4ba012a8ed9c defined the "header" (pointer to the stats data) of from_serialized_data() as a const, even though it is fine (and expected!) for the callback to modify the shared memory entry when loading the stats at startup. While on it, this commit updates the callback to_serialized_data() in the test module test_custom_stats to make the data extracted from the "header" parameter a const since it should never be modified: the stats are written to disk and no modifications are expected in the shared memory entry. This clarifies the API contract of these new callbacks. Reported-By: Peter Eisentraut <peter@eisentraut.org> Author: Michael Paquier <michael@paquier.xyz> Co-authored-by: Sami Imseih <samimseih@gmail.com> Discussion: https://postgr.es/m/d87a93b0-19c7-4db6-b9c0-d6827e7b2da1@eisentraut.org