How can you make ASDF stop trying to load a nonexistent file? - common-lisp

Question

On Debian, I had a bunch of cruft installed in /usr/lib/sbcl/site-systems that wouldn't load because the FASLs didn't match the version of SBCL that is actually installed.
For some reason, none of these files were associated with any Debian package (this is an old computer that has been running the same Debian install for over a decade– it's on Debian Sid).
I deleted the bad systems one at a time, and for most of them, Quicklisp did the right thing and downloaded the Quicklisp version. Sometimes, ASDF would insist that the system should exist at its previous path, but restarting SBCL got past that problem.
But for one system, ASDF has persistently cached the location of its .asd file as being in the /usr/lib/sbcl/site-systems/ directory. Loading this system is impossible because ASDF will not look anywhere else, even after restarting SBCL.
I tried looking in all the paths specified in various config files under /etc/common-lisp. None of those files contain a reference to the now-missing library.
I've resorted to doing a grep -rli across all the files under /usr. I don't expect that to complete in less than a day, and it might not find anything, in which case I'll be forced to grep the whole hard drive, which might take a whole week. Hopefully, the cache isn't compressed, because then I'll never find it.
Does anyone happen to know how ASDF persists the paths of files?

Answer 1

After a lot of excruciating debugging, I discovered that the files in /usr/lib/sbcl/site-systems/ actually do exist. They're broken symlinks.
The files I deleted were in the similar-looking path /usr/lib/sbcl/site/, to which the symlinks pointed.
Removing the symlinks fixed all the loading errors.

Answer 2

A couple of ideas about troubleshooting Quicklisp, particularly if your getting bizarre behavior.:
If you use Quicklisp for any length of time you'll probably eventually use local packages, found here by default, ~/quicklisp/local-projects It's valid to symlink you're projects into that directory. If you ever rename one of your projects, of course, don't forget to create a new symlink and delete the old one
Likewise, if you rename a local project, also delete the system index which Quicklisp will then recreate the next time it runs: ~/quicklisp/local-projects/system-index.txt Doesn't hurt to delete it from time to time just to keep your system fresh.
your *.fasl files can become stale too, deleting the system cache forces quicklisp to recompile everything. On an Ubuntu system running SBCL that would mean deleting the contents of:
rm -rf ~/.cache/common-lisp
Try updating the Quicklisp client
(ql:update-client)
Potentially deleting and reinstalling Quicklisp itself at ~/quicklisp can be necessary. (It's possible to inadvertently edit source files when your debugging and using Swanks lookup definition feature, breaking installed packages that used to work. Not that I would ever have done something as careless as that.)
Also, don't forget that ASDF decends into directories looking for *.asd files. If you have a stray one that's improperly structured that can cause havoc on you build system. (See my comment above about registering local projects to Quicklisp)
Finally, don't forget to check your lisp init file, e.g. .sbclrc for any debugging or quick and dirty hacks you might have left there and forgot about.
These are all things that have worked for me at one time or another, hopefully I'm not perpetuating legend and cant on things have have long since been fixed!