Autosaves are generated every other preview if post is unchanged

[mboynes]

I'm trying to work with revisioned post meta and noticed an interesting quirk in core today that I'd categorize as "unexpected behavior." While this probably wouldn't come up very often, I think it can cause a bit of a headache when it does.

To replicate:

1. Create a new post, give it a title, and publish it.
2. Once the screen reloads, click "Preview Changes". Don't make any changes to the post.
   • In loading the preview, WordPress created an autosave of this post
3. Return to the WordPress Admin and click "Preview Changes" again without making any changes.
   • In loading the preview, WordPress deleted the autosave it created in step 2

Every other preview will add an autosave and every other one will delete it. If at any point you do change the post, the autosave won't get deleted.

In the code, what appears to be happening is, wp_create_post_autosave() checks to see if an autosave exists. If it does, the code checks to see if the autosave is different from the post, and if so, it deletes the autosave. If an autosave doesn't exist, the function then creates one (using _wp_put_post_revision()) -- *without checking to see if the post has changed*. It seems to me that there should be an intermediate function, e.g. _wp_maybe_put_post_revision(), which would check to see if the post has changed, and only then call _wp_put_post_revision().

This probably isn't a big deal, because it requires someone to attempt to preview (twice) without making changes. However, it's still a bug and it might come up in other ways.

Verified in trunk and 4.2.2.