[bw_pages] - Infinite recursion processing post content

[bobbingwide]

There's a new block in the Gutenberg plugin called post-content. I've discovered multiple ways of causing Gutenberg to go into an infinitely recursive loop. While trying to reason what the output should be when recursion is detected, I created a test case using the [bw_pages] shortcode which was equivalent to the scenario I'd tried using FSE. Two pages that would recursively embed each other ad infinitum.
Imagine my surprise when I discovered that my shortcode suffered the same problem.

Steps to reproduce

1. Create two identical pages with the same shortcode. One called A, the other called C.

[bw_pages post_type=page format="T d e/C" post_parent=0 orderby=title numberposts=3]

2. Create two other pages B and C - with normal content.
3. Attempt to view either A or C.
4. Wait a long time.

Details

In order to create a display that's a bit more understandable I decided it might be easier to
create 4 pages to be used in the test scenario.
A and C contain the problematic shortcode.
B and D are just for a bit of padding.

Page	Content
A	[bw_pages] shortcode
B	plain content
C	[bw_pages] shortcode
D	plain content

Expected output

Note: Both [bw_pages] and core/post-content attempt to prevent the recursion by excluding themselves from the result set.
When the shortcode's format parameter doesn't include C - for Content - then the problem shouldn't occur.
Try this with E for Excerpt.

When viewing A, with format="T e / C" the processing should be:

• A's content before the shortcode should be displayed
• A runs [bw_pages] which returns B, C and D
• B is formatted: Title edit and the rendered content
• C is formatted: Title edit
  • And content is expanded. [bw_pages] returns A, B and D
  • A is formatted: Title edit
  • Recursion is detected
  • B is formatted as before
  • D is formatted
• C's content after the shortcode is rendered
• D is formatted

Here's the screen capture for A

And here's the screen capture for C

Additional notes
I added another line of code into the bw_pages() function that prevents double expansion of the top level post.

// Don't process the current post inside the loop
bw_process_this_post( $cp );

[bobbingwide]

In order to make sense of the output I added class=A or class=C parameters to the shortcodes and the following custom CSS

div.A {background-color: #aaa;border: 1px solid grey;padding-left: 1em;}
div.C { background-color: #ccc;border: 1px dotted grey;padding-left: 2em; padding-bottom: 0.5em;}
ul.C, ul.A { display: block; list-style: none;}
.A li, .C li { margin-right: 5px; display: inline-block; }
body { font-size: 12px;}
p { margin: 2px}

Posts A and C also contained a bw_list shortcode, that creates the simple list of posts: eg B C D for A and A B D for C.

[bw_list post_type=page format="L e/C" post_parent=0 orderby=title numberposts=3 class=A]

[bobbingwide]

Scenario 3 - 3 pages with the same shortcode

The screenshots above showed the output when the queries were limited to 3 posts.

I added post E the similar to A and C,

Post	bw_pages shortcode
A	[bw_pages post_type=page format="L e/C" post_parent=0 orderby=title numberposts=4 class=A]
C	[bw_pages post_type=page format="L e/C" post_parent=0 orderby=title numberposts=3 class=C]
E	[bw_pages post_type=page format="T e/C" post_parent=0 orderby=title numberposts=5 class=E exclude=-1]

Results viewing E:

Note: Use of exclude=-1 on E means that its query returns itself.
In nested evaluation we see:

Content not available; already processed. post_content E (915) 915,867

The new function bw_report_recursion_error() provides additional information when WP_DEBUG is true.
eg post_content A (867) 915,867,872
where

• post_content indicates this is the post_content field
• A is the title of the post that's considered to have gone recursive
• (867) is the post ID
• 915,867,872 shows the current contents of the global $processed_posts

[bobbingwide]

We should still test what happens when the [bw_pages] shortcode is used in the Excerpt ( before more ) and the format= includes E for excerpt.