bitset: fix index size calculation

Bitset index size calculation uses the cardinality of the 'flag' bitset,
but when the bitset index is empty, i.e., uninitialized, the
'flag' bitset is not allocated, hence we should simply return 0.

Closes #5809

NO_DOC=bugfix

(cherry picked from commit d542a01a)

changelogs/unreleased/gh-5809-bitset-index-len-crash.md

+## bugfix/core
+
+* Fixed empty bitset index crash on `len` call (gh-5809).

src/lib/bitset/index.h

@@ -294,7 +294,8 @@ tt_bitset_index_contains_value(struct tt_bitset_index *index, size_t value);
 inline size_t
 tt_bitset_index_size(const struct tt_bitset_index *index)
 {
-	return tt_bitset_cardinality(index->bitsets[0]);
+	return index->capacity == 0 ? 0 :
+	       tt_bitset_cardinality(index->bitsets[0]);
 }
 
 /**

test/box-luatest/gh_5809_bitset_index_len_crash_test.lua

+local t = require('luatest')
+local server = require('luatest.server')
+
+local g = t.group('')
+
+g.before_all(function(cg)
+    cg.server = server:new{alias = 'dflt'}
+    cg.server:start()
+end)
+
+g.after_all(function(cg)
+    cg.server:drop()
+end)
+
+-- Checks size of empty, i.e., uninitialized bitset index works correctly.
+g.test_empty_uninit_bitset_index_len = function()
+    g.server:exec(function()
+        local t = require('luatest')
+
+        local s = box.schema.space.create('s')
+        s:create_index('pk')
+        s:create_index('bitset', {type = 'bitset'})
+        t.assert_equals(s.index.bitset:len(), 0)
+    end)
+end

test/unit/bitset_index.c

@@ -45,6 +45,13 @@ void test_size_and_count(void)
 
 	struct tt_bitset_index index;
 	tt_bitset_index_create(&index, realloc);
+	/*
+	 * Checks size of empty, i.e., uninitialized bitset index works
+	 * correctly.
+	 */
+	fail_unless(tt_bitset_index_size(&index) == 0);
+	/* Checks bit number overflow works correctly. */
+	fail_unless(tt_bitset_index_count(&index, UINT64_MAX) == 0);
 
 	enum { P = 10, SIZE = (1 << P) + 1 };
 	for(size_t i = 0; i < SIZE; i++) {