fixup: make blocking client func fail on unicoap thread

Author: carl-tud

sys/net/application_layer/unicoap/client/client.c

@@ -328,6 +328,24 @@ int unicoap_send_request_sync_copy(unicoap_message_t* request,
     assert(buffer);
     assert(buffer_capacity > 0);
     assert(response);
+
+    /* Prevent this function from deadlocking the unicoap thread. No one besides the unicoap
+     * thread can unlock the mutex below (via the _sync_callback). You cannot have
+     * a function that waits for the response to arrive while blocking the very same
+     * thread would process the response.
+     *
+     * This is particularly true for applications running unicoap in sync mode. */
+    if (thread_getpid() == _unicoap_pid) {
+        if (IS_ACTIVE(CONFIG_UNICOAP_ASSIT)) {
+            unicoap_assist(API_MISUSE("Attempted to call blocking '%s' on unicoap thread, "
+                                      "would block processing loop")
+                               FIXIT("Use 'unicoap_send_request_async' instead"),
+                           __func__);
+        }
+        _CLIENT_DEBUG("Attempted to open request on unicoap thread, not blocking.");
+        assert(false);
+        return -1;
+    }
 
     _sync_copy_args_t args = (_sync_copy_args_t) {
         .response = response,

sys/net/application_layer/unicoap/include/private/state.h

@@ -153,6 +153,7 @@ void unicoap_event_cancel(unicoap_scheduled_event_t* event);
 *          messaging-layer state for transmissions. */
 #define UNICOAP_HAVE_MESSAGING_STATE \
     IS_USED(MODULE_UNICOAP_DRIVER_RFC7252_COMMON)
+/* MARK: unicoap_driver_extension_point */
 
 /**
  * @brief A type used to retain state spanning across a single or multiple message exchanges