The issue I'm seeing, I believe, concerns the behavior of async-upload.php when called from an admin-side post page. I ran into an issue where I had multiple editors logged into the admin side of my site, changing and uploading a number of posts, and for some reason the server's CPU block (4 procs) pegged. When this occurred (multiple times over across the course of several days before we caught on to the problem) several php errors were thrown and recorded in my server error logs. These errors look like the following:
Script timed out before returning headers: async-upload.php, referer: wp-admin/post.php?post=75064&action=edit&message=1 Script timed out before returning headers: async-upload.php, referer: wp-admin/post.php?post=75262&action=edit&message=10 Script timed out before returning headers: async-upload.php, referer: wp-admin/post.php?post=75269&action=edit&message=6
and were all over the place.
I'm not sure if these time outs occurred when the admin-side post pages were loaded or saved, but the upshot is that the posts ended up losing a lot of data in the process. I have a number of meta boxes set up on each of these post pages that are populated with saved custom field values (meta). When a post is saved, I've defined a custom save function that checks for current values that should be saved and those that should be deleted based on what is submitted through the WP form within these custom meta boxes I've defined. Somewhere in the load/save process while my editors were going to town, it seems that the values that should have been present in my custom meta boxes weren't there and so they all got deleted when my editors saved the posts.
I've verified that this condition doesn't occur at any time other than when the server CPU is capping (at 100%) and apparently it also only happens when the above php errors are being reported.
Thus, in order to debug my problem any further, I need to know what happens when the async-upload.php file terminates early because of a time out. Rather than go digging into the core WP code, I thought I'd ask here first to see if an expert could give me some information that I didn't have, as I know that digging into the core WP files and trying to figure out what they're doing is probably going to take me (an admitted non-expert) a bundle of time.
As this error is only happening on the admin side and also only when the server CPU is capping, I'm not going to leave a link for my site.