Allow modules to be hotloaded

Reload a module when it's script file got changed or when one of it's
imported modules got reloaded.

This doesn't work for dynamic `import`s.

source/scriptinterface/ModuleLoader.cpp

@@ -114,16 +114,19 @@ VfsPath GetBaseFilename(const VfsPath& filename)
 	return file.DecodeUTF8();
 }
 
+template<typename Requester>
 [[nodiscard]] JSObject* CompileModule(const ScriptRequest& rq, ModuleLoader::RegistryType& registry,
-	const VfsPath& filePath)
+	const VfsPath& filePath, Requester&& requester)
 {
 	const VfsPath normalizedPath{filePath.fileSystemPath().lexically_normal().generic_string()};
 	const auto insertResult = registry.try_emplace(normalizedPath, rq, normalizedPath);
-	return std::get<1>(*std::get<0>(insertResult)).m_ModuleObject;
+	ModuleLoader::CompiledModule& compiledModule{std::get<1>(*std::get<0>(insertResult))};
+	compiledModule.AddRequester(std::forward<Requester>(requester));
+	return compiledModule.m_ModuleObject;
 }
 
-[[nodiscard]] JSObject* Resolve(const ScriptRequest& rq,
-	ModuleLoader::RegistryType& registry, JS::HandleObject moduleRequest)
+[[nodiscard]] JSObject* Resolve(const ScriptRequest& rq, ModuleLoader::RegistryType& registry,
+	JS::HandleValue referencingModule, JS::HandleObject moduleRequest)
 {
 	std::string includeString;
 	const JS::RootedValue pathValue{rq.cx,
@@ -131,7 +134,11 @@ VfsPath GetBaseFilename(const VfsPath& filename)
 	if (!Script::FromJSVal(rq, pathValue, includeString))
 		throw std::logic_error{"The module-name to import isn't a string."};
 
-	return CompileModule(rq, registry, includeString);
+	std::string includingModule;
+	if (!Script::FromJSProperty(rq, referencingModule, "path", includingModule))
+		throw std::logic_error{"The importing module doesn't have a \"path\" property."};
+
+	return CompileModule(rq, registry, includeString, includingModule);
 }
 
 [[nodiscard]] JSObject* Evaluate(const ScriptRequest& rq, JS::HandleObject mod)
@@ -152,6 +159,39 @@ VfsPath GetBaseFilename(const VfsPath& filename)
 	return &val.toObject();
 }
 
+Status FileChangedHook(void* param, const VfsPath& changedFile)
+{
+	ModuleLoader::RegistryType& registry{*static_cast<ModuleLoader::RegistryType*>(param)};
+
+	const VfsPath proposedBasePath{GetBaseFilename(changedFile)};
+
+	std::vector<VfsPath> modulesToErase{proposedBasePath.empty() ? changedFile : proposedBasePath};
+	std::vector<std::reference_wrapper<ModuleLoader::Result>> queries;
+	while (!modulesToErase.empty())
+	{
+		const VfsPath path{modulesToErase.back()};
+		modulesToErase.pop_back();
+		const VfsPath pathWithExtension{path.ChangeExtension(".js")};
+		const auto it = registry.find(pathWithExtension);
+		if (it == registry.end())
+			continue;
+
+		ModuleLoader::CompiledModule compiledModule{std::move(std::get<1>(*it))};
+		registry.erase(it);
+
+		const auto [additionalModules, callbacks] = compiledModule.GetRequesters();
+		modulesToErase.insert(modulesToErase.end(),
+			additionalModules.begin(), additionalModules.end());
+
+		queries.insert(queries.end(), callbacks.begin(), callbacks.end());
+	}
+
+	for (ModuleLoader::Result& result : queries)
+		result.Resume();
+
+	return INFO::OK;
+}
+
 template<bool reject>
 bool Call(JSContext* cx, const unsigned argc, JS::Value* vp)
 {
@@ -228,7 +268,34 @@ ModuleLoader::CompiledModule::CompiledModule(const ScriptRequest& rq, const VfsP
 	JS::SetModulePrivate(m_ModuleObject, modInfo);
 }
 
