mod_rewrite REQUEST_FILENAME doesn't contain absolute path

I have a problem with a file test operation in a mod_rewrite RewriteCond entry which is testing whether %{REQUEST_FILENAME} exists. It seems that rather than %{REQUEST_FILENAME} being an absolute path, I'm getting a path which is rooted at the DocumentRoot instead.

Configuration

I have this inside a <VirtualHost> block in my apache 2.2.9 configuration:

RewriteEngine on
RewriteLog /tmp/rewrite.log
RewriteLogLevel 5

#push virtually everything through our dispatcher script
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^/([^/]*)/?([^/]*) /dispatch.php?_c=$1&_m=$2 [qsa,L]

Diagnostics attempted

That rule is a common enough idiom for routing requests for non-existent files or directories through a script. Trouble is, it's firing even if a file does exist.

If I remove the rule, I can request normal files just fine. But with the rule in place, these requests get directed to dispatch.php

Rewrite log trace

Here's what I see in the rewrite.log

init rewrite engine with requested uri /test.txt
applying pattern '^/([^/]*)/?([^/]*)' to uri '/test.txt'
RewriteCond: input='/test.txt' pattern='!-f' => matched
RewriteCond: input='/test.txt' pattern='!-d' => matched
rewrite '/test.txt' -> '/dispatch.php?_c=test.txt&_m='
split uri=/dispatch.php?_c=test.txt&_m= -> uri=/dispatch.php, args=_c=test.txt&_m=
local path result: /dispatch.php
prefixed with document_root to /path/to/my/public_html/dispatch.php
go-ahead with /path/to/my/public_html/dispatch.php [OK]

So, it looks to me like the REQUEST_FILENAME is being presented as a path from the document root, rather than the file system root, which is presumably why the file test operator fails.

Any pointers for resolving this gratefully received...

Took me a while to find out as well, but the reason is mentioned in the mod_rewrite documentation:

REQUEST_FILENAME The full local filesystem path to the file or script matching the request, if this has already been determined by the server at the time REQUEST_FILENAME is referenced. Otherwise, such as when used in virtual host context, the same value as REQUEST_URI.

That's why it works in .htaccess but not in the conf file of the vhost. (And that's also why adding the DOCUMENT_ROOT works around the problem).

I resolved this by explicitly writing the document root into the condition

RewriteCond /path/to/my/public_html%{REQUEST_FILENAME} !-f
RewriteCond /path/to/my/public_html%{REQUEST_FILENAME} !-d

RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d

This won't work as intended in a virtualhost (or server) context because the directives are processed before the request has been mapped to the filesystem, unlike when used in a directory (or .htaccess) context. Before the request has been mapped to the filesystem, REQUEST_FILENAME is the same as REQUEST_URI, ie. the root-relative URL-path.

However, you can use a lookahead instead to get the absolute filename that the request will eventually map to. For example:

RewriteCond %{LA-U:REQUEST_FILENAME} !-f
RewriteCond %{LA-U:REQUEST_FILENAME} !-d