checkout: Always replace existing content

The combination of the "honor whiteout" and "union" flags
are intended to basically be "merge trees like overlayfs does".
But we were missing this case in order to support e.g. replacing
a symlink with a directory.

src/libostree/ostree-repo-checkout.c

@@ -1036,6 +1036,16 @@ checkout_tree_at_recurse (OstreeRepo *self, OstreeRepoCheckoutAtOptions *options
       contents_csum_v = NULL; /* iter_loop freed it */
     }
 
+  /* In this mode, we're flattening in a manner similar to overlayfs, so ensure
+   * any content that was there before is gone.
+   */
+  if (!is_opaque_whiteout && options->process_whiteouts
+      && options->overwrite_mode == OSTREE_REPO_CHECKOUT_OVERWRITE_UNION_FILES)
+    {
+      if (!glnx_shutil_rm_rf_at (destination_parent_fd, destination_name, NULL, error))
+        return FALSE;
+    }
+
   /* First, make the directory.  Push a new scope in case we end up using
    * setfscreatecon().
    */

tests/basic-test.sh

@@ -1159,6 +1159,14 @@ if touch overlay/baz/.wh.cow && touch overlay/.wh.deeper && touch overlay/baz/an
     assert_has_file overlay-co/baz
     test -L overlay-co/anewdir
 
+    rm overlay-co overlay -rf
+    mkdir -p overlay/somelink
+    $OSTREE --repo=repo commit ${COMMIT_ARGS} -b overlay-symlink-convert --tree=dir=overlay
+    for branch in test2 overlay-symlink-convert; do
+        $OSTREE --repo=repo checkout --union --whiteouts ${branch} overlay-co
+    done
+    test -d overlay-co/somelink || fatal "should replace symlink with dir"
+    rm overlay-co overlay -rf
     echo "ok whiteouts enabled"
 
     # Now double check whiteouts are not processed without --whiteouts