archive: fix corrupt listings when listing a zip directory more than once

The zip archiver handed out its cached directory tree directly. Any
caller which filters a listing in place (as the core listing code
does) altered the cache, so later listings of the same directory could
be corrupted.

Return a copy of the cached listing instead.

backend/archive/zip/zip.go

@@ -9,6 +9,7 @@ import (
 	"io"
 	"os"
 	"path"
+	"slices"
 	"strings"
 	"time"
 
@@ -217,7 +218,8 @@ func (f *Fs) List(ctx context.Context, dir string) (entries fs.DirEntries, err e
 		return nil, fs.ErrorDirNotFound
 	}
 	fs.Debugf(f, "dir=%q, entries=%v", dir, entries)
-	return entries, nil
+	// Return a copy as callers may filter the listing in place
+	return slices.Clone(entries), nil
 }
 
 // NewObject finds the Object at remote.

backend/archive/zip/zip_internal_test.go

@@ -128,3 +128,30 @@ func TestReadZipRootNamedEntry(t *testing.T) {
 	remotes := allRemotes(t, f)
 	assert.Equal(t, []string{"good.txt"}, remotes)
 }
+
+// Listings are served from a cache which must survive callers
+// filtering the returned slice in place, as fs/list does.
+func TestListCacheNotAliased(t *testing.T) {
+	ctx := context.Background()
+	dir := t.TempDir()
+	name := writeZip(t, dir, "test.zip", "a.txt", "b.txt", "c.txt")
+
+	localFs, err := cache.Get(ctx, dir)
+	require.NoError(t, err)
+	f, err := New(ctx, localFs, name, "", "")
+	require.NoError(t, err)
+
+	entries, err := f.List(ctx, "")
+	require.NoError(t, err)
+	require.Len(t, entries, 3)
+	// Compact in place, dropping the first entry
+	copy(entries, entries[1:])
+
+	entries, err = f.List(ctx, "")
+	require.NoError(t, err)
+	var remotes []string
+	for _, entry := range entries {
+		remotes = append(remotes, entry.Remote())
+	}
+	assert.Equal(t, []string{"a.txt", "b.txt", "c.txt"}, remotes)
+}