Allow for setting max_keys to wrap Acc

Acc in response is now of form {Reason, Acc} not just Acc so that the application can understand the reason for the results ending - and take appropriate action (e.g. restart again from the LastKey to return more results).

src/leveled_bookie.erl

@@ -906,12 +906,20 @@ book_headfold(Pid, Tag, {range, Bucket, KeyRange}, FoldAccT, JournalCheck, SnapP
     book_returnfolder(Pid, RunnerType).
 
 %% @doc as book_headfold/7, but with the addition of a Last Modified Date
-%% Range and Max Head Count.  For version 2 objects this will filter out
+%% Range and Max Object Count.  For version 2 objects this will filter out
 %% all objects with a highest Last Modified Date that is outside of the range.
 %% All version 1 objects will be included in the result set regardless of Last
 %% Modified Date.
-%% The Max Head Count will stop the fold once the count has been reached on
+%% The Max Object Count will stop the fold once the count has been reached on
-%% this store only
+%% this store only.  The Max Object Count if provided will mean that on
+%% completion of the fold the accumulator will be wrapped in a tuple to
+%% indicate the reason for completion:
+%% - {no_more_keys, Acc} if the end of the range was reached wihtout hitting
+%% the Max Object Count limit
+%% - {max_count, Acc} if the Max Object Count limit was reached before 
+%% reaching the end of the range
+%% If MaxObjectCount is false then the Acc will be returned not wrapped in a
+%% tuple
 -spec book_headfold(pid(), Tag, Limiter, FoldAccT, JournalCheck, SnapPreFold,
                         SegmentList, LastModRange, MaxObjectCount) ->
                            {async, Runner} when

src/leveled_penciller.erl

@@ -1405,14 +1405,21 @@ keyfolder(IMMiter, SSTiter, StartKey, EndKey, {AccFun, Acc}) ->
 
 keyfolder(_Iterators, _KeyRange, {_AccFun, Acc}, 
                     {_SegmentList, _LastModRange, MaxKeys}) when MaxKeys == 0 ->
-    Acc;
+    {max_count, Acc};
 keyfolder({[], SSTiter}, KeyRange, {AccFun, Acc}, 
                     {SegmentList, LastModRange, MaxKeys}) ->
     {StartKey, EndKey} = KeyRange,
     case find_nextkey(SSTiter, StartKey, EndKey, 
                         SegmentList, element(1, LastModRange)) of
         no_more_keys ->
-            Acc;
+            case MaxKeys > 0 of
+                true ->
+                    % Need to single this query ended not because the
+                    % MaxKeys was reached
+                    {no_more_keys, Acc};
+                false ->
+                    Acc
+            end;
         {NxSSTiter, {SSTKey, SSTVal}} ->
             {Acc1, MK1} = 
                 maybe_accumulate(SSTKey, SSTVal, Acc, AccFun,

src/leveled_runner.erl

@@ -461,10 +461,10 @@ get_nextbucket(NextBucket, NextKey, Tag, LedgerSnapshot, BKList, {C, L}) ->
                                                     ExtractFun,
                                                     null),
     case R of
-        null ->
+        {no_more_keys, null} ->
             leveled_log:log("B0008",[]),
             BKList;
-        {{B, K}, V} ->
+        {_, {{B, K}, V}} ->
             case leveled_codec:is_active({Tag, B, K, null}, V, Now) of
                 true ->
                     leveled_log:log("B0009",[B]),