realpath_does_not_absolutize_non_existing_relative_paths() {
+	relative_path := os.join_path('one', 'nonexisting_folder', '..', 'something')
+	assert os.real_path(relative_path).contains('..')
+	assert os.real_path(relative_path) == relative_path
+}
+
 fn test_tmpdir() {
 	t := os.temp_dir()
 	assert t.len > 0