bug#16361: compile cache confused about file identity

[Zefram]

The automatic cache of compiled versions of scripts in guile-2.0.9
identifies scripts mainly by name, and partially by mtime.  This is not
actually sufficient: it is easily misled by a pathname that refers to
different files at different times.  Test case:

$ echo '(display "aaa\n")' >t13
$ echo '(display "bbb\n")' >t14
$ guile-2.0 t13
;;; note: auto-compilation is enabled, set GUILE_AUTO_COMPILE=0
;;;       or pass the --no-auto-compile argument to disable.
;;; compiling /home/zefram/usr/guile/t13
;;; compiled 
/home/zefram/.cache/guile/ccache/2.0-LE-8-2.0/home/zefram/usr/guile/t13.go
aaa
$ mv t14 t13
$ guile-2.0 t13
aaa

You can see that the mtime is not fully used here: the cache is misapplied
even if there is a delay of seconds between the creations of the two
script files.  The cache's mtime check will only notice a mismatch if
the script currently seen under the supplied name was modified later
than when the previous script was *compiled*.

Obviously, in this test case the cache could trivially distinguish the
two script files by looking at the inode numbers.  On its own the inode
number isn't sufficient, but exact match on device, inode number, and
mtime would be far superior to the current behaviour, only going wrong
in the presence of deliberate timestamp manipulation.  As a bonus, if
the cache were actually *keyed* by inode number and device, rather than
by pathname, it would retain the caching of compilation across renamings
of the script.

Or, even better, the cache could be keyed by a cryptographic hash of the
file contents.  This would be immune even to timestamp manipulation, and
would preserve the cached compilation even across the script being copied
to a fresh file or being edited and reverted.  This would be a cache
worthy of the name.  The only downside is the expense of computing the
hash, but I expect this is small compared to the expense of compilation.

Debian incarnation of this bug report:
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=734178

-zefram