add fold/3 to wterl.erl and fix dialyzer warnings

The backend fold_objects was failing because it was trying to use
wterl:fold_keys/3, which doesn't fold over values. Add fold/3 to
wterl.erl to fix this.

Also clean up a few dialyzer warnings.

src/riak_kv_wterl_backend.erl

@@ -246,7 +246,7 @@ fold_objects(FoldObjectsFun, Acc, Opts, #state{conn=ConnRef, table=Table}) ->
                 {ok, SRef} = wterl:session_open(ConnRef),
                 {ok, Cursor} = wterl:cursor_open(SRef, Table),
                 try
-                    wterl:fold_keys(Cursor, FoldFun, Acc)
+                    wterl:fold(Cursor, FoldFun, Acc)
                 catch
                     {break, AccFinal} ->
                         AccFinal
@@ -265,8 +265,12 @@ fold_objects(FoldObjectsFun, Acc, Opts, #state{conn=ConnRef, table=Table}) ->
 %% @doc Delete all objects from this wterl backend
 -spec drop(state()) -> {ok, state()} | {error, term(), state()}.
 drop(#state{table=Table, session=SRef}=State) ->
-    ok = wterl:session_truncate(SRef, Table),
-    {ok, State}.
+    case wterl:session_truncate(SRef, Table) of
+        ok ->
+            {ok, State};
+        Error ->
+            {error, Error, State}
+    end.
 
 %% @doc Returns true if this wterl backend contains any
 %% non-tombstone values; otherwise returns false.

src/wterl.erl

@@ -58,7 +58,8 @@
          session_verify/2,
          session_verify/3,
          config_to_bin/1,
-         fold_keys/3]).
+         fold_keys/3,
+         fold/3]).
 
 -type config() :: binary().
 -type config_list() :: [{atom(), any()}].
@@ -92,7 +93,7 @@ init() ->
                   Path ->
                       Path
               end,
-    erlang:load_nif(filename:join(PrivDir, ?MODULE), 0).
+    erlang:load_nif(filename:join(PrivDir, atom_to_list(?MODULE)), 0).
 
 -spec conn_open(string(), config()) -> {ok, connection()} | {error, term()}.
 conn_open(_HomeDir, _Config) ->
@@ -102,7 +103,7 @@ conn_open(_HomeDir, _Config) ->
 conn_close(_ConnRef) ->
     ?nif_stub.
 
--spec session_open(connection()) -> {ok, reference()} | {error, term()}.
+-spec session_open(connection()) -> {ok, session()} | {error, term()}.
 session_open(_ConnRef) ->
     ?nif_stub.
 
@@ -234,7 +235,9 @@ cursor_update(_Cursor, _Key, _Value) ->
 cursor_remove(_Cursor, _Key, _Value) ->
     ?nif_stub.
 
--spec fold_keys(cursor(), fun(), any()) -> any().
+-type fold_keys_fun() :: fun((Key::binary(), any()) -> any()).
+
+-spec fold_keys(cursor(), fold_keys_fun(), any()) -> any().
 fold_keys(Cursor, Fun, Acc0) ->
     fold_keys(Cursor, Fun, Acc0, cursor_next_key(Cursor)).
 fold_keys(_Cursor, _Fun, Acc, not_found) ->
@@ -242,6 +245,15 @@ fold_keys(_Cursor, _Fun, Acc, not_found) ->
 fold_keys(Cursor, Fun, Acc, {ok, Key}) ->
     fold_keys(Cursor, Fun, Fun(Key, Acc), cursor_next_key(Cursor)).
 
+-type fold_fun() :: fun(({Key::binary(), Value::binary()}, any()) -> any()).
+
+-spec fold(cursor(), fold_fun(), any()) -> any().
+fold(Cursor, Fun, Acc0) ->
+    fold(Cursor, Fun, Acc0, cursor_next(Cursor)).
+fold(_Cursor, _Fun, Acc, not_found) ->
+    Acc;
+fold(Cursor, Fun, Acc, {ok, Key, Value}) ->
+    fold(Cursor, Fun, Fun({Key, Value}, Acc), cursor_next(Cursor)).
 
 %%
 %% Configuration type information.

src/wterl_conn.erl

@@ -36,7 +36,7 @@
 -export([init/1, handle_call/3, handle_cast/2, handle_info/2,
          terminate/2, code_change/3]).
 
--record(state, {conn :: reference(),
+-record(state, {conn :: wterl:connection(),
                 monitors :: ets:tid()}).
 
 %% ====================================================================