Fix incorrect index selection when sort specified

Mango has an apparently long-standing bug whereby if an
index is deemed valid for sorting a query but is not valid
according to mango_idx:is_usable, the query planner would
fall back to _all_docs silently, therefore ignoring the
sort order specified in the query.

This reverses the index selection logic so that the
list of usable indexes is generated prior to filtering
them based on the sort order specified in the query.

Similar logic is applied to use_index, allowing us
to generate a more specific error message when a user
specifies an index which isn't valid for the current
selector.

src/mango/src/mango_error.erl

@@ -33,6 +33,12 @@ info(mango_idx, {no_usable_index, no_index_matching_name}) ->
         <<"no_usable_index">>,
         <<"No index matches the index specified with \"use_index\"">>
     };
+info(mango_idx, {no_usable_index, no_usable_index_matching_name}) ->
+    {
+        400,
+        <<"no_usable_index">>,
+        <<"The index specified with \"use_index\" is not usable for the query.">>
+    };
 info(mango_idx, {no_usable_index, missing_sort_index}) ->
     {
         400,

src/mango/src/mango_idx.erl

@@ -70,13 +70,33 @@ get_usable_indexes(Db, Selector0, Opts) ->
         ?MANGO_ERROR({no_usable_index, no_index_matching_name})
     end,
 
-    SortIndexes = mango_idx:for_sort(FilteredIndexes, Opts),
-    if SortIndexes /= [] -> ok; true ->
-        ?MANGO_ERROR({no_usable_index, missing_sort_index})
+    UsableFilter = fun(I) -> mango_idx:is_usable(I, Selector) end,
+    UsableIndexes0 = lists:filter(UsableFilter, FilteredIndexes),
+
+    UsableIndexes1 = case has_use_index(Opts) of
+        true ->
+            UsableFilteredIndexes = mango_cursor:maybe_filter_indexes_by_ddoc(UsableIndexes0, Opts),
+            if UsableFilteredIndexes /= [] -> ok; true ->
+                ?MANGO_ERROR({no_usable_index, no_usable_index_matching_name})
+            end,
+            UsableFilteredIndexes;
+        false ->
+            UsableIndexes0
     end,
 
-    UsableFilter = fun(I) -> mango_idx:is_usable(I, Selector) end,
-    lists:filter(UsableFilter, SortIndexes).
+    % If a sort was specified we have to find an index that
+    % can satisfy the request.
+    case has_sort(Opts) of
+        true ->
+            SortIndexes = mango_idx:for_sort(UsableIndexes1, Opts),
+            if SortIndexes /= [] -> ok; true ->
+                ?MANGO_ERROR({no_usable_index, missing_sort_index})
+            end,
+            SortIndexes;
+        false ->
+            UsableIndexes1
+    end.
+
 
 recover(Db) ->
     {ok, DDocs0} = mango_util:open_ddocs(Db),
@@ -93,17 +113,33 @@ recover(Db) ->
     end, DDocs)}.
 
 
-for_sort(Indexes, Opts) ->
-    % If a sort was specified we have to find an index that
-    % can satisfy the request.
+has_use_index(Opts) ->
+    erlang:display(<<"sort opts">>),
+    erlang:display(Opts),
+    case lists:keyfind(sort, 1, Opts) of
+        {use_index, _} ->
+            true;
+        _ ->
+            false
+    end.
+
+
+has_sort(Opts) ->
     case lists:keyfind(sort, 1, Opts) of
+        {sort, {[]}} ->
+            false;
         {sort, {SProps}} when is_list(SProps) ->
-            for_sort_int(Indexes, {SProps});
+            true;
         _ ->
-            Indexes
+            false
     end.
 
 
+for_sort(Indexes, Opts) ->
+    {sort, {SProps}} = lists:keyfind(sort, 1, Opts),
+    for_sort_int(Indexes, {SProps}).
+
+
 for_sort_int(Indexes, Sort) ->
     Fields = mango_sort:fields(Sort),
     FilterFun = fun(Idx) ->

src/mango/test/02-basic-find-test.py

@@ -222,6 +222,20 @@ def test_sort(self):
         docs2 = list(reversed(sorted(docs1, key=lambda d: d["age"])))
         assert docs1 is not docs2 and docs1 == docs2
 
+    def test_sort_desc_complex(self):
+        docs = self.db.find({
+            "company": {"$lt": "M"},
+            "manager": {"$exists": True},
+            "$or": [
+                {"company": "Dreamia"},
+                {"manager": True}
+            ]
+        }, sort=[{"company":"desc"}])
+
+        companies_returned = list(d["company"] for d in docs)
+        desc_companies = sorted(companies_returned, reverse=True)
+        self.assertEqual(desc_companies, companies_returned)
+
     def test_fields(self):
         selector = {"age": {"$gt": 0}}
         docs = self.db.find(selector, fields=["user_id", "location.address"])

src/mango/test/13-users-db-find-test.py

@@ -44,7 +44,7 @@ def test_multi_cond_or(self):
 
     def test_sort(self):
         self.db.create_index(["order", "name"])
-        selector = {"name": {"$gt": "demo01"}}
+        selector = {"name": {"$gt": "demo01"}, "order": {"$exists": True}}
         docs1 = self.db.find(selector, sort=[{"order": "asc"}])
         docs2 = list(sorted(docs1, key=lambda d: d["order"]))
         assert docs1 is not docs2 and docs1 == docs2