mod_realdoc is an Apache module which does a realpath on the docroot symlink and sets the absolute path as the real document root for the remainder of the request.
It executes as soon as Apache is finished reading the request from the client.
The realpath frequency can be adjusted in httpd.conf using:
<IfModule mod_realdoc.c>
RealpathEvery 2
</IfModule>
By resolving the configured symlinked docroot directory to an absolute path at the start of a request we can safely switch this symlink to point to another directory on a deploy. Requests that started before the symlink change will continue to execute on the previous symlink target and therefore will not be vulnerable to deploy race conditions.