-ModuleLoader::Future::Future(const ScriptRequest& rq, ModuleLoader& loader, const VfsPath& modulePath):
+[[nodiscard]] std::tuple<const std::vector<VfsPath>&,
+	const std::vector<std::reference_wrapper<ModuleLoader::Result>>&>
+	ModuleLoader::CompiledModule::GetRequesters() const
+{
+	return {m_Importer, m_Callbacks};
+}
+
+void ModuleLoader::CompiledModule::AddRequester(VfsPath importer)
+{
+	m_Importer.push_back(std::move(importer));
+}
+
+void ModuleLoader::CompiledModule::AddRequester(Result& callback)
+{
+	m_Callbacks.push_back(callback);
+}
+
+void ModuleLoader::CompiledModule::RemoveRequester(Result* toErase)
+{
+	m_Callbacks.erase(std::remove_if(m_Callbacks.begin(), m_Callbacks.end(),
+		[&](Result& elem)
+		{
+			return &elem == toErase;
+		}), m_Callbacks.end());
+}
+
+ModuleLoader::Future::Future(const ScriptRequest& rq, RegistryType& registry, Result& result,
+	VfsPath modulePath):
 	m_Status{Evaluating{{rq.cx, nullptr}, {rq.cx, JS_NewObject(rq.cx, &callbackClass<false>)},
 		{rq.cx, JS_NewObject(rq.cx, &callbackClass<true>)}}}
 {
@@ -240,7 +307,7 @@ ModuleLoader::Future::Future(const ScriptRequest& rq, ModuleLoader& loader, cons
 	// - Accessing values which are not yet exported results in an error. These errors might implicitly be
 	//	dropped.
 
-	JS::RootedObject mod{rq.cx, CompileModule(rq, loader.m_Registry, modulePath)};
+	JS::RootedObject mod{rq.cx, CompileModule(rq, registry, modulePath, result)};
 	JS::RootedObject promise{rq.cx, Evaluate(rq, mod)};
 	Evaluating& evaluatingStatus{std::get<Evaluating>(m_Status)};
 	evaluatingStatus.moduleNamespace = JS::GetModuleNamespace(rq.cx, mod);
@@ -285,6 +352,16 @@ ModuleLoader::Future::~Future()
 	std::rethrow_exception(std::move(error));
 }
 
+[[nodiscard]] bool ModuleLoader::Future::IsWaiting() const noexcept
+{
+	return std::holds_alternative<WaitingForFileChange>(m_Status);
+}
+
+void ModuleLoader::Future::SetWaiting() noexcept
+{
+	m_Status.emplace<WaitingForFileChange>();
+}
+
 void ModuleLoader::Future::SetReservedSlot(JS::Value privateValue) noexcept
 {
 	Evaluating* evaluatingStatus{std::get_if<Evaluating>(&m_Status)};
@@ -296,10 +373,90 @@ void ModuleLoader::Future::SetReservedSlot(JS::Value privateValue) noexcept
 		JS::SetReservedSlot(evaluatingStatus->reject, 0, privateValue);
 }
 
-[[nodiscard]] ModuleLoader::Future ModuleLoader::LoadModule(const ScriptRequest& rq,
+ModuleLoader::Result::iterator::iterator(Result& backReference):
+	backRef{&backReference}
+{}
+
+[[nodiscard]] ModuleLoader::Future& ModuleLoader::Result::iterator::operator*() const
+{
+	return backRef->m_Storage;
+}
+
+[[nodiscard]] ModuleLoader::Future* ModuleLoader::Result::iterator::operator->() const
+{
+	return &(**this);
+}
+
+ModuleLoader::Result::iterator& ModuleLoader::Result::iterator::operator++()
+{
+	backRef->m_Storage.SetWaiting();
+	return *this;
+}
+
+ModuleLoader::Result::iterator& ModuleLoader::Result::iterator::operator++(int)
+{
+	++(*this);
+	// All iterator of this `LoadModuleResult` refere to the same `LoadModuleResult`.
+	return *this;
+}
+
+[[nodiscard]] bool ModuleLoader::Result::iterator::operator==(const iterator&)
+{
+	return false;
+}
+
+[[nodiscard]] bool ModuleLoader::Result::iterator::operator!=(const iterator&)
+{
+	return true;
+}
+
+ModuleLoader::Result::Result(const ScriptRequest& rq, const VfsPath& modulePath):
+	m_Cx{rq.cx},
+	m_Registry{rq.GetScriptInterface().GetModuleLoader().m_Registry},
+	m_ModulePath{modulePath},
+	m_Storage{rq, m_Registry, *this, m_ModulePath}
+{
+}
+
+ModuleLoader::Result::~Result()
+{
+	const auto modIter = m_Registry.find(m_ModulePath);
+	if (modIter == m_Registry.end())
+		return;
+
+	std::get<1>(*modIter).RemoveRequester(this);
+}
+
+[[nodiscard]] ModuleLoader::Result::iterator ModuleLoader::Result::begin() noexcept
+{
+	return ModuleLoader::Result::iterator{*this};
+}
+
+[[nodiscard]] ModuleLoader::Result::iterator ModuleLoader::Result::end() const noexcept
+{
+	return ModuleLoader::Result::iterator{};
+}
+
+void ModuleLoader::Result::Resume()
+{
+	if (m_Storage.IsWaiting())
+		m_Storage = ModuleLoader::Future{m_Cx, m_Registry, *this, m_ModulePath};
+}
+
+ModuleLoader::ModuleLoader()
+{
+	RegisterFileReloadFunc(FileChangedHook, static_cast<void*>(&m_Registry));
+}
+
+ModuleLoader::~ModuleLoader()
+{
+	UnregisterFileReloadFunc(FileChangedHook, static_cast<void*>(&m_Registry));
+}
+
+[[nodiscard]] ModuleLoader::Result ModuleLoader::LoadModule(const ScriptRequest& rq,
 	const VfsPath& modulePath)
 {
-	return Future{rq, *this, modulePath};
+	return Result{rq, modulePath};
 }
 
