extmod/nimble: Improve the flow control for l2cap recv path.

If the _IRQ_L2CAP_RECV handler does the actual consumption of the incoming
data (i.e. via l2cap_recvinto), rather than setting a flag for
non-scheduler-context to handle it later, then two things can happen:

- It can starve the VM (i.e. the scheduled task never terminates).  This is
  because calling l2cap_recvinto will empty the rx buffer, which will grant
  more credits to the channel (an HCI command), meaning more data can
  arrive.  This means that the loop in hal_uart.c that keeps reading HCI
  data from the uart and executing NimBLE events as they are created will
  not terminate, preventing other VM code from running.

- There's no flow control (i.e. data will arrive too quickly).  The channel
  shouldn't be given credits until after we return from scheduler context.

It's preferable that no work is done in scheduler/IRQ context.  But to
prevent this being a problem this commit changes l2cap_recvinto so that if
it is called in IRQ context, and the Python handler empties the rx buffer,
then don't grant credits until the Python handler is complete.
Signed-off-by: Jim Mussared <jim.mussared@gmail.com>

extmod/nimble/modbluetooth_nimble.c

@@ -1355,6 +1355,7 @@ typedef struct _mp_bluetooth_nimble_l2cap_channel_t {
     struct os_mbuf_pool sdu_mbuf_pool;
     struct os_mempool sdu_mempool;
     struct os_mbuf *rx_pending;
+    bool irq_in_progress;
     uint16_t mtu;
     os_membuf_t sdu_mem[];
 } mp_bluetooth_nimble_l2cap_channel_t;
@@ -1441,7 +1442,23 @@ STATIC int l2cap_channel_event(struct ble_l2cap_event *event, void *arg) {
             chan->chan->coc_rx.sdu = sdu_rx;
 
             ble_l2cap_get_chan_info(event->receive.chan, &info);
+
+            // Don't allow granting more credits until after the IRQ is handled.
+            chan->irq_in_progress = true;
+
             mp_bluetooth_gattc_on_l2cap_recv(event->receive.conn_handle, info.scid);
+            chan->irq_in_progress = false;
+
+            // If all data has been consumed by the IRQ handler, then now allow
+            // more credits. If the IRQ handler doesn't consume all available data
+            // then rx_pending will be still set.
+            if (!chan->rx_pending) {
+                struct os_mbuf *sdu_rx = chan->chan->coc_rx.sdu;
+                assert(sdu_rx);
+                if (sdu_rx) {
+                    ble_l2cap_recv_ready(chan->chan, sdu_rx);
+                }
+            }
             break;
         }
         case BLE_L2CAP_EVENT_COC_TX_UNSTALLED: {
@@ -1508,6 +1525,7 @@ STATIC int create_l2cap_channel(uint16_t mtu, mp_bluetooth_nimble_l2cap_channel_
 
     chan->mtu = mtu;
     chan->rx_pending = NULL;
+    chan->irq_in_progress = false;
 
     int err = os_mempool_init(&chan->sdu_mempool, buf_blocks, L2CAP_BUF_BLOCK_SIZE, chan->sdu_mem, "l2cap_sdu_pool");
     if (err != 0) {
@@ -1635,13 +1653,17 @@ int mp_bluetooth_l2cap_recvinto(uint16_t conn_handle, uint16_t cid, uint8_t *buf
                 os_mbuf_free_chain(chan->rx_pending);
                 chan->rx_pending = NULL;
 
-                // We've already given the channel a new mbuf in l2cap_channel_event above, so
-                // re-use that mbuf in the call to ble_l2cap_recv_ready. This will just
-                // give the channel more credits.
-                struct os_mbuf *sdu_rx = chan->chan->coc_rx.sdu;
-                assert(sdu_rx);
-                if (sdu_rx) {
-                    ble_l2cap_recv_ready(chan->chan, sdu_rx);
+                // If we're in the call stack of the l2cap_channel_event handler, then don't
+                // re-enable receiving yet (as we need to complete the rest of IRQ handler first).
+                if (!chan->irq_in_progress) {
+                    // We've already given the channel a new mbuf in l2cap_channel_event above, so
+                    // re-use that mbuf in the call to ble_l2cap_recv_ready. This will just
+                    // give the channel more credits.
+                    struct os_mbuf *sdu_rx = chan->chan->coc_rx.sdu;
+                    assert(sdu_rx);
+                    if (sdu_rx) {
+                        ble_l2cap_recv_ready(chan->chan, sdu_rx);
+                    }
                 }
             } else {
                 // Trim the used bytes from the start of the mbuf.