TODO

BUGS TO FIX TO MAKE IT ACTUALLY WORK
- Even if apache:apache owns DocumentRoot, SELinux denies by default.
  Todd Zullinger suggested:
  semanage fcontext -a -t httpd_sys_content_t "/InstantMirrorPath(/.*)?"
  Investigate this.
  Unfortunately, we have no automatable solution to this.

TODO
- If multiple InstantMirror instances are downloading a large file, the
  first instance (the master) appends to the local file while the other
  instances (slaves) just copy from the local file to their clients.
  The current implementation is lame and slaves are throttled by the
  download speed of the master's client.  With clever use of select,
  the master avoid blocking on writes to its client, allowing it to
  download the upstream URL and append to the local file as fast as
  possible.
- Implement upstream fallback
- Check files in local cache against upstream web server.  If 404, delete locally.
- Parse repomd.xml and other *.xml to intelligently know when to expire files
  instead of querying the upstream server during every client download.
- Offline mode: if the upstream server is unavailable but the file exists locally,
  InstantMirror lets Apache serve the local file.  This doesn't work so well for
  directory requests, though.