mod_mirrorbrain does not handle filename correctly if inside of symlinked dir

[sme-gmbh]

Let's assume we have the following standard httpd setup with mirrorbrain activated:
DocumentRoot /var/www/htdocs/
FollowSymlinks yes

Inside of that directory we place a symlink pointing to a directory with longer path:
/var/www/htdocs/mysymlink -> /mnt/www/somelongdir/someotherdir/

We place file "foo.txt" in that folder and try to wget that from the webserver.

The following will happen:
mod_mirrorbrain tries to strip the DocumentRoot part of the filename.
But it does not strip "/mnt/storage/somedir/someotherdir/" as it should do, it strips the number of char according to the length of "/var/www/htdocs/".

This results in an invalid lookup path, giving a strange error log:

[Fri Dec 22 14:21:54.376008 2017] [mirrorbrain:notice] [pid 6000:tid 140026726733568] [client xx.xx.xx.xx:34122] [mod_mirrorbrain] 'ngdir/someotherdir/mysymlink/foo.txt': no usable mirrors after classification. Have to deliver directly.

The problem ist here in mod_mirrorbrain.c:
/* the leading directory needs to be stripped from the file path /
/ a directory from Apache always ends in '/'; a result from realpath() doesn't */
filename = realfile + strlen(mirror_base) + 1;

This does not care about symlinks.

[sme-gmbh]

This works at least for me:
https://github.com/sme-gmbh/mirrorbrain/tree/url-source-rework

[darix]

JFYI: If you dont object, I will pull this fix into gh:openSUSE/mirrorbrain. (which is our fork with all the fixes and features we did over the last few years)