[wp-trac] [WordPress Trac] #53723: 503 Errors in Posts with Complex Gutenberg Content

[WordPress Trac]

#53723: 503 Errors in Posts with Complex Gutenberg Content
--------------------------+-----------------------------
 Reporter:  aut0poietic   |      Owner:  (none)
     Type:  defect (bug)  |     Status:  new
 Priority:  normal        |  Milestone:  Awaiting Review
Component:  General       |    Version:
 Severity:  normal        |   Keywords:
  Focuses:                |
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 Trying to track down the source of an error I'm seeing across the
 sites/servers I host that appeared after the 5.8 upgrade.

 Symptoms so far are either:

 - A 503 Service Unavailable error
 - A Blank page. Server cutting off output at random point on each site,
 often in the middle of tag names or attributes (where there's no PHP,
 static content).

 This is happening on specific pages/posts. It is happening when viewing
 the page/post on the front end AND if you try to edit the post/page in the
 editor via the WP Admin.

 In both error cases, there are no PHP errors displayed, outside of the
 servers that were displaying the 503 error.

 In narrowing down the error, I tried removing filters from `the_content`
 on a hunch and found that if I removed the filter below, the pages started
 working without error:

 {{{
 remove_filter( 'the_content', 'do_blocks', 9 );
 }}}

 Once there, I tried setting `wp_posts.post_content` manually and narrowed
 the issue down to specific sets of blocks: Nested blocks with an HTML
 block that contains SVGs that have large-ish paths. Here's a gist with 3
 different `post_content` HTML.

 https://gist.github.com/aut0poietic/9df32c4383bd8934db59671b77a673ee

 The example `post_content-broken.html` triggers the error consistently on
 all of my sites across multiple servers. The other examples are the same
 block content, with minor changes that DO NOT trigger the error.

 Our Ops folks could find almost nothing in the logs, spare this FastCGI
 error:

 {{{
 [Tue Jul 20 15:36:00.465094 2021] [proxy_fcgi:error] [pid 3207915:tid
 140096612689664] [client 172.31.1.17:58998] AH01067: Failed to read
 FastCGI header
 }}}

 Digging around, it seems like `WP_Block_Parser` may be having issues with
 the content.

 For giggles, I tried adding this code to a fresh site on a completely
 unrelated server (Flywheel Local) and couldn't trigger the issue. So
 there's obviously something on these servers that are contributing to the
 issue, I just can't narrow down what that could be.

 The server info from Site Health is pretty standard:
 {{{
 server_architecture: Linux 4.18.0-240.22.1.el8_3.x86_64 x86_64
 httpd_software: Apache
 php_version: 7.2.24 64bit
 php_sapi: fpm-fcgi
 max_input_variables: 4000
 time_limit: 600
 memory_limit: 256M
 max_input_time: 300
 }}}

 I can post the full site health if needed. It goes without saying that I
 used the standard troubleshooting practices to test, including:

 - Disable all plugins
 - Set the theme to twentytwenty
 - Reset Permalinks


 Not sure where to go with this next. Help?

 Examples demonstrating the content from the Gist in action:

 https://help.okfosters.org/testing-broken/
 https://help.okfosters.org/testing-working-remove-group/
 https://help.okfosters.org/testing-working-remove-line-15/

 If you need any more information, please let me know.

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Ticket URL: <https://core.trac.wordpress.org/ticket/53723>
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