[wp-trac] [WordPress Trac] #41358: Shutdown hooks can significantly slow down REST API responses

[WordPress Trac]

#41358: Shutdown hooks can significantly slow down REST API responses
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 Reporter:  mikejolley             |      Owner:
     Type:  defect (bug)           |     Status:  new
 Priority:  normal                 |  Milestone:  Awaiting Review
Component:  REST API               |    Version:
 Severity:  normal                 |   Keywords:
  Focuses:  rest-api, performance  |
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 If you have a site with some slow, maybe deferred actions hooked into
 `shutdown`, these actions will slow the response from the REST API
 unnecessarily.

 To test, simply add the following then do a request to the WP Rest API:

 {{{#!php
 <?php
 add_action( 'shutdown', 'delay_shutdown' );

 function delay_shutdown() { sleep( 3); }
 }}}

 The response will not be sent out until all of these shutdown hooks are
 completed. In our case, we found that some heavy sync functions ran on
 shutdown would cause our API calls to WooCommerce API timed out, when
 really the response could have been sent much earlier so these calls could
 run afterwards.

 This is semi-related to https://core.trac.wordpress.org/ticket/27122 in
 that the fix could be along those lines.

 This isn't a solution we feel should be merged into WooCommerce itself
 because it would be more consistent to have it in WP itself, but see
 https://github.com/woocommerce/woocommerce/pull/16158/files

 So on shutdown, it ends any sessions, uses `fastcgi_finish_request` if
 available, and falls back to flushing. The response is sent and received,
 and shutdown continues on in the background. The response is sent before
 the `sleep(3`) in this test case.

 This could be applied to just REST API requests, or everywhere, depending
 on everyones feedback. If REST API only, it may make sense to fire an
 action after the response is sent to avoid `shutdown` hook altogether.

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Ticket URL: <https://core.trac.wordpress.org/ticket/41358>
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