gspawn: Use close_range() if available to close FDs between fork/exec

It’s landed in kernel 5.9: http://lkml.iu.edu/hypermail/linux/kernel/2008.0/02649.html

Note, this is untested because I currently don’t have kernel 5.9. We can
fix anything up if it breaks once the new syscall is wrapped in glibc.

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>

glib/gspawn.c

@@ -1327,6 +1327,11 @@ safe_closefrom (int lowfd)
    * simple wrapper of the fcntl command.
    */
   (void) fcntl (lowfd, F_CLOSEM);
+#elif defined(HAVE_CLOSE_RANGE)
+  /* close_range() is available in Linux since kernel 5.9, and on FreeBSD at
+   * around the same time. It was designed for use in async-signal-safe
+   * situations: https://bugs.python.org/issue38061 */
+  (void) close_range (lowfd, G_MAXUINT);
 #else
   (void) safe_fdwalk (close_func, GINT_TO_POINTER (lowfd));
 #endif

meson.build

@@ -473,6 +473,7 @@ if host_system == 'windows'
 endif
 
 functions = [
+  'close_range',
   'endmntent',
   'endservent',
   'fallocate',