LibJS: Add JS::SafeFunction, like Function but protects captures from GC

SafeFunction automatically registers its closure memory area in a place
where the JS garbage collector can find it.

This means that you can capture JS::Value and arbitrary pointers into
the GC heap in closures, as long as you're using a SafeFunction, and the
GC will not zap those values!

There's probably some performance impact from this, and there's a lot of
things that could be nicer/smarter about it, but let's build something
that ensures safety first, and we can worry about performance later. :^)

1 files changed, 250 insertions, 0 deletions

diff --git a/Userland/Libraries/LibJS/SafeFunction.h b/Userland/Libraries/LibJS/SafeFunction.h
new file mode 100644
index 0000000000..4dfb9599b1
--- /dev/null
+++ b/Userland/Libraries/LibJS/SafeFunction.h
@@ -0,0 +1,250 @@
+/*
+ * Copyright (c) 2016 Apple Inc. All rights reserved.
+ * Copyright (c) 2021, Gunnar Beutner <gbeutner@serenityos.org>
+ * Copyright (c) 2022, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Function.h>
+
+namespace JS {
+
+void register_safe_function_closure(void*, size_t);
+void unregister_safe_function_closure(void*, size_t);
+
+template<typename>
+class SafeFunction;
+
+template<typename Out, typename... In>
+class SafeFunction<Out(In...)> {
+    AK_MAKE_NONCOPYABLE(SafeFunction);
+
+public:
+    SafeFunction() = default;
+    SafeFunction(std::nullptr_t)
+    {
+    }
+
+    ~SafeFunction()
+    {
+        clear(false);
+    }
+
+    void register_closure()
+    {
+        if (auto* wrapper = callable_wrapper())
+            register_safe_function_closure(wrapper, m_size);
+    }
+
+    void unregister_closure()
+    {
+        if (auto* wrapper = callable_wrapper())
+            unregister_safe_function_closure(wrapper, m_size);
+    }
+
+    template<typename CallableType>
+    SafeFunction(CallableType&& callable) requires((AK::IsFunctionObject<CallableType> && IsCallableWithArguments<CallableType, In...> && !IsSame<RemoveCVReference<CallableType>, SafeFunction>))
+    {
+        init_with_callable(forward<CallableType>(callable));
+    }
+
+    template<typename FunctionType>
+    SafeFunction(FunctionType f) requires((AK::IsFunctionPointer<FunctionType> && IsCallableWithArguments<RemovePointer<FunctionType>, In...> && !IsSame<RemoveCVReference<FunctionType>, SafeFunction>))
+    {
+        init_with_callable(move(f));
+    }
+
+    SafeFunction(SafeFunction&& other)
+    {
+        move_from(move(other));
+    }
+
+    // Note: Despite this method being const, a mutable lambda _may_ modify its own captures.
+    Out operator()(In... in) const
+    {
+        auto* wrapper = callable_wrapper();
+        VERIFY(wrapper);
+        ++m_call_nesting_level;
+        ScopeGuard guard([this] {
+            if (--m_call_nesting_level == 0 && m_deferred_clear)
+                const_cast<SafeFunction*>(this)->clear(false);
+        });
+        return wrapper->call(forward<In>(in)...);
+    }
+
+    explicit operator bool() const { return !!callable_wrapper(); }
+
+    template<typename CallableType>
+    SafeFunction& operator=(CallableType&& callable) requires((AK::IsFunctionObject<CallableType> && IsCallableWithArguments<CallableType, In...>))
+    {
+        clear();
+        init_with_callable(forward<CallableType>(callable));
+        return *this;
+    }
+
+    template<typename FunctionType>
+    SafeFunction& operator=(FunctionType f) requires((AK::IsFunctionPointer<FunctionType> && IsCallableWithArguments<RemovePointer<FunctionType>, In...>))
+    {
+        clear();
+        if (f)
+            init_with_callable(move(f));
+        return *this;
+    }
+
+    SafeFunction& operator=(std::nullptr_t)
+    {
+        clear();
+        return *this;
+    }
+
+    SafeFunction& operator=(SafeFunction&& other)
+    {
+        if (this != &other) {
+            clear();
+            move_from(move(other));
+        }
+        return *this;
+    }
+
+private:
+    class CallableWrapperBase {
+    public:
+        virtual ~CallableWrapperBase() = default;
+        // Note: This is not const to allow storing mutable lambdas.
+        virtual Out call(In...) = 0;
+        virtual void destroy() = 0;
+        virtual void init_and_swap(u8*, size_t) = 0;
+    };
+
+    template<typename CallableType>
