[nrf fromlist] Bluetooth: Host: Invoke tx callbacks when channel is deleted

When bt_l2cap_send_pdu() succeeds, it transfers buffer ownership to the
stack, which must eventually invoke the provided callback. This contract
is honored in all paths where transmission becomes impossible:

- Normal transmission: callback invoked with err=0 after HCI Number of
  Completed Packets event (tx_notify_process)
- Send errors (after tx allocated): callback invoked with err=-ESHUTDOWN
  via conn_tx_destroy
- Send errors (before tx allocated): callback invoked with the specific
  error code in send_buf error_return path
- Connection disconnect: callbacks invoked with err=-ESHUTDOWN via
  process_unack_tx -> conn_tx_destroy for all PDUs in tx_pending

However, when a channel is deleted (l2cap_chan_del), PDUs remaining in
the tx_queue are dropped without invoking their callbacks, violating the
ownership contract.

Fix this by extracting and invoking any non-NULL callbacks from the
closure stored in buf->user_data before releasing the buffers. The
callback is invoked with err=-ESHUTDOWN, making this path analogous to
process_unack_tx: both drain queues of unsent PDUs when transmission
becomes impossible due to external events (channel deletion vs connection
disconnect). The only difference is the buffer lifecycle stage - in
l2cap_chan_del, PDUs are still in tx_queue (closure in buf->user_data),
while in process_unack_tx, they've progressed to tx_pending (callback in
bt_conn_tx struct).

Note: conn_tx_destroy() cannot be used here because no bt_conn_tx struct
has been allocated yet - the closure is still in buf->user_data.

Upstream PR #: 97056

Signed-off-by: Aleksander Wasaznik <[email protected]>

Author: alwa-nordic

subsys/bluetooth/host/l2cap.c

@@ -273,6 +273,21 @@ static void l2cap_chan_del(struct bt_l2cap_chan *chan)
 	 * `l2cap_chan_destroy()` as it is not called for fixed channels.
 	 */
 	while ((buf = k_fifo_get(&le_chan->tx_queue, K_NO_WAIT))) {
+		bt_conn_tx_cb_t cb = closure_cb(buf->user_data);
+
+		if (cb) {
+			void *user_data = closure_data(buf->user_data);
+
+			/* When bt_l2cap_send_pdu() succeeds, the stack takes ownership
+			 * and must invoke the callback eventually. Since these PDUs will
+			 * never be transmitted, invoke the callback now with an error.
+			 * Note: We cannot use conn_tx_destroy() here because no bt_conn_tx
+			 * struct has been allocated yet - the closure is still in the
+			 * buf->user_data.
+			 */
+			cb(chan->conn, user_data, -ESHUTDOWN);
+		}
+
 		net_buf_unref(buf);
 	}