fix: use sane sleep on primary coordinator main loop

The primary coordinator main loop shall not use zero sleep
when heartbeat has been missed. Use half of the heartbeat
interval in that case.

karapace/master_coordinator.py

@@ -227,6 +227,8 @@ def run(self) -> None:
             try:
                 if not self.kafka_client:
                     if self.init_kafka_client() is False:
+                        # If Kafka client is not initialized sleep a bit
+                        time.sleep(0.5)
                         continue
                 if not self.sc:
                     self.init_schema_coordinator()
@@ -236,7 +238,13 @@ def run(self) -> None:
                 self.sc.ensure_active_group()
                 self.sc.poll_heartbeat()
                 LOG.debug("We're master: %r: master_uri: %r", self.sc.are_we_master, self.sc.master_url)
-                time.sleep(min(_hb_interval, self.sc.time_to_next_heartbeat()))
+                # In cases when heartbeat is missed the sleep min sleep time would be 0
+                # from `time_to_next_heartbeat`. In that case halve the heartbeat interval for
+                # some sane sleep instead of running the loop without sleep for a while.
+                sleep_time = min(_hb_interval, self.sc.time_to_next_heartbeat())
+                if not sleep_time:
+                    sleep_time = _hb_interval / 2
+                time.sleep(sleep_time)
             except:  # pylint: disable=bare-except
                 LOG.exception("Exception in master_coordinator")
                 time.sleep(1.0)