memtx: validate only cmp_def parts in pagination

Currently, we check that tuple passed as position fits space format.
However, for pagination purposes, it's enough to validate only parts of
tuple that are used in cmp_def. And, it allows not to use space format -
we will need it in read view. So the patch replaces tuple validation with
validation of its cmp_def parts only.

Closes #8511
Part of tarantool/tarantool-ee#285

NO_DOC=bugfix

(cherry picked from commit 275130ff)

changelogs/unreleased/gh-8511-pagination-validate-cmp-def.md

+## bugfix/core
+
+* Loosen requirements for tuple which is used as position - passed to
+  `index:tuple_pos()` or to option `after` of `index:select`. Now, only
+  key parts of current and primary indexes are validated (gh-8511).

src/box/index.cc

@@ -217,11 +217,11 @@ box_index_tuple_position(uint32_t space_id, uint32_t index_id,
 			 "position by tuple");
 		return -1;
 	}
-	if (tuple_validate_raw(space->format, tuple) != 0) {
+	struct key_def *cmp_def = index->def->cmp_def;
+	if (tuple_validate_key_parts_raw(cmp_def, tuple) != 0) {
 		diag_set(ClientError, ER_ITERATOR_POSITION);
 		return -1;
 	}
-	struct key_def *cmp_def = index->def->cmp_def;
 	uint32_t key_size;
 	const char *key = tuple_extract_key_raw(tuple, tuple_end, cmp_def,
 						MULTIKEY_NONE, &key_size);

test/engine-luatest/pagination_test.lua

@@ -350,10 +350,20 @@ tree_g.test_invalid_positions = function(cg)
         s:replace{1, 'Zero'}
 
         local tuples
+        -- Tuple is not suitable for format, but suitable for index parts.
+        -- This should be OK.
         local pos = {1, 2}
         local flag, err = pcall(function()
             s:select(1, {limit=1, iterator='GE', after=pos})
         end)
+        t.assert_equals(flag, true)
+        t.assert_equals(err, nil)
+
+        -- Now let's check tuple which is not compatible with index parts
+        pos = {'ABC', 2}
+        flag, err = pcall(function()
+            s:select(1, {limit=1, iterator='GE', after=pos})
+        end)
         t.assert_equals(flag, false)
         t.assert_equals(err.code, box.error.ITERATOR_POSITION)