 /**
@@ -322,13 +479,14 @@ void ModuleLoader::Future::SetReservedSlot(JS::Value privateValue) noexcept
 	return true;
 }
 
-[[nodiscard]] JSObject* ModuleLoader::ResolveHook(JSContext* cx, JS::HandleValue,
-	JS::HandleObject moduleRequest) noexcept
+[[nodiscard]] JSObject* ModuleLoader::ResolveHook(JSContext* cx, JS::HandleValue referencingPrivate,
+	JS::HandleObject request) noexcept
 {
 	try
 	{
 		const ScriptRequest rq{cx};
-		return Resolve(rq, rq.GetScriptInterface().GetModuleLoader().m_Registry, moduleRequest);
+		return Resolve(rq, rq.GetScriptInterface().GetModuleLoader().m_Registry, referencingPrivate,
+			request);
 	}
 	catch (const std::exception& e)
 	{
@@ -349,7 +507,7 @@ void ModuleLoader::Future::SetReservedSlot(JS::Value privateValue) noexcept
 	try
 	{
 		JS::RootedObject mod{rq.cx, Resolve(rq, rq.GetScriptInterface().GetModuleLoader().m_Registry,
-			moduleRequest)};
+			referencingPrivate, moduleRequest)};
 		JS::RootedObject evaluationPromise{rq.cx, Evaluate(rq, mod)};
 		return JS::FinishDynamicModuleImport(rq.cx, evaluationPromise, referencingPrivate,
 			moduleRequest, promise);

source/scriptinterface/ModuleLoader.h

@@ -21,9 +21,12 @@
 #include "lib/file/vfs/vfs_path.h"
 #include "scriptinterface/ScriptTypes.h"
 
+#include <cstddef>
 #include <exception>
+#include <functional>
 #include <unordered_map>
 #include <variant>
+#include <vector>
 
 class ScriptContext;
 class ScriptRequest;
@@ -35,80 +38,162 @@ class ModuleLoader
 public:
 	friend ScriptContext;
 
-	class CompiledModule
-	{
-	public:
-		CompiledModule(const ScriptRequest& rq, const VfsPath& filePath);
-		JS::PersistentRootedObject m_ModuleObject;
-	};
+	class CompiledModule;
+	class Future;
+	class Result;
 
 	using RegistryType = std::unordered_map<VfsPath, CompiledModule>;
 
-	class Future
-	{
-	public:
-		struct Evaluating
-		{
-			JS::PersistentRootedObject moduleNamespace;
-			JS::PersistentRootedObject fulfill;
-			JS::PersistentRootedObject reject;
-		};
-		struct Fulfilled
-		{
-			JS::PersistentRootedObject moduleNamespace;
-		};
-		struct Rejected
-		{
-			std::exception_ptr error;
-		};
-		struct Invalid {};
-		using Status = std::variant<Evaluating, Fulfilled, Rejected, Invalid>;
-
-		explicit Future(const ScriptRequest& rq, ModuleLoader& loader, const VfsPath& modulePath);
-		Future() = default;
-		Future(const Future&) = delete;
-		Future& operator=(const Future&) = delete;
-		Future(Future&& other) noexcept;
-		Future& operator=(Future&& other) noexcept;
-		~Future();
-
-		[[nodiscard]] bool IsDone() const noexcept;
-
-		/**
-		 * Throws if the evaluation of the module failed.
-		 * @return The module namespace. All exported values are a property
-		 *	of this object. @c default is a property with name "default".
-		 */
-		[[nodiscard]] JSObject* Get();
-
-	private:
-		// It's save to not require a `JS::HandleValue` here.
-		void SetReservedSlot(JS::Value privateValue) noexcept;
-
-		Status m_Status{Invalid{}};
-	};
+	ModuleLoader();
+	ModuleLoader(const ModuleLoader&) = delete;
+	ModuleLoader& operator=(const ModuleLoader&) = delete;
+	ModuleLoader(ModuleLoader&&) = delete;
+	ModuleLoader& operator=(ModuleLoader&&) = delete;
+	~ModuleLoader();
 
