ports/sh: fix AC/ON locking after some time

The Python program being run is in charge of keyboard events with the
gint module. Most programs don't care though, and simply let events
accumulate until the queue is full.

The async filter is able to receive events even when the queue is full.
However, it filtered only AC/ON presses, not releases, so the releases
were sent back to the driver to queue. This was impossible as the queue
was full, so the release was never recorded. This failure then repeated
at every tick, forever.

Since the key was never properly released, further presses were just
seen as a continuation of the current press and thus did not produce
any new event, so the async filter was no longer called and the Python
program could no longer be interrupted.

README.md

@@ -20,6 +20,8 @@ Bugs to fix:
 - Fix not resetting the shell when importing a file from command-line
 - Fix current working directory not changing during a module import (for
   relative imports)
+- Fix accumulated events being processed when the program "ends" (if we can
+  detect that)
 
 Python features:
 - Compare features with existing implementations and other brands

ports/sh/main.c

@@ -113,13 +113,17 @@ static char *path_to_module(char const *path)
    interrupt MicroPython instead. */
 static bool async_filter(key_event_t ev)
 {
-    if(mp_interrupt_char < 0)
-        return true;
-    if(ev.type == KEYEV_DOWN && ev.key == KEY_ACON) {
-        /* This function is designed to be called asynchronously. */
-        mp_sched_keyboard_interrupt();
+    /* Gobble all events related to AC/ON to make sure that the keyboard driver
+       treats them as handled. Otherwise, we'd run the risk of filling the
+       event queue (if the user doesn't read from it) thus preventing the
+       driver from handling AC/ON releases, which disables further presses. */
+    if(mp_interrupt_char >= 0 && ev.key == KEY_ACON) {
+        /* This function supports asynchronous calls, by design. */
+        if(ev.type == KEYEV_DOWN)
+            mp_sched_keyboard_interrupt();
         return false;
     }
+
     return true;
 }