Fix and improve couch_btree testing

Previously, it turns out, we didn't actually test the reverse sorted keys.
Instead we just renamed the `Reversed = Sorted` and then tested the `Sorted`
case.

But it's even worse than that because in order to "force" the test to pass we
had effectively disable the check so make it count element whether they were
there or not (Count + 1) in this snippet:

```diff
    FoldFun = fun(Element, {[HAcc | TAcc], Count}) ->
         case Element == HAcc of
             true -> {ok, {TAcc, Count + 1}};
-            _ -> {ok, {TAcc, Count + 1}}
+            _ -> {ok, {TAcc, Count}}
         end
     end,
```

Luckily, it seems we survived with the code being correct as far as fixing the
test didn't reveal any actual btree logic errors.

In addition, we were missing tests trying out various chunk sizes, so added two
extra cases: one with much smaller chunks, and another with much larger chunks.

Author: nickva

src/couch/test/eunit/couch_btree_tests.erl

@@ -38,6 +38,26 @@ setup_kvs_with_cache(_) ->
     ]),
     {Fd, Btree}.
 
+setup_kvs_with_small_chunk_size(_) ->
+    % Less than a 1/4 than current default 1279
+    config:set("couchdb", "btree_chunk_size", "300", false),
+    {ok, Fd} = couch_file:open(?tempfile(), [create, overwrite]),
+    {ok, Btree} = couch_btree:open(nil, Fd, [
+        {compression, none},
+        {reduce, fun reduce_fun/2}
+    ]),
+    {Fd, Btree}.
+
+setup_kvs_with_large_chunk_size(_) ->
+    % About 4x than current default 1279
+    config:set("couchdb", "btree_chunk_size", "5000", false),
+    {ok, Fd} = couch_file:open(?tempfile(), [create, overwrite]),
+    {ok, Btree} = couch_btree:open(nil, Fd, [
+        {compression, none},
+        {reduce, fun reduce_fun/2}
+    ]),
+    {Fd, Btree}.
+
 setup_red() ->
     {_, EvenOddKVs} = lists:foldl(
         fun(Idx, {Key, Acc}) ->
@@ -114,7 +134,7 @@ sorted_kvs_test_() ->
 rsorted_kvs_test_() ->
     Sorted = [{Seq, rand:uniform()} || Seq <- lists:seq(1, ?ROWS)],
     Funs = kvs_test_funs(),
-    Reversed = Sorted,
+    Reversed = lists:reverse(Sorted),
     {
         "BTree with backward sorted keys",
         {
@@ -167,6 +187,42 @@ sorted_kvs_with_cache_test_() ->
         }
     }.
 
+sorted_kvs_small_chunk_size_test_() ->
+    Funs = kvs_test_funs(),
+    Sorted = [{Seq, rand:uniform()} || Seq <- lists:seq(1, ?ROWS)],
+    {
+        "BTree with a small chunk size and sorted keys",
+        {
+            setup,
+            fun() -> test_util:start_couch() end,
+            fun test_util:stop/1,
+            {
+                foreachx,
+                fun setup_kvs_with_small_chunk_size/1,
+                fun teardown/2,
+                [{Sorted, Fun} || Fun <- Funs]
+            }
+        }
+    }.
+
+sorted_kvs_large_chunk_size_test_() ->
+    Funs = kvs_test_funs(),
+    Sorted = [{Seq, rand:uniform()} || Seq <- lists:seq(1, ?ROWS)],
+    {
+        "BTree with a large chunk size and sorted keys",
+        {
+            setup,
+            fun() -> test_util:start_couch() end,
+            fun test_util:stop/1,
+            {
+                foreachx,
+                fun setup_kvs_with_large_chunk_size/1,
+                fun teardown/2,
+                [{Sorted, Fun} || Fun <- Funs]
+            }
+        }
+    }.
+
 reductions_test_() ->
     {
         "BTree reductions",
@@ -642,18 +698,15 @@ test_key_access(Btree, List) ->
     FoldFun = fun(Element, {[HAcc | TAcc], Count}) ->
         case Element == HAcc of
             true -> {ok, {TAcc, Count + 1}};
-            _ -> {ok, {TAcc, Count + 1}}
+            _ -> {ok, {TAcc, Count}}
         end
     end,
     Length = length(List),
     Sorted = lists:sort(List),
     {ok, _, {[], Length}} = couch_btree:foldl(Btree, FoldFun, {Sorted, 0}),
-    {ok, _, {[], Length}} = couch_btree:fold(
-        Btree,
-        FoldFun,
-        {Sorted, 0},
-        [{dir, rev}]
-    ),
+    Reversed = lists:reverse(Sorted),
+    RevOpts = [{dir, rev}],
+    {ok, _, {[], Length}} = couch_btree:fold(Btree, FoldFun, {Reversed, 0}, RevOpts),
     ok.
 
 test_lookup_access(Btree, KeyValues) ->