Initial head_only features

Initial commit to add head_only mode to leveled.  This allows leveled to receive batches of object changes, but where those objects exist only in the Penciller's Ledger (once they have been persisted within the Ledger).

The aim is to reduce significantly the cost of compaction.  Also, the objects ar enot directly accessible (they can only be accessed through folds).  Again this makes life easier during merging in the LSM trees (as no bloom filters have to be created).

test/end_to_end/tictac_SUITE.erl

@@ -8,7 +8,8 @@
             recent_aae_noaae/1,
             recent_aae_allaae/1,
             recent_aae_bucketaae/1,
-            recent_aae_expiry/1
+            recent_aae_expiry/1,
+            basic_headonly/1
             ]).
 
 all() -> [
@@ -17,7 +18,8 @@ all() -> [
             recent_aae_noaae,
             recent_aae_allaae,
             recent_aae_bucketaae,
-            recent_aae_expiry
+            recent_aae_expiry,
+            basic_headonly
             ].
 
 -define(LMD_FORMAT, "~4..0w~2..0w~2..0w~2..0w~2..0w").
@@ -1010,6 +1012,81 @@ recent_aae_expiry(_Config) ->
     true = length(DL4_0) == 0.
 
 
+basic_headonly(_Config) ->
+    % Load some AAE type objects into Leveled using the read_only mode.  This
+    % should allow for the items to be added in batches.  Confirm that the 
+    % journal is garbage collected as expected, and that it is possible to 
+    % perform a fold_heads style query 
+    ObjectCount = 100000,
+
+    RootPathHO = testutil:reset_filestructure("testHO"),
+    StartOpts1 = [{root_path, RootPathHO},
+                    {max_pencillercachesize, 16000},
+                    {sync_strategy, sync},
+                    {head_only, true}],
+    {ok, Bookie1} = leveled_bookie:book_start(StartOpts1),
+    {B1, K1, V1, S1, MD} = {"Bucket",
+                                "Key1.1.4567.4321",
+                                "Value1",
+                                [],
+                                [{"MDK1", "MDV1"}]},
+    {TestObject, TestSpec} = testutil:generate_testobject(B1, K1, V1, S1, MD),
+    {unsupported_message, put} = 
+        testutil:book_riakput(Bookie1, TestObject, TestSpec),
+    
+    ObjectSpecFun =
+        fun(Op) -> 
+            fun(N) ->
+                Bucket = <<"B", N:32/integer>>,
+                Key = <<"K", N:32/integer>>,
+                <<SegmentID:20/integer, _RestBS/bitstring>> = 
+                    crypto:hash(md5, term_to_binary({Bucket, Key})),
+                <<Hash:32/integer, _RestBN/bitstring>> =
+                    crypto:hash(md5, <<N:32/integer>>),
+                {Op, <<SegmentID:32/integer>>, Bucket, Key, Hash}
+            end 
+        end,
+    
+    ObjectSpecL = lists:map(ObjectSpecFun(add), lists:seq(1, ObjectCount)),
+    ok = load_objectspecs(ObjectSpecL, 32, Bookie1),
+
+    FoldFun = 
+        fun(_B, _K, V, {HashAcc, CountAcc}) ->
+            {HashAcc bxor V, CountAcc + 1}
+        end,
+    InitAcc = {0, 0},
+
+    RunnerDefinition = 
+        {foldheads_allkeys, h, {FoldFun, InitAcc}, false, false, false},
+    {async, Runner1} = 
+        leveled_bookie:book_returnfolder(Bookie1, RunnerDefinition),
+
+    SW1 = os:timestamp(),
+    {AccH1, AccC1} = Runner1(),
+    io:format("AccH and AccC of ~w ~w in ~w microseconds ~n", 
+                [AccH1, AccC1, timer:now_diff(os:timestamp(), SW1)]),
+
+    true = AccC1 == ObjectCount, 
+
+    ok = leveled_bookie:book_close(Bookie1).
+
+
+load_objectspecs([], _SliceSize, _Bookie) ->
+    ok;
+load_objectspecs(ObjectSpecL, SliceSize, Bookie) 
+                                    when length(ObjectSpecL) < SliceSize ->
+    load_objectspecs(ObjectSpecL, length(ObjectSpecL), Bookie);
+load_objectspecs(ObjectSpecL, SliceSize, Bookie) ->
+    {Head, Tail} = lists:split(SliceSize, ObjectSpecL),
+    case leveled_bookie:book_mput(Bookie, Head) of
+        ok ->
+            load_objectspecs(Tail, SliceSize, Bookie);
+        pause ->
+            timer:sleep(10),
+            load_objectspecs(Tail, SliceSize, Bookie)
+    end.
+
+
 
 load_and_check_recentaae(Book1A, Book1B, Book1C, Book1D,
                             SW_StartLoad, TreeSize, UnitMins,