Unpleasant bug with attachments

Hi @crypto,
sorry for the delay.

I’ll try to explain better what’s happening.
Every attachment, in WordPress, is “made” of two parts: a physical file inside (usually) the wp-content/uploads folder, and a post of type attachment which contains the URL of the physical file. This attachment can have a “normal” post (or page, or whatever) as its parent, and thus be “attached” to it. Or it can be without parent, as it happens when you upload an attachment from the Media Library instead of from within a post edit screen.
When you use a gallery shortcode like that one, WordPress gets all the attachments attached to the current post and uses their physical files, as long as they are images, to display them somehow. If the post doesn’t have any image attached, the gallery is empty.

Enter my plugin, which may or may not copy the attachments of a post.

• with versions up to 2.6, when attachments were copied, only the database entries (the posts of type attachment having the current post has parent) were cloned, but not the physical files. This meant that you could have many attachments linking to the same physical file, which is wrong because deleting an attachment would have deleted the physical file for every other attachment pointing to that.
  But it worked with that shortcode, because the new post had its own attachments, distinct from but pointing to the same physical files of the attachments of the original post.
• since version 3.0, when attachments are copied, not only the database entries are clone, but also the physical files. It’s as if you are uploading the same images that you uploaded in the original post: so you have the original post and the copy with distinct attachment, pointing to different physical files that happen to be identical, but not the same ones. Obviously it doubles the space used on the server.
  If you disable the Attachments setting instead, the attachments are not cloned with the post. Usually it’s not big deal as long as images are embedded in the content, or there are hyperlinks pointing to uploaded documents, or you have selected a featured image: that’s because all this things use the ID of the attachments, or the URLs of the physical file, regardless of whether the attachments are attached to the original or to another post, or are un-attached at all.
  But it’s a problem with that shortcode because the clone post does not have any attachment, so it shows nothing. Pay attention that if you create a new post by hand, and you copy the content of another post including the shortcode, it doesn’t display anything because the post doesn’t have anything attached.

So I stick to my claim that v3.2 is not buggy in that regard, but behaves exactly how I intended it to, and how I think it’s the expected way.
That said, hoping it’s all clear, what about the solution if you’re using that kind of shortcode?

1. Reverting to v2.6 could be an easy solution, but as I said it would be an unsafe and buggy one, so I don’t recommend it.
2. Turning on the Attachments setting makes that shortcode work, but at the price of duplicating files in the uploads folder, using more hosting space, as you experienced.
3. Turning off the the Attachments setting, you need to fix the shortcode just as you would have to if you copied the post by hand: creating a gallery by explicitly selecting the images you want it to display, regardless of the post to which they are attached.
   Not that if you do it in the original post already, when you copy it you won’t need to edit the gallery since the shortcode would already have the list of images IDs to display, e.g. [gallery ids="4,7,90,330" columns="1" link="none" size="full"].
4. Another possible solution would be to add some code to my plugin to edit the gallery shortcodes automatically when cloning a post to add the IDs of the images attached to the original post, instead of doing it by hand as I told above. But it’s not so simple and probably it wouldn’t make it in the next version as I have some other, more general priorities
5. There could be some other solutions, tailor-made to your case, which would involve some coding (for example by editing your template).

I suggest you solution 3, which is the most straightforward though it requires to explicitly choose the images for every gallery: it should be what’s already happening when you add a gallery shortcode using the “Add Media” button… How come you don’t have any ids parameter to your shortcode?

Sorry if the reply is to loong, I hope I’ve explained the issue clearly enough and helped you choose the best solution for you…

Thanks for reporting, feel free to write for any doubt or bug or question!