memtx: raise an error if argument of select() is a "clear" dict

Currently, select/pairs/get look at an :array part of passed table,
that is why different problems occurs when passed to select() table
is not a regular array.

Now we will check if passed table is not "clear dict"
(not empty table without array part). Invalid queries
like s:select{key = 'value'} will fail with an appropriate
error, but the problems with dicts containing array part
still remain (for example, s:select{1, key = 'value'} is
the same as s:select{1}).

Closes #6167

changelogs/unreleased/gh-6167-select-with-dict.md

+## bugfix/memtx
+
+* Now memtx will raise an error if "clear" dictionary was passed to s:select() (gh-6167)

src/box/lua/tuple.lua

@@ -79,8 +79,13 @@ local tuple_encode = function(tmpbuf, obj)
     elseif is_tuple(obj) then
         encode_r(tmpbuf, obj, 1)
     elseif type(obj) == "table" then
-        encode_array(tmpbuf, #obj)
-        for i = 1, #obj, 1 do
+        local obj_size = #obj
+        if obj_size == 0 and not rawequal(next(obj), nil) then
+            -- dictionary cannot represent a tuple
+            return box.error(box.error.TUPLE_NOT_ARRAY)
+        end
+        encode_array(tmpbuf, obj_size)
+        for i = 1, obj_size, 1 do
             encode_r(tmpbuf, obj[i], 1)
         end
     else

test/box/gh-6167-select-with-dict.result

+-- test-run result file version 2
+s = box.schema.create_space('test')
+ | ---
+ | ...
+_ = s:create_index('pk')
+ | ---
+ | ...
+dict = {key = 'value'}
+ | ---
+ | ...
+-- There must be errors saying tuple must be array
+s:select(dict)
+ | ---
+ | - error: Tuple/Key must be MsgPack array
+ | ...
+s:pairs(dict)
+ | ---
+ | - error: Tuple/Key must be MsgPack array
+ | ...
+s:get(dict)
+ | ---
+ | - error: Tuple/Key must be MsgPack array
+ | ...
+s:drop()
+ | ---
+ | ...

test/box/gh-6167-select-with-dict.test.lua

+s = box.schema.create_space('test')
+_ = s:create_index('pk')
+dict = {key = 'value'}
+-- There must be errors saying tuple must be array
+s:select(dict)
+s:pairs(dict)
+s:get(dict)
+s:drop()