fix lua result serialization causing various issues

for example, loading a library which returns a table of functions would
fail with an inexplicable (to the user) error about serialization. This
is now fixed. We no longer use a single result type, instead there are
void results and normal results, and the former simply never has to
worry about the result.

This was likely causing even more issues; if the `sol::object` was
populated before, it hitting the dtor in another path would, again, like
we've seen before, corrupt the lua stack.

include/TLuaResult.h

@@ -37,19 +37,54 @@ struct TDetachedLuaValue {
 };
 std::ostream& operator<<(std::ostream& os, const TDetachedLuaValue& value);
 
-struct TLuaResult {
+/// Result for operations that stay within one Lua state (e.g. script loads).
+/// This type intentionally does not store any Lua references or serialized Lua values.
+struct TLuaVoidResult {
     struct Snapshot {
         bool Error;
         bool Ready;
         std::string ErrorMessage;
-        /// CAUTION: Accessing this object causes the Lua stack of the owning state
-        /// to be accessed. Only call this from the owning Lua state. Otherwise,
-        /// use `DetachedSnapshot`.
-        sol::object Result;
         TLuaStateId StateId;
-        std::string Function;
     };
 
+    explicit TLuaVoidResult(TLuaStateId StateId)
+        : mStateId(std::move(StateId)) {}
+
+    /// Marks this result as success & sets it as ready, waking all waiting threads.
+    void MarkReadySuccess();
+    /// Marks this result as erroneous & sets it as ready, waking all waiting threads.
+    void MarkReadyError(sol::protected_function_result Res);
+    /// Marks this result as erroneous & sets it as ready, waking all waiting threads.
+    void MarkReadyError(std::string Res);
+    /// Wait (suspend) until this result is ready.
+    void WaitUntilReady();
+    bool IsReady() const;
+    bool IsError() const;
+    Snapshot GetSnapshot() const;
+
+private:
+    mutable std::mutex mMutex {};
+    bool mReady { false };
+    bool mError { false };
+    std::string mErrorMessage;
+    TLuaStateId mStateId;
+    std::condition_variable mReadyCondition {};
+
+    void SetErrorMessageFromResult(sol::protected_function_result& Res) {
+        if (Res.valid()) {
+            beammp_lua_errorf("Error was not an error");
+        }
+        if (Res.get_type() == sol::type::string) {
+            mErrorMessage = Res.get<sol::error>().what();
+        } else {
+            mErrorMessage = "(unknown error; error object is not inspectable)";
+        }
+    }
+
+    void MarkAsReady();
+};
+
+struct TLuaResult {
     struct DetachedSnapshot {
         bool Error;
         bool Ready;
@@ -66,33 +101,29 @@ struct TLuaResult {
 
     /// Marks this result as success & sets it as ready, waking all threads waiting on it.
     void MarkReadySuccess(sol::object Res);
+    /// Marks this result as successful and ready without serializing a Lua value.
+    void MarkReadySuccessNoResult();
     /// Marks this result as erroneous & sets it as ready, waking all threads waiting on it.
     void MarkReadyError(sol::protected_function_result Res);
     /// Marks this result as erroneous & sets it as ready, waking all threads waiting on it.
     void MarkReadyError(std::string Res);
-    /// Wait (suspend) until this result is ready. Use `GetSnapshot` to get the results.
+    /// Wait (suspend) until this result is ready. Use `GetDetachedSnapshot` to get the result.
     void WaitUntilReady();
     bool IsReady() const;
     bool IsError() const;
     TLuaStateId OwnerState() const;
     void SetOwnerState(TLuaStateId StateId);
 
-    /// To get the result or error, while in the owning state, use this function.
-    /// Pass your lua state ID so that the function can verify the safety of the access.
-    /// If you have no state ID, use `GetDetachedSnapshot`.
-    /// Accesses the Lua state directly when using the result.
-    Snapshot GetSnapshot(TLuaStateId CallingState) const;
     /// Returns a snapshot with, if not an error, a serialized version of the result
-    /// object. In contrast to `GetSnapshot`, this does not access the Lua stack when
-    /// accessing `.Result`.
+    /// object. This never stores or returns raw Lua references and is safe to use
+    /// across threads.
     DetachedSnapshot GetDetachedSnapshot() const;
 
 private:
     mutable std::mutex mMutex {};
     bool mReady { false };
-    bool mError;
+    bool mError { false };
     std::string mErrorMessage;
-    sol::object mResult { sol::lua_nil };
     TDetachedLuaValue mDetachedResult;
     TLuaStateId mStateId;
     std::string mFunction;