Rewrite existing but broken symlinks with .htacces? - .htaccess

Question

I would like to use a rewrite rule that is executed when a symlink exists but is broken.
So the scenario's would be:
Symlink does not exist: normal 404/403 error.
Symlink exists but is broken: generate-cache.php is called.
Symlink exists and is working: target file is loaded normally.
For example:
## Symlink does not exist.
GET /links/cache/secret.jpg
404 Not Found
## Symlink is broken.
GET /links/cache/secret.jpg
Links to /images/cache/secret.jpg
Because it's broken, rewrites to: generate-cache.php?path=cache/secret.jpg
200 OK
## Symlink works.
GET /links/cache/secret.jpg
Links to /images/cache/secret.jpg
200 OK
Update: I want to avoid using PHP to do these checks, because it causes a performance bottleneck. Outputting the file through PHP if it exists causes PHP to lock. Also I have no option to use multiple PHP threads or install additional apache modules.

Answer 1

I don't know of a way of testing for a broken symlink in mod_rewrite (-l checks for the existence of a symlink, but doesn't attempt to follow it), which may mean you'd need to write some kind of callback in PHP (or some other language).
An alternative approach would be to rewrite all requests, and build this logic in PHP:
if the file exists in the cache directory, set appropriate headers and use readfile() to output the data
if the symlink exists (or just an empty file with the right name in a "control" directory; I presume you have some other process creating the symlinks, so this could be amended to touch files instead), do appropriate generation
if the symlink/control file doesn't exist, send a 404 header and immediately exit
Another variation, slightly more efficient, would be to let Apache serve the cached image directly if it exists, and rewrite to PHP for steps 2 and 3. Something like this:
RewriteCond %{DOCUMENT_ROOT}%{REQUEST_FILENAME} !-f
RewriteRule /links/cache/(.*) generate-cache.php?path=$1
And in PHP
if ( ! file_exists('cache_control/' . $_GET['path'] )
{
header('HTTP/1.1 404 Not Found');
exit;
}
else
{
// Control file exists, so this is an allowable file; carry on...
generate_file_by_whatever_magic_you_have( 'links/cache/' . $_GET['path'] );
header('Content-Type: image/jpeg'); // May need to support different types
readfile( 'links/cache/' . $_GET['path'] );
exit;
}
Assuming you can replace the symlinks with control files, and the names match up directly (i.e. the target of your symlink can be "guessed" from its name), you could move the control file check into mod_rewrite as well:
# If the requested file doesn't exist (if it does, let Apache serve it)
RewriteCond %{DOCUMENT_ROOT}%{REQUEST_FILENAME} !-f
# Match the basic path format and capture image name into %1
RewriteCond %{REQUEST_FILENAME} /links/cache/(.*)
# Check if a cache control file exists with that image name
RewriteCond %{DOCUMENT_ROOT}/cache_control/%1 -f
# If so, serve via PHP; if not, no rewrite will happen, so Apache will return a 404
RewriteRule /links/cache/(.*) generate-cache.php?path=$1