[ccache] confusion with ccache, distcc, and ssh host key changes

[J. Bruce Fields]

Thanks to the recent Debian openssl discovery, all the host keys on my
build machines changed.  It took me an embarassingly long time to figure
out what why that made my distcc builds fail: ccache was caching
distcc's "host key verification failed" messages and returning them long
after I'd completely wiped out and replaced .ssh/known_hosts on the main
compile machine.  A 'ccache -C' solved the problem.

I probably should have figured out sooner why I kept getting complaints
about lines in known_hosts even after known_hosts was completely empty!
So, mainly this is a case of me being a little slow.  But it *was*
awfully confusing.

Would it be possible for ccache to be any smarter about this sort of
thing?  Or could I have configured this some way to avoid this (or make
the problem more obvious)?

It seems to me that the combination of distcc and ssh is probably prone
to errors that aren't really permanent, so perhaps this isn't the first
time this has happened.

--b.