Do not report failure for empty appends

Summary:
Stop reporting an error if we fail when handling an empty append that
fail because the previous log entry is missing. This log entry can
create excessive log entries when clusters are malfunctioning.

Reviewed By: jaher

Differential Revision: D65510926

fbshipit-source-id: 88231b0e1700080461c976a27b15233f76679934

src/wa_raft_server.erl

@@ -2559,8 +2559,9 @@ append_entries(State, PrevLogIndex, PrevLogTerm, Entries, EntryCount, #raft_stat
             % If the log entry is not found, then ignore and notify the leader of what log entry
             % is required by this follower in the reply.
             ?RAFT_COUNT({raft, State, 'heartbeat.skip.missing_previous_log_entry'}),
-            ?LOG_WARNING("Server[~0p, term ~0p, ~0p] skips appending ~0p log entries in range ~0p to ~0p because previous log entry at ~0p is not available in local log covering ~0p to ~0p.",
-                [Name, CurrentTerm, State, EntryCount, PrevLogIndex + 1, PrevLogIndex + EntryCount, PrevLogIndex, wa_raft_log:first_index(View), wa_raft_log:last_index(View)], #{domain => [whatsapp, wa_raft]}),
+            EntryCount =/= 0 andalso
+                ?LOG_WARNING("Server[~0p, term ~0p, ~0p] skips appending ~0p log entries in range ~0p to ~0p because previous log entry at ~0p is not available in local log covering ~0p to ~0p.",
+                    [Name, CurrentTerm, State, EntryCount, PrevLogIndex + 1, PrevLogIndex + EntryCount, PrevLogIndex, wa_raft_log:first_index(View), wa_raft_log:last_index(View)], #{domain => [whatsapp, wa_raft]}),
             {ok, false, wa_raft_log:last_index(View), Data};
         {error, Reason} ->
             ?RAFT_COUNT({raft, State, 'heartbeat.skip.failed_to_read_previous_log_entry'}),