Input: Ensure clearTimer won't compute a lower nfds value than one of…

… our opened fds (#1685)

By making sure in openInputDevice that the top member of the array also happens to be the highest fd number.

input/input.c

@@ -170,6 +170,16 @@ static int openInputDevice(lua_State* L)
         }
     }
 
+    // NOTE: Make sure the top member has the highest fd number, for clearTimer's sake when it recomputes nfds
+    if (fd_idx > 0) {
+        int prev_fd   = inputfds[fd_idx - 1];
+        int opened_fd = inputfds[fd_idx];
+        if (opened_fd < prev_fd) {
+            inputfds[fd_idx - 1] = opened_fd;
+            inputfds[fd_idx]     = prev_fd;
+        }
+    }
+
     // We're done w/ inputdevice, pop it
     lua_settop(L, 0);
     // Pass the fd to Lua, we might need it for FFI ioctl shenanigans

input/timerfd-callbacks.h

@@ -172,7 +172,9 @@ static int clearTimer(lua_State* L)
     timerfd_list_delete_node(&timerfds, expired_node);
 
     // Re-compute nfds...
-    nfds = inputfds[fd_idx - 1U] + 1;  // NOTE: Assumes that we've got at least one fd open, which should always hold true.
+    // NOTE: Assumes that we've got at least one fd open, which should always hold true.
+    // NOTE: Also assumes that the top fd in the array is the one with the highest fd number, which openInputDevice makes sure of.
+    nfds = inputfds[fd_idx - 1U] + 1;
     for (timerfd_node_t* restrict node = timerfds.head; node != NULL; node = node->next) {
         if (node->fd >= nfds) {
             nfds = node->fd + 1;