Handling collision between standard i/o file descriptors and newly created ones

Though unlikely to happen, it may happen that newly created file
descriptor take the value 0 (stdin), 1 (stdout) or 2 (stderr) if one
of the standard ones have been dismissed in between. So, it may
confuse the program if it is unaware of this change.

The point of this patch is to avoid a reasign of standard file
descriptors on newly created ones.

Closes issue #16

glib/glib-unix.c

@@ -108,6 +108,17 @@ g_unix_open_pipe (int     *fds,
     ecode = pipe2 (fds, pipe2_flags);
     if (ecode == -1 && errno != ENOSYS)
       return g_unix_set_error_from_errno (error, errno);
+    /* Don't reassign pipes to stdin, stdout, stderr if closed meanwhile */
+    else if (fds[0] < 3 || fds[1] < 3)
+      {
+        int old_fds[2] = { fds[0], fds[1] };
+        gboolean result = g_unix_open_pipe (fds, flags, error);
+        close (old_fds[0]);
+        close (old_fds[1]);
+
+        if (!result)
+          g_unix_set_error_from_errno (error, errno);
+      }
     else if (ecode == 0)
       return TRUE;
     /* Fall through on -ENOSYS, we must be running on an old kernel */
@@ -116,6 +127,19 @@ g_unix_open_pipe (int     *fds,
   ecode = pipe (fds);
   if (ecode == -1)
     return g_unix_set_error_from_errno (error, errno);
+  /* Don't reassign pipes to stdin, stdout, stderr if closed meanwhile */
+  else if (fds[0] < 3 || fds[1] < 3)
+    {
+      int old_fds[2] = { fds[0], fds[1] };
+      gboolean result = g_unix_open_pipe (fds, flags, error);
+      close (old_fds[0]);
+      close (old_fds[1]);
+
+      if (!result)
+        g_unix_set_error_from_errno (error, errno);
+
+      return result;
+    }
 
   if (flags == 0)
     return TRUE;