This must be called before calling assetsFromDir().

Please clarify more on this because the current placement looks fine to me since:

• assetsFromDir opens the files (via assets.NewFileAsset) and returns []assets.CopyableFile. We don’t have anything to close until you have that slice.
• We register t.Cleanup immediately after obtaining actualFiles. That’s the earliest possible point we can close them reliably.
• t.Cleanup runs after the subtest finishes (even if it fails), so descriptors are released in all cases.
• Moving the cleanup registration “before” the call would be impossible (the slice doesn’t exist yet) unless we refactored assetsFromDir to accept a callback, which is unnecessary for a test.

So if assetsFromDir fails, we don't have anything to cleanup? Can return incomplete results and fail?

If it either return slice or fail without opening anything we are fine with current code. If it can open some files and fail, we can either clean up in assetsFromDir, or here by registring the cleanup before we call:

var actualFiles []Xxx
t.Cleanup(...)
actualFiles = ...

But it will probably better to cleanup in assetsFromDir() or a test helper using it. Test helpers should get a t *testing.T argument and handle cleanup and test failures internally so the test code is simpler.

I introduced a collectAssets(t, root, dest, flatten) test helper that:

• Calls assetsFromDir once and immediately registers a t.Cleanup to close any FileAsset descriptors it opened (including partial slices if the walk fails).
• Keeps error handling + cleanup localized so each subtest only sets up files and validates the mapping. This addresses the earlier concern about calling cleanup “early enough” without modifying production code. Moving cleanup into assetsFromDir would change its resource lifecycle semantics for other callers, so keeping it in a test helper preserves current behavior while preventing leaks on Windows. Let me know if you’d prefer an additional error-path test; happy to add one if we can deterministically trigger a walk failure.

Hey @msft-linliu , investigating moving t.Cleanup, yes we can move t.Cleanup earlier, but the practical benefit is very small. Right now cleanup is already registered before we branch on err, so opened assets are closed even when assetsFromDir returns an error. Moving it “further up” only helps in the very narrow case where we want the cleanup registered before a potential panic inside assetsFromDir. If that function panics before returning, we still won’t have access to any partially created assets to close; the slice would remain empty. So this is mostly a stylistic change.

The moved-up version would look like this

func collectAssets(t *testing.T, root, dest string, flatten bool) []assets.CopyableFile {
    t.Helper()

    files := []assets.CopyableFile{} // declare first so closure captures it
    t.Cleanup(func() {
        for _, f := range files {
            if cerr := f.Close(); cerr != nil {
                t.Logf("warning: closing asset %s failed: %v", f.GetSourcePath(), cerr)
            }
        }
    })

    // now populate files
    fs, err := assetsFromDir(root, dest, flatten)
    files = fs

    if err != nil {
        t.Fatalf("assetsFromDir(%q, %q, flatten=%v) unexpected error: %v", root, dest, flatten, err)
    }
    return files
}