 	/**
 	 * Load the specified module and all module it imports recursively.
 	 *
 	 * @param rq @c globalThis is taken from this @c ScriptRequest.
-	 * @param modulePath The path to the file which should be loaded as a module.
-	 * @return A future that is fulfilled when the evaluation of the module
-	 *	completes.
+	 * @param modulePath The path to the file which should be loaded as a
+	 *	module.
+	 * @return A range of futures. The compilation of the first future is
+	 *	already started. The evaluation of the subsequent futures start once
+	 *	the module file is edited.
 	 */
-	[[nodiscard]] Future LoadModule(const ScriptRequest& rq, const VfsPath& modulePath);
+	[[nodiscard]] Result LoadModule(const ScriptRequest& rq, const VfsPath& modulePath);
 
 private:
 	// Functions used by the `ScriptContext`.
 	[[nodiscard]] static bool MetadataHook(JSContext* cx, JS::HandleValue privateValue,
 		JS::HandleObject metaObject) noexcept;
-	[[nodiscard]] static JSObject* ResolveHook(JSContext* cx, JS::HandleValue,
+	[[nodiscard]] static JSObject* ResolveHook(JSContext* cx, JS::HandleValue referencingPrivate,
 		JS::HandleObject moduleRequest) noexcept;
 	[[nodiscard]] static bool DynamicImportHook(JSContext* cx, JS::HandleValue referencingPrivate,
 		JS::HandleObject moduleRequest, JS::HandleObject promise) noexcept;
 
 	RegistryType m_Registry;
 };
+
+class ModuleLoader::CompiledModule
+{
+public:
+	CompiledModule(const ScriptRequest& rq, const VfsPath& filePath);
+
+	std::tuple<const std::vector<VfsPath>&,
+		const std::vector<std::reference_wrapper<Result>>&> GetRequesters() const;
+
+	void AddRequester(VfsPath importer);
+	void AddRequester(Result& callback);
+	void RemoveRequester(Result* toErase);
+
+	JS::PersistentRootedObject m_ModuleObject;
+private:
+	std::vector<VfsPath> m_Importer;
+	std::vector<std::reference_wrapper<Result>> m_Callbacks;
+};
+
+/**
+ * The future is fulfilled once the evaluation of the module
+ * completes.
+ * Note: The evaluation might not be started yet.
+ */
+class ModuleLoader::Future
+{
+	friend Result;
+public:
+	struct Evaluating
+	{
+		JS::PersistentRootedObject moduleNamespace;
+		JS::PersistentRootedObject fulfill;
+		JS::PersistentRootedObject reject;
+	};
+	struct Fulfilled
+	{
+		JS::PersistentRootedObject moduleNamespace;
+	};
+	struct Rejected
+	{
+		std::exception_ptr error;
+	};
+	struct WaitingForFileChange {};
+	struct Invalid {};
+	using Status = std::variant<Evaluating, Fulfilled, Rejected, WaitingForFileChange, Invalid>;
+
+	explicit Future(const ScriptRequest& rq, RegistryType& reqistry, Result& result, VfsPath modulePath);
+	Future() = default;
+	Future(const Future&) = delete;
+	Future& operator=(const Future&) = delete;
+	Future(Future&& other) noexcept;
+	Future& operator=(Future&& other) noexcept;
+	~Future();
+
+	[[nodiscard]] bool IsDone() const noexcept;
+
+	/**
+	 * Throws if the evaluation of the module failed.
+	 * @return The module namespace. All exported values are a property
+	 *	of this object. @c default is a property with name "default".
+	 */
+	[[nodiscard]] JSObject* Get();
+
+private:
+	[[nodiscard]] bool IsWaiting() const noexcept;
+	void SetWaiting() noexcept;
+
+	// It's save to not require a `JS::HandleValue` here.
+	void SetReservedSlot(JS::Value privateValue) noexcept;
+
+	Status m_Status{Invalid{}};
+};
+
+class ModuleLoader::Result
+{
+	class iterator;
+public:
+	explicit Result(const ScriptRequest& rq, const VfsPath& modulePath);
+	Result(const Result&) = delete;
+	Result& operator=(const Result&) = delete;
+	Result(Result&&) = delete;
+	Result& operator=(Result&&) = delete;
+	~Result();
+
+	[[nodiscard]] iterator begin() noexcept;
+	[[nodiscard]] iterator end() const noexcept;
+
+	void Resume();
+
+private:
+	JSContext* m_Cx;
+	RegistryType& m_Registry;
+	VfsPath m_ModulePath;
+	Future m_Storage;
+};
+
+class ModuleLoader::Result::iterator
+{
+public:
+	using difference_type = std::ptrdiff_t;
+	using value_type = Future;
+	using pointer = value_type*;
+	using reference = value_type&;
+	using iterator_category = std::input_iterator_tag;
+
+	explicit iterator() = default;
+	explicit iterator(Result& backReference);
+
+	[[nodiscard]] reference operator*() const;
+	[[nodiscard]] pointer operator->