copying a file with a tree path doesn't set timestamps correctly

[smira]

This issue appeared while debugging issue with buildkit creating images with wrong timestamps. It is a custom LLB frontend which outputs an instruction like:

COPY --timestamp=<FIXED> /src/file /dst/path1/path2/file

In the output, the /dst/path1/path2/file has proper timestamp <FIXED>, while /dst, /dst/path1, /dst/path1 all have wrong timestamps (current timestamps).

While debugging this, I got down to this library and put a unit-test (to copy_test.go) which fails:

func TestCopyFileWithPathTimestamp(t *testing.T) {
	timestamp := time.Unix(0, 0)
	apply := fstest.Apply(
		fstest.CreateDir("/foo/", 0755),
		fstest.CreateFile("/foo/bar", []byte{}, 0644),
	)

	t1 := t.TempDir()
	t2 := t.TempDir()

	require.NoError(t, apply.Apply(t1))
	require.NoError(t, Copy(context.TODO(), t1, "/foo/bar", t2, "/test1/test2/test3/bar", WithCopyInfo(CopyInfo{
		CopyDirContents: true,
		Utime:           &timestamp,
	})))

	for _, s := range []string{"/test1/test2/test3/bar", "/test1/test2/test3", "/test1/test2", "/test1"} {
		stat, _ := os.Stat(filepath.Join(t2, s))
		require.Equal(t, timestamp, stat.ModTime(), "path: %s", s)
	}
}

This test shows exactly same as I observed with buildkit - the final file has a correct timestamp, while all intermediate directories have a current timestamp.

The reason is that MkdirAll recursively calls itself, and when it creates the parent /path1, the /path1 timestamp will be set correctly with Utimes, but then MkdirAll will create /path1/path2, which will reset the timestamp of /path1, and so on.

The second issue is in the Copy function itself - when it creates the file, it doesn't revert the timestamp of the parent directory.

So I couldn't come up with an easy fix, so creating this issue to get some ideas how to fix it.

For me the most clean way would be to set the timestamp after all copy operations for all the created items to avoid dealing with parent-child file creation.