bug#38396: internal fds leak across exec

[Zefram]

Where Guile opens a file in order to read code from it, that file
descriptor ought to be private to Guile, in the sense that it should
not be passed on to another program across exec.  But it is so leaked,
as can be seen especially clearly using Linux's /proc/*/fd feature:

$ cat /tmp/t0
(system
  (string-append "ls -l /proc/" (number->string (getpid))
    "/fd; ls -l /proc/self/fd"))
$ guile-2.2.6 --no-auto-compile -s /tmp/t0
total 0
lrwx------ 1 zefram zefram 64 Nov 27 05:21 0 -> /dev/pts/4
lrwx------ 1 zefram zefram 64 Nov 27 05:21 1 -> /dev/pts/4
lrwx------ 1 zefram zefram 64 Nov 27 05:21 2 -> /dev/pts/4
lr-x------ 1 zefram zefram 64 Nov 27 05:21 3 -> pipe:[59357425]
l-wx------ 1 zefram zefram 64 Nov 27 05:21 4 -> pipe:[59357425]
lr-x------ 1 zefram zefram 64 Nov 27 05:21 5 -> pipe:[59357426]
l-wx------ 1 zefram zefram 64 Nov 27 05:21 6 -> pipe:[59357426]
lr-x------ 1 zefram zefram 64 Nov 27 05:21 7 -> /tmp/t0
total 0
lrwx------ 1 zefram zefram 64 Nov 27 05:21 0 -> /dev/pts/4
lrwx------ 1 zefram zefram 64 Nov 27 05:21 1 -> /dev/pts/4
lrwx------ 1 zefram zefram 64 Nov 27 05:21 2 -> /dev/pts/4
lr-x------ 1 zefram zefram 64 Nov 27 05:21 3 -> /proc/3649/fd
lr-x------ 1 zefram zefram 64 Nov 27 05:21 7 -> /tmp/t0
$

Observe that the Guile process has open both the script file and a couple
of pipes.  The ls process also has a file descriptor on the script file,
though it does not have the pipes open.  Clearly Guile has leaked the
script fd, but has avoided leaking the pipe fds.

The script fd ought to have the FD_CLOEXEC flag set.  Preferably,
that flag should be set immediately upon opening the file, by means of
the O_CLOEXEC flag.  I note that pipes opened by the thread system are
already opened with O_CLOEXEC.

Guile 2.0 behaves the same as 2.2 in this respect.  Guile 1.8 leaks
both the script fd and the pipe fds.  Guile 1.6 doesn't open any pipes
in this test, and also leaks the script fd.

-zefram