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.

2 files changed, 42 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--;
   }
 }
 
diff --git a/test/unit-test/test/device/usbd/test_usbd.c b/test/unit-test/test/device/usbd/test_usbd.c
index 7f3c3f5b2..935a20221 100644
--- a/test/unit-test/test/device/usbd/test_usbd.c
+++ b/test/unit-test/test/device/usbd/test_usbd.c
@@ -271,6 +271,44 @@ void test_usbd_control_in_zlp(void)
 }
 
 //--------------------------------------------------------------------+
+// SETUP dropped by full event queue
+//--------------------------------------------------------------------+
+
+// When the event queue is full, queue_event() drops the SETUP event. The queued-setup
+// counter must not keep the dropped SETUP's increment: a leaked count makes the handler
+// skip every later SETUP ("other SETUP in queue") forever, leaving EP0 permanently deaf.
+void test_usbd_setup_dropped_by_full_queue_recovers(void)
+{
+  // fillers drain through usbd_reset -> class reset
+  mscd_reset_Ignore();
+
+  // fill the queue to the brim, then post one more SETUP: queue_event() drops it
+  for (unsigned i = 0; i < CFG_TUD_TASK_QUEUE_SZ; i++) {
+    dcd_event_bus_signal(rhport, DCD_EVENT_UNPLUGGED, false);
+  }
+  dcd_event_setup_received(rhport, (uint8_t*) &req_get_desc_device, false);
+
+  // drain all fillers (each tud_task pass handles at most CFG_TUD_TASK_EVENTS_PER_RUN
+  // events); the dropped SETUP never arrives
+  for (unsigned i = 0; i < (CFG_TUD_TASK_QUEUE_SZ / CFG_TUD_TASK_EVENTS_PER_RUN) + 1; i++) {
+    tud_task();
+  }
+
+  // the next SETUP must still be answered
+  desc_device = (uint8_t const*) &data_desc_device;
+  dcd_event_setup_received(rhport, (uint8_t*) &req_get_desc_device, false);
+
+  dcd_edpt_xfer_ExpectWithArrayAndReturn(rhport, 0x80, (uint8_t*) &data_desc_device, sizeof(tusb_desc_device_t), sizeof(tusb_desc_device_t), false, true);
+  dcd_event_xfer_complete(rhport, EDPT_CTRL_IN, sizeof(tusb_desc_device_t), 0, false);
+
+  dcd_edpt_xfer_ExpectAndReturn(rhport, EDPT_CTRL_OUT, NULL, 0, false, true);
+  dcd_event_xfer_complete(rhport, EDPT_CTRL_OUT, 0, 0, false);
+  dcd_edpt0_status_complete_ExpectWithArray(rhport, &req_get_desc_device, 1);
+
+  tud_task();
+}
+
+//--------------------------------------------------------------------+
 // Control OUT data stage host overrun
 //--------------------------------------------------------------------+