Apache 2.4 + PHP 7.2 prefork mpm + file uploads lead to partial upload errors after 20 seconds

I’ve been noticing an issue with a web application we have where our application’s code that handles file uploads would intermittently encounter file upload error 3. I wasn’t sure how our users were triggering this error but I do know that the ones who did would be uploading files through an unstable or slow internet connection (mobile hotspots, public wifi, etc). So I tested this by using Chrome Dev Tools’ throttling feature (fast 3g) and uploading a 10mb file that would take somewhere around a minute or more to complete. We use the Dropzonejs library to handle uploads by the way with no chunking whatsoever. After exactly 22 – 23 seconds, the connection always seems to get aborted but Apache still proceeds to handle the incomplete request body it receives and passes it to PHP, leading to a partial upload error.

I can’t seem to figure out what is causing this to happen. PHP config has max_execution_time and max_input_time set to 0 and -1 respectively. Post and upload max sizes are set relatively high and well, the file upload size doesn’t even seem to matter. As long as the upload request takes longer than 22 – 23 seconds, the problem occurs. I tried disabling mod_reqtimeout and it didn’t make a difference. Other things I’ve tried are tinkering with the apache Timeout value and disabling keepalive and it still gives me issues a little after 20 seconds always (that time comes from the browser’s network tab).

I don’t see anything in apache’s error logs and the access logs make these requests appear legit, since apache still continues to handle the incomplete request like normal.

I initially thought it might be dropzonejs killing the ajax request connection but I have also tested the same code on my local dev environment which uses laradock (software versions will be slightly different. Still both apache 2.4 and php 7.2) and I cannot replicate the issue, so it couldn’t be a client-side problem.