bug#15648: 24.2.50; gnutls SSL connection to IMAP server causes emacs to crash completely

[Stefan Monnier]

>    Specifically, gnutls-negotiate uses the Magic File associated
>    primitive file-exists-p to test for the existence of a file but
>    then passes off the unmodified (and non-existent) Magic File path

Good point.  The problem is the use of file-exists-p (which tests if
Elisp code can reach this file) instead of another function along the
lines of file-accessible-directory-p, which checks whether the file name
is reachable to C-level system primitives.

"Unmodified" is not a real problem, although we might want to use some
function to "canonicalize" the file name so as to increase the chances
that the file is reachable to C-level system primitives.

>    either (1) by using the Magic File aware primitive expand-file-name

expand-file-name is not the right function, even if in some cases it may
happen to help.

> 2. At the C-code level, there is a bug in that passing a
>    non-existent/invalid file path may cause the entire Emacs code to
>    crash.

Right, this is the most serious of the two problems.


        Stefan