usbd: don't leak the queued-setup counter when the event queue is full

A SETUP arriving while the event queue is full is silently dropped by
queue_event(), but _usbd_queued_setup has already been incremented. The
leaked count makes the event handler skip every subsequent SETUP
("Skipped since there is other SETUP in queue") forever: EP0 stays deaf
until tud_init() while the device otherwise looks alive - enumerated,
endpoints armed. Undo the increment when the enqueue fails.

Unit test: fill the queue so a SETUP is dropped, then verify the next
SETUP still completes a GET_DESCRIPTOR control transfer.

1 files changed, 4 insertions, 2 deletions

diff --git a/src/device/usbd.c b/src/device/usbd.c
index 5471e132d..b77b766dd 100644
--- a/src/device/usbd.c
+++ b/src/device/usbd.c
@@ -1473,8 +1473,10 @@ TU_ATTR_FAST_FUNC void dcd_event_handler(dcd_event_t const* event, bool in_isr)
       break;
   }
 
-  if (send) {
-    queue_event(event, in_isr);
+  if (send && !queue_event(event, in_isr) && event->event_id == DCD_EVENT_SETUP_RECEIVED) {
+    // dropped by a full queue: undo the increment, else every later SETUP is skipped as
+    // "other SETUP in queue" and EP0 is deaf until re-init
+    _usbd_queued_setup--;
   }
 }