Data loss - menu items unintentionally delete themselves if a node is updated via code (not via form UI) . Test case attached.

Although mostly a node is expected to be edited through the node edit form, occasionally we update then through code. Eg, with views_bulk_operations or, in todays case, with Feeds.

Normally, that would be a case of

1. $node = node_load($nid);
2. $node->whatever = $changed_value;
3. node_save($node);

Now, you would not expect that to throw away information. And - it doesn't.
However if you add the innocuous step

• node_object_prepare($node);

into the mix (as feeds.module does, menu.module:menu_node_save() decides to delete the existing menu item altogether.
This is bad.

Attached is an extra test case for menu.module that verifies that node forms behave correctly with menu settings (this seems to have no test case at the moment) and then demonstrates the problem - producing a fail.

The real fix (I believe) is to get menu_node_prepare() to set $node->menu['enabled'] to TRUE if a menu link was in fact loaded. Currently it's a temporary flag that only exists in FAPI state, not as real data. This would prevent the unexpected and destructive behavior.

I found this error when using feeds.module, and also when writing my own VBO action (using hook_action_info()) that needed to load menu info for a node in order to inspect it. Just loading node menu data to look at it causes that menu to go away.

It's a rare edge case, and can be worked around by the caller (who has to add $node->menu['enabled'] to the node before saving it again) but it's unintuative and wrong to allow this to happen as a side-effect here.

To replicate:

    // Programatically load the node.
    $node = node_load($nid1);

    // If we run this, data loss happens:
    node_object_prepare($node);
    // 'Preparing' an object should be a safe act.
    // I needed to run prepare to READ the $node->menu data.

    // Make a small change to the node. (dummy example)
    $node->title = "Node title changed again";

    // Save.
    node_save($node);
    // Your menu item is now deleted

Even if nobody thinks this data loss is a problem, here's a new test case that exercises the node form menu settings.