The SDAM spec states the following for heartbeatFrequencyMS:

For both multi- and single-threaded drivers, the driver MUST NOT permit users to configure it less than minHeartbeatFrequencyMS (500ms).

MONGOC_TOPOLOGY_MIN_HEARTBEAT_FREQUENCY_MS is defined as 500, but that value is never used to validate the heartbeatFrequencyMS option. It is only referenced mongoc_topology_select_server_id() for computing the allowed scan time in single-threaded mode and _mongoc_topology_run_background() when computing a shorter interval for a requested scan.

Should libmongoc report some error when a value smaller than 500 is specified for heartbeatFrequencyMS? Alternatively, are invalid heartbeatFrequencyMS values ignored based on the aforementioned topology scanner code, which references MONGOC_TOPOLOGY_MIN_HEARTBEAT_FREQUENCY_MS?