rgw/rgw_lua_utils: fix memory leak in luaL_error() formatting

Previously, error messages passed to luaL_error() were formatted using
std::string concatenation. Since luaL_error() never returns (it throws
a Lua exception via longjmp), the allocated std::string memory was
leaked, as detected by AddressSanitizer:

```
Direct leak of 105 byte(s) in 1 object(s) allocated from:
    #0 0x7fc5f1921a2d in operator new(unsigned long) /usr/src/debug/gcc/gcc/libsanitizer/asan/asan_new_delete.cpp:86
    ceph#1 0x563bd89144c7 in std::__new_allocator<char>::allocate(unsigned long, void const*) /usr/include/c++/15.1.1/bits/new_allocator.h:151
    ceph#2 0x563bd89144c7 in std::allocator<char>::allocate(unsigned long) /usr/include/c++/15.1.1/bits/allocator.h:203
    ceph#3 0x563bd89144c7 in std::allocator_traits<std::allocator<char> >::allocate(std::allocator<char>&, unsigned long) /usr/include/c++/15.1.1/bits/alloc_traits.h:614
    ceph#4 0x563bd89144c7 in std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::_S_allocate(std::allocator<char>&, unsigned long) /usr/include/c++/15.1.1/bits/basic_string.h:142
    ceph#5 0x563bd89144c7 in std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::_M_create(unsigned long&, unsigned long) /usr/include/c++/15.1.1/bits/basic_string.tcc:164
    ceph#6 0x563bd896ae1b in std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::_M_mutate(unsigned long, unsigned long, char const*, unsigned long) /usr/include/c++/15.1.1/bits/basic_string.tcc:363
    ceph#7 0x563bd896b256 in std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::_M_append(char const*, unsigned long) /usr/include/c++/15.1.1/bits/basic_string.tcc:455
    ceph#8 0x563bd896b2bb in std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::append(char const*) /usr/include/c++/15.1.1/bits/basic_string.h:1585
    ceph#9 0x563bd943c2f2 in std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > std::operator+<char, std::char_traits<char>, std::allocator<char> >(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&&, char const*) /usr/include/c++/15.1.1/bits/basic_string.h:3977
    ceph#10 0x563bd943c2f2 in rgw::lua::lua_state_guard::runtime_hook(lua_State*, lua_Debug*) /home/kefu/dev/ceph/src/rgw/rgw_lua_utils.cc:245
    ceph#11 0x7fc5f139f8ef  (/usr/lib/liblua.so.5.4+0xe8ef) (BuildId: b7533e2973d4b0d82e10fc29973ec5a8d355d2b8)
    ceph#12 0x7fc5f139fbfe  (/usr/lib/liblua.so.5.4+0xebfe) (BuildId: b7533e2973d4b0d82e10fc29973ec5a8d355d2b8)
    ceph#13 0x7fc5f13b26fe  (/usr/lib/liblua.so.5.4+0x216fe) (BuildId: b7533e2973d4b0d82e10fc29973ec5a8d355d2b8)
    ceph#14 0x7fc5f139f581  (/usr/lib/liblua.so.5.4+0xe581) (BuildId: b7533e2973d4b0d82e10fc29973ec5a8d355d2b8)
    ceph#15 0x7fc5f139b735  (/usr/lib/liblua.so.5.4+0xa735) (BuildId: b7533e2973d4b0d82e10fc29973ec5a8d355d2b8)
    ceph#16 0x7fc5f139ba8f  (/usr/lib/liblua.so.5.4+0xaa8f) (BuildId: b7533e2973d4b0d82e10fc29973ec5a8d355d2b8)
    ceph#17 0x7fc5f139f696 in lua_pcallk (/usr/lib/liblua.so.5.4+0xe696) (BuildId: b7533e2973d4b0d82e10fc29973ec5a8d355d2b8)
    ceph#18 0x563bd8a925ef in rgw::lua::request::execute(rgw::sal::Driver*, RGWREST*, OpsLogSink*, req_state*, RGWOp*, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&) /home/kefu/dev/ceph/src/rgw/rgw_lua_request.cc:824
    ceph#19 0x563bd8952e3d in TestRGWLua_LuaRuntimeLimit_Test::TestBody() /home/kefu/dev/ceph/src/test/rgw/test_rgw_lua.cc:1628
    ceph#20 0x563bd8a37817 in void testing::internal::HandleSehExceptionsInMethodIfSupported<testing::Test, void>(testing::Test*, void (testing::Test::*)(), char const*) /home/kefu/dev/ceph/src/googletest/googletest/src/gtest.cc:2653
    ceph#21 0x563bd8a509b5 in void testing::internal::HandleExceptionsInMethodIfSupported<testing::Test, void>(testing::Test*, void (testing::Test::*)(), char const*) (/home/kefu/dev/ceph/build/bin/unittest_rgw_lua+0x11199b5) (BuildId: b2628caba5290d882d25f7bea166f058b682bc85)`
```

This change replaces std::string formatting with stack-allocated buffer
and std::to_chars() to eliminate the memory leak.

Note: We cannot format int64_t directly through luaL_error() because
lua_pushfstring() does not support long long or int64_t format specifiers,
even in Lua 5.4 (see https://www.lua.org/manual/5.4/manual.html#lua_pushfstring).
Since libstdc++ uses int64_t for std::chrono::milliseconds::rep, we use
std::to_chars() for safe, efficient conversion without heap allocation.

The maximum runtime limit was a configuration introduced by 3e3cb15.

Fixes: https://tracker.ceph.com/issues/71595
Signed-off-by: Kefu Chai <[email protected]>

Author: tchaikov

src/rgw/rgw_lua_utils.cc

@@ -241,9 +241,16 @@ void lua_state_guard::runtime_hook(lua_State* L, lua_Debug* ar) {
       std::chrono::duration_cast<std::chrono::milliseconds>(now - start_time);
 
   if (elapsed > max_runtime) {
-    std::string err = "Lua runtime limit exceeded: total elapsed time is " +
-                      std::to_string(elapsed.count()) + " ms";
-    luaL_error(L, "%s", err.c_str());
+    // +1 for max digit, +1 for null terminator
+    constexpr size_t max_digits = std::numeric_limits<decltype(elapsed)::rep>::digits10 + 2;
+    char elapsed_str[max_digits] = {};
+    auto [ptr, ec] = std::to_chars(std::begin(elapsed_str), std::end(elapsed_str), elapsed.count());
+    // buffer too small, should never happen though
+    assert(ec == std::errc{});
+    *ptr = '\0';
+    // luaL_error() never returns, so we have to format the number in the hard way instead of
+    // using std::to_string or fmt::to_string
+    luaL_error(L, "Lua runtime limit exceeded: total elapsed time is %s ms", elapsed_str);
   }
 }