Fix lua_*upvalue() when upvalue names aren't in debug info (#787)

`lua_getupvalue()` and `lua_setupvalue()` don't behave as expected when
working with Lua closure whose `Proto` has no debug info. The code
currently uses `sizeupvalues` to do bounds checking of upvalue indices,
but that's the size of the upvalue _names_ array. It will always be `0`
if the `Proto` doesn't have debug info.

This uses `nups` instead, and just returns `""` as the upvalue name if
we don't have one, same as for C closures.

Co-authored-by: Harold Cindy <HaroldCindy@users.noreply.github.com>

VM/src/lapi.cpp

@@ -1285,10 +1285,12 @@ static const char* aux_upvalue(StkId fi, int n, TValue** val)
     else
     {
         Proto* p = f->l.p;
-        if (!(1 <= n && n <= p->sizeupvalues))
+        if (!(1 <= n && n <= p->nups)) // not a valid upvalue
             return NULL;
         TValue* r = &f->l.uprefs[n - 1];
         *val = ttisupval(r) ? upvalue(r)->v : r;
+        if (!(1 <= n && n <= p->sizeupvalues)) // don't have a name for this upvalue
+            return "";
         return getstr(p->upvalues[n - 1]);
     }
 }

tests/Conformance.test.cpp

@@ -693,6 +693,34 @@ TEST_CASE("Debugger")
         CHECK(stephits > 100); // note; this will depend on number of instructions which can vary, so we just make sure the callback gets hit often
 }
 
+TEST_CASE("NDebugGetUpValue")
+{
+    lua_CompileOptions copts = defaultOptions();
+    copts.debugLevel = 0;
+    // Don't optimize away any upvalues
+    copts.optimizationLevel = 0;
+
+    runConformance(
+        "ndebug_upvalues.lua",
+        nullptr,
+        [](lua_State* L) {
+            lua_checkstack(L, LUA_MINSTACK);
+
+            // push the second frame's closure to the stack
+            lua_Debug ar = {};
+            REQUIRE(lua_getinfo(L, 1, "f", &ar));
+
+            // get the first upvalue
+            const char* u = lua_getupvalue(L, -1, 1);
+            REQUIRE(u);
+            // upvalue name is unknown without debug info
+            CHECK(strcmp(u, "") == 0);
+            CHECK(lua_tointeger(L, -1) == 5);
+            lua_pop(L, 2);
+        },
+        nullptr, &copts, /* skipCodegen */ false);
+}
+
 TEST_CASE("SameHash")
 {
     extern unsigned int luaS_hash(const char* str, size_t len); // internal function, declared in lstring.h - not exposed via lua.h

tests/conformance/ndebug_upvalues.lua

@@ -0,0 +1,13 @@
+-- This file is part of the Luau programming language and is licensed under MIT License; see LICENSE.txt for details
+-- This tests that the lua_*upval() APIs work correctly even with debug info disabled
+local foo = 5
+function clo_test()
+    -- so `foo` gets captured as an upval
+    print(foo)
+    -- yield so we can look at clo_test's upvalues
+    coroutine.yield()
+end
+
+clo_test()
+
+return 'OK'