FunctionWrapper.h

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/* Copyright (C) 2024 Wildfire Games.
 * This file is part of 0 A.D.
 *
 * 0 A.D. is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 2 of the License, or
 * (at your option) any later version.
 *
 * 0 A.D. is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with 0 A.D.  If not, see <http://www.gnu.org/licenses/>.
 */
 
#ifndef INCLUDED_FUNCTIONWRAPPER
#define INCLUDED_FUNCTIONWRAPPER
 
#include "ScriptConversions.h"
#include "ScriptExceptions.h"
#include "ScriptRequest.h"
 
#include <fmt/format.h>
#include <tuple>
#include <type_traits>
#include <stdexcept>
#include <utility>
 
class ScriptInterface;
 
/**
 * This class introduces templates to conveniently wrap C++ functions in JSNative functions.
 * This _is_ rather template heavy, so compilation times beware.
 * The C++ code can have arbitrary arguments and arbitrary return types, so long
 * as they can be converted to/from JS using Script::ToJSVal (FromJSVal respectively),
 * and they are default-constructible (TODO: that can probably changed).
 * (This could be a namespace, but I like being able to specify public/private).
 */
class ScriptFunction
{
private:
    ScriptFunction() = delete;
    ScriptFunction(const ScriptFunction&) = delete;
    ScriptFunction(ScriptFunction&&) = delete;
 
    /**
     * In JS->C++ calls, types are converted using FromJSVal,
     * and this requires them to be default-constructible (as that function takes an out parameter)
     * thus constref needs to be removed when defining the tuple.
     * Exceptions are:
     *  - const ScriptRequest& (as the first argument only, for implementation simplicity).
     *  - const ScriptInterface& (as the first argument only, for implementation simplicity).
     *  - JS::HandleValue
     */
    template<typename T>
    using type_transform = std::conditional_t<
        std::is_same_v<const ScriptRequest&, T> || std::is_same_v<const ScriptInterface&, T>,
        T,
        std::remove_const_t<typename std::remove_reference_t<T>>
    >;
 
    /**
     * Convenient struct to get info on a [class] [const] function pointer.
     * TODO VS19: I ran into a really weird bug with an auto specialisation on this taking function pointers.
     * It'd be good to add it back once we upgrade.
     */
    template <class T> struct args_info;
 
    template<typename R, typename ...Types>
    struct args_info<R(*)(Types ...)>
    {
        static constexpr const size_t nb_args = sizeof...(Types);
        using return_type = R;
        using object_type = void;
        using arg_types = std::tuple<type_transform<Types>...>;
    };
 
    template<typename C, typename R, typename ...Types>
    struct args_info<R(C::*)(Types ...)> : public args_info<R(*)(Types ...)> { using object_type = C; };
    template<typename C, typename R, typename ...Types>
    struct args_info<R(C::*)(Types ...) const> : public args_info<R(C::*)(Types ...)> {};
 
    struct IteratorResultError : std::runtime_error
    {
        IteratorResultError(const std::string& property) :
            IteratorResultError{property.c_str()}
        {}
        IteratorResultError(const char* property) :
            std::runtime_error{fmt::format("Failed to get `{}` from an `IteratorResult`.", property)}
        {}
        using std::runtime_error::runtime_error;
    };
 
    ///////////////////////////////////////////////////////////////////////////
    ///////////////////////////////////////////////////////////////////////////
 
    /**
     * DoConvertFromJS takes a type, a JS argument, and converts.
     * The type T must be default constructible (except for HandleValue, which is handled specially).
     * (possible) TODO: this could probably be changed if FromJSVal had a different signature.
     * @param wentOk - true if the conversion succeeded and wentOk was true before, false otherwise.
     */
    template<size_t idx, typename T>
    static T DoConvertFromJS(const ScriptRequest& rq, JS::CallArgs& args, bool& wentOk)
    {
        // No need to convert JS values.
        if constexpr (std::is_same_v<T, JS::HandleValue>)
        {
            // Default-construct values that aren't passed by JS.
            // TODO: this should perhaps be removed, as it's distinct from C++ default values and kind of tricky.
            if (idx >= args.length())
                return JS::UndefinedHandleValue;
            else
            {
                // GCC (at least < 9) & VS17 prints warnings if arguments are not used in some constexpr branch.
                UNUSED2(rq); UNUSED2(args); UNUSED2(wentOk);
                return args[idx]; // This passes the null handle value if idx is beyond the length of args.
            }
        }
        else
        {
            // Default-construct values that aren't passed by JS.
            // TODO: this should perhaps be removed, as it's distinct from C++ default values and kind of tricky.
            if (idx >= args.length())
                return {};
            else
            {
                T ret;
                wentOk &= Script::FromJSVal<T>(rq, args[idx], ret);
                return ret;
            }
        }
    }
 
    /**
     * Wrapper: calls DoConvertFromJS for each element in T.
     */
    template<typename... T, size_t... idx>
    static std::tuple<T...> DoConvertFromJS(std::index_sequence<idx...>, const ScriptRequest& rq,
        JS::CallArgs& args, bool& wentOk)
    {
        return {DoConvertFromJS<idx, T>(rq, args, wentOk)...};
    }
 
    /**
     * ConvertFromJS is a wrapper around DoConvertFromJS, and handles specific cases for the
     * first argument (ScriptRequest, ...).
     *
     * Trick: to unpack the types of the tuple as a parameter pack, we deduce them from the function signature.
     * To do that, we want the tuple in the arguments, but we don't want to actually have to default-instantiate,
     * so we'll pass a nullptr that's static_cast to what we want.
     */
    template<typename ...Types>
    static std::tuple<Types...> ConvertFromJS(const ScriptRequest& rq, JS::CallArgs& args, bool& wentOk,
        std::tuple<Types...>*)
    {
        return DoConvertFromJS<Types...>(std::index_sequence_for<Types...>(), rq, args, wentOk);
    }
 
    // Overloads for ScriptRequest& first argument.
    template<typename ...Types>
    static std::tuple<const ScriptRequest&, Types...> ConvertFromJS(const ScriptRequest& rq,
        JS::CallArgs& args, bool& wentOk, std::tuple<const ScriptRequest&, Types...>*)
    {
        return std::tuple_cat(std::tie(rq), DoConvertFromJS<Types...>(
            std::index_sequence_for<Types...>(), rq, args, wentOk));
    }
 
    // Overloads for ScriptInterface& first argument.
    template<typename ...Types>
    static std::tuple<const ScriptInterface&, Types...> ConvertFromJS(const ScriptRequest& rq,
        JS::CallArgs& args, bool& wentOk, std::tuple<const ScriptInterface&, Types...>*)
    {
        return std::tuple_cat(std::tie(rq.GetScriptInterface()),
            DoConvertFromJS<Types...>(std::index_sequence_for<Types...>(), rq, args, wentOk));
    }
 
    ///////////////////////////////////////////////////////////////////////////
    ///////////////////////////////////////////////////////////////////////////
 
    /**
     * Wrap std::apply for the case where we have an object method or a regular function.
     */
    template <auto callable, typename T, typename tuple>
    static typename args_info<decltype(callable)>::return_type call(T* object, tuple& args)
    {
        if constexpr(std::is_same_v<T, void>)
        {
            // GCC (at least < 9) & VS17 prints warnings if arguments are not used in some constexpr branch.
            UNUSED2(object);
            return std::apply(callable, args);
        }
        else
            return std::apply(callable, std::tuple_cat(std::forward_as_tuple(*object), args));
    }
 
    ///////////////////////////////////////////////////////////////////////////
    ///////////////////////////////////////////////////////////////////////////
 
    struct IgnoreResult_t {};
    static inline IgnoreResult_t IgnoreResult;
 
    /**
     * Converts any number of arguments to a `JS::MutableHandleValueVector`.
     * If `idx` is empty this function does nothing. For that case there is a
     * `[[maybe_unused]]` on `argv`. GCC would issue a
     * "-Wunused-but-set-parameter" warning.
     * For references like `rq` this warning isn't issued.
     */
    template<typename... Types, size_t... idx>
    static void ToJSValVector(std::index_sequence<idx...>, const ScriptRequest& rq,
        [[maybe_unused]] JS::MutableHandleValueVector argv, const Types&... params)
    {
        (Script::ToJSVal(rq, argv[idx], params), ...);
    }
 
    /**
     * Wrapper around calling a JS function from C++.
     * Arguments are const& to avoid lvalue/rvalue issues, and so can't be used as out-parameters.
     * In particular, the problem is that Rooted are deduced as Rooted, not Handle, and so can't be copied.
     * This could be worked around with more templates, but it doesn't seem particularly worth doing.
     */
    template<typename R, typename ...Args>
    static bool Call_(const ScriptRequest& rq, JS::HandleValue val, const char* name, R& ret, const Args&... args)
    {
        JS::RootedObject obj(rq.cx);
        if (!JS_ValueToObject(rq.cx, val, &obj) || !obj)
            return false;
 
        // Fetch the property explicitly - this avoids converting the arguments if it doesn't exist.
        JS::RootedValue func(rq.cx);
        if (!JS_GetProperty(rq.cx, obj, name, &func) || func.isUndefined())
            return false;
 
        JS::RootedValueVector argv(rq.cx);
        ignore_result(argv.resize(sizeof...(Args)));
        ToJSValVector(std::index_sequence_for<Args...>{}, rq, &argv, args...);
 
        bool success;
        if constexpr (std::is_same_v<R, JS::MutableHandleValue>)
            success = JS_CallFunctionValue(rq.cx, obj, func, argv, ret);
        else
        {
            JS::RootedValue jsRet(rq.cx);
            success = JS_CallFunctionValue(rq.cx, obj, func, argv, &jsRet);
            if constexpr (!std::is_same_v<R, IgnoreResult_t>)
            {
                if (success)
                    Script::FromJSVal(rq, jsRet, ret);
            }
            else
                UNUSED2(ret); // VS2017 complains.
        }
        // Even if everything succeeded, there could be pending exceptions
        return !ScriptException::CatchPending(rq) && success;
    }
 
    ///////////////////////////////////////////////////////////////////////////
    ///////////////////////////////////////////////////////////////////////////
public:
    template <typename T>
    using ObjectGetter = T*(*)(const ScriptRequest&, JS::CallArgs&);
 
    // TODO: the fact that this takes class and not auto is to work around an odd VS17 bug.
    // It can be removed with VS19.
    template <class callableType>
    using GetterFor = ObjectGetter<typename args_info<callableType>::object_type>;
 
    /**
     * The meat of this file. This wraps a C++ function into a JSNative,
     * so that it can be called from JS and manipulated in Spidermonkey.
     * Most C++ functions can be directly wrapped, so long as their arguments are
     * convertible from JS::Value and their return value is convertible to JS::Value (or void)
     * The C++ function may optionally take const ScriptRequest& or ScriptInterface& as its first argument.
     * The function may be an object method, in which case you need to pass an appropriate getter
     *
     * Optimisation note: the ScriptRequest object is created even without arguments,
     * as it's necessary for IsExceptionPending.
     *
     * @param thisGetter to get the object, if necessary.
     */
    template <auto callable, GetterFor<decltype(callable)> thisGetter = nullptr>
    static bool ToJSNative(JSContext* cx, unsigned argc, JS::Value* vp)
    {
        using ObjType = typename args_info<decltype(callable)>::object_type;
 
        JS::CallArgs args = JS::CallArgsFromVp(argc, vp);
        ScriptRequest rq(cx);
 
        // If the callable is an object method, we must specify how to fetch the object.
        static_assert(std::is_same_v<typename args_info<decltype(callable)>::object_type, void> || thisGetter != nullptr,
                      "ScriptFunction::Register - No getter specified for object method");
 
// GCC 7 triggers spurious warnings
#ifdef __GNUC__
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Waddress"
#endif
        ObjType* obj = nullptr;
        if constexpr (thisGetter != nullptr)
        {
            obj = thisGetter(rq, args);
            if (!obj)
                return false;
        }
#ifdef __GNUC__
#pragma GCC diagnostic pop
#endif
 
        bool wentOk = true;
        typename args_info<decltype(callable)>::arg_types outs = ConvertFromJS(rq, args, wentOk,
            static_cast<typename args_info<decltype(callable)>::arg_types*>(nullptr));
        if (!wentOk)
            return false;
 
        /**
         * TODO: error handling isn't standard, and since this can call any C++ function,
         * there's no simple obvious way to deal with it.
         * For now we check for pending JS exceptions, but it would probably be nicer
         * to standardise on something, or perhaps provide an "errorHandler" here.
         */
        if constexpr (std::is_same_v<void, typename args_info<decltype(callable)>::return_type>)
            call<callable>(obj, outs);
        else if constexpr (std::is_same_v<JS::Value, typename args_info<decltype(callable)>::return_type>)
            args.rval().set(call<callable>(obj, outs));
        else
            Script::ToJSVal(rq, args.rval(), call<callable>(obj, outs));
 
        return !ScriptException::IsPending(rq);
    }
 
    /**
     * Call a JS function @a name, property of object @a val, with the arguments @a args.
     * @a ret will be updated with the return value, if any.
     * @return the success (or failure) thereof.
     */
    template<typename R, typename ...Args>
    static bool Call(const ScriptRequest& rq, JS::HandleValue val, const char* name, R& ret, const Args&... args)
    {
        return Call_(rq, val, name, ret, std::forward<const Args>(args)...);
    }
 
    // Specialisation for MutableHandleValue return.
    template<typename ...Args>
    static bool Call(const ScriptRequest& rq, JS::HandleValue val, const char* name, JS::MutableHandleValue ret, const Args&... args)
    {
        return Call_(rq, val, name, ret, std::forward<const Args>(args)...);
    }
 
    /**
     * Call a JS function @a name, property of object @a val, with the arguments @a args.
     * @return the success (or failure) thereof.
     */
    template<typename ...Args>
    static bool CallVoid(const ScriptRequest& rq, JS::HandleValue val, const char* name, const Args&... args)
    {
        return Call(rq, val, name, IgnoreResult, std::forward<const Args>(args)...);
    }
 
    /**
     * Call a JS function @a name, property of object @a val, with the argument @a args. Repeatetly
     * invokes @a yieldCallback with the yielded value.
     * @return the final value of the generator.
     */
    template<typename Callback>
    static JS::Value RunGenerator(const ScriptRequest& rq, JS::HandleValue val, const char* name,
        JS::HandleValue arg, Callback yieldCallback)
    {
        JS::RootedValue generator{rq.cx};
        if (!ScriptFunction::Call(rq, val, name, &generator, arg))
            throw std::runtime_error{fmt::format("Failed to call the generator `{}`.", name)};
 
        const auto continueGenerator = [&](const char* property, auto... args) -> JS::Value
            {
                JS::RootedValue iteratorResult{rq.cx};
                if (!ScriptFunction::Call(rq, generator, property, &iteratorResult, args...))
                    throw std::runtime_error{fmt::format("Failed to call `{}`.", name)};
                return iteratorResult;
            };
 
        JS::PersistentRootedValue error{rq.cx, JS::UndefinedValue()};
        while (true)
        {
            JS::RootedValue iteratorResult{rq.cx, error.isUndefined() ? continueGenerator("next") :
                continueGenerator("throw", std::exchange(error, JS::UndefinedValue()))};
 
            try
            {
                JS::RootedObject iteratorResultObject{rq.cx, &iteratorResult.toObject()};
 
                bool done;
                if (!Script::FromJSProperty(rq, iteratorResult, "done", done, true))
                    throw IteratorResultError{"done"};
 
                JS::RootedValue value{rq.cx};
                if (!JS_GetProperty(rq.cx, iteratorResultObject, "value", &value))
                    throw IteratorResultError{"value"};
 
                if (done)
                    return value;
 
                yieldCallback(value);
            }
            catch (const std::exception& e)
            {
                JS::RootedValue global{rq.cx, rq.globalValue()};
                if (!ScriptFunction::Call(rq, global, "Error", &error, e.what()))
                    throw std::runtime_error{"Failed to construct `Error`."};
            }
        }
    }
 
    /**
     * Return a function spec from a C++ function.
     */
    template <auto callable, GetterFor<decltype(callable)> thisGetter = nullptr>
    static JSFunctionSpec Wrap(const char* name,
        const u16 flags = JSPROP_ENUMERATE | JSPROP_READONLY | JSPROP_PERMANENT)
    {
        return JS_FN(name, (&ToJSNative<callable, thisGetter>), args_info<decltype(callable)>::nb_args, flags);
    }
 
    /**
     * Return a JSFunction from a C++ function.
     */
    template <auto callable, GetterFor<decltype(callable)> thisGetter = nullptr>
    static JSFunction* Create(const ScriptRequest& rq, const char* name,
        const u16 flags = JSPROP_ENUMERATE | JSPROP_READONLY | JSPROP_PERMANENT)
    {
        return JS_NewFunction(rq.cx, &ToJSNative<callable, thisGetter>, args_info<decltype(callable)>::nb_args, flags, name);
    }
 
    /**
     * Register a function on the native scope (usually 'Engine').
     */
    template <auto callable, GetterFor<decltype(callable)> thisGetter = nullptr>
    static void Register(const ScriptRequest& rq, const char* name,
        const u16 flags = JSPROP_ENUMERATE | JSPROP_READONLY | JSPROP_PERMANENT)
    {
        JS_DefineFunction(rq.cx, rq.nativeScope, name, &ToJSNative<callable, thisGetter>, args_info<decltype(callable)>::nb_args, flags);
    }
 
    /**
     * Register a function on @param scope.
     * Prefer the version taking ScriptRequest unless you have a good reason not to.
     * @see Register
     */
    template <auto callable, GetterFor<decltype(callable)> thisGetter = nullptr>
    static void Register(JSContext* cx, JS::HandleObject scope, const char* name,
        const u16 flags = JSPROP_ENUMERATE | JSPROP_READONLY | JSPROP_PERMANENT)
    {
        JS_DefineFunction(cx, scope, name, &ToJSNative<callable, thisGetter>, args_info<decltype(callable)>::nb_args, flags);
    }
};
 
#endif // INCLUDED_FUNCTIONWRAPPER