Ensure background tasks are serviced during a lengthy sendall

Author: jepler

shared-bindings/socketpool/Socket.c

@@ -283,6 +283,13 @@ STATIC mp_obj_t _socketpool_socket_sendall(mp_obj_t self_in, mp_obj_t buf_in) {
         }
         bufinfo.len -= ret;
         bufinfo.buf += ret;
+        if (bufinfo.len > 0) {
+            RUN_BACKGROUND_TASKS;
+            // Allow user to break out of sendall with a KeyboardInterrupt.
+            if (mp_hal_is_interrupted()) {
+                return 0;
+            }
+        }
     }
     return mp_const_none;
 }