+    class CallableWrapper final : public CallableWrapperBase {
+        AK_MAKE_NONMOVABLE(CallableWrapper);
+        AK_MAKE_NONCOPYABLE(CallableWrapper);
+
+    public:
+        explicit CallableWrapper(CallableType&& callable)
+            : m_callable(move(callable))
+        {
+        }
+
+        Out call(In... in) final override
+        {
+            return m_callable(forward<In>(in)...);
+        }
+
+        void destroy() final override
+        {
+            delete this;
+        }
+
+        // NOLINTNEXTLINE(readability-non-const-parameter) False positive; destination is used in a placement new expression
+        void init_and_swap(u8* destination, size_t size) final override
+        {
+            VERIFY(size >= sizeof(CallableWrapper));
+            new (destination) CallableWrapper { move(m_callable) };
+        }
+
+    private:
+        CallableType m_callable;
+    };
+
+    enum class FunctionKind {
+        NullPointer,
+        Inline,
+        Outline,
+    };
+
+    CallableWrapperBase* callable_wrapper() const
+    {
+        switch (m_kind) {
+        case FunctionKind::NullPointer:
+            return nullptr;
+        case FunctionKind::Inline:
+            return bit_cast<CallableWrapperBase*>(&m_storage);
+        case FunctionKind::Outline:
+            return *bit_cast<CallableWrapperBase**>(&m_storage);
+        default:
+            VERIFY_NOT_REACHED();
+        }
+    }
+
+    void clear(bool may_defer = true)
+    {
+        bool called_from_inside_function = m_call_nesting_level > 0;
+        // NOTE: This VERIFY could fail because a Function is destroyed from within itself.
+        VERIFY(may_defer || !called_from_inside_function);
+        if (called_from_inside_function && may_defer) {
+            m_deferred_clear = true;
+            return;
+        }
+        m_deferred_clear = false;
+        auto* wrapper = callable_wrapper();
+        if (m_kind == FunctionKind::Inline) {
+            VERIFY(wrapper);
+            wrapper->~CallableWrapperBase();
+            unregister_closure();
+        } else if (m_kind == FunctionKind::Outline) {
+            VERIFY(wrapper);
+            wrapper->destroy();
+            unregister_closure();
+        }
+        m_kind = FunctionKind::NullPointer;
+    }
+
+    template<typename Callable>
+    void init_with_callable(Callable&& callable)
+    {
+        VERIFY(m_call_nesting_level == 0);
+        VERIFY(m_kind == FunctionKind::NullPointer);
+        using WrapperType = CallableWrapper<Callable>;
+        if constexpr (sizeof(WrapperType) > inline_capacity) {
+            *bit_cast<CallableWrapperBase**>(&m_storage) = new WrapperType(forward<Callable>(callable));
+            m_kind = FunctionKind::Outline;
+        } else {
+            new (m_storage) WrapperType(forward<Callable>(callable));
+            m_kind = FunctionKind::Inline;
+        }
+        m_size = sizeof(WrapperType);
+        register_closure();
+    }
+
+    void move_from(SafeFunction&& other)
+    {
+        VERIFY(m_call_nesting_level == 0);
+        VERIFY(other.m_call_nesting_level == 0);
+        VERIFY(m_kind == FunctionKind::NullPointer);
+        auto* other_wrapper = other.callable_wrapper();
+        m_size = other.m_size;
+        switch (other.m_kind) {
+        case FunctionKind::NullPointer:
+            break;
+        case FunctionKind::Inline:
+            other.unregister_closure();
+            other_wrapper->init_and_swap(m_storage, inline_capacity);
+            m_kind = FunctionKind::Inline;
+            register_closure();
+            break;
+        case FunctionKind::Outline:
+            *bit_cast<CallableWrapperBase**>(&m_storage) = other_wrapper;
+            m_kind = FunctionKind::Outline;
+            break;
+        default:
+            VERIFY_NOT_REACHED();
+        }
+        other.m_kind = FunctionKind::NullPointer;
+    }
+
+    FunctionKind m_kind { FunctionKind::NullPointer };
+    bool m_deferred_clear { false };
+    mutable Atomic<u16> m_call_nesting_level { 0 };
+    size_t m_size { 0 };
+
+    // Empirically determined to fit most lambdas and functions.
+    static constexpr size_t inline_capacity = 4 * sizeof(void*);
+    alignas(max(alignof(CallableWrapperBase), alignof(CallableWrapperBase*))) u8 m_storage[inline_capacity];
+};
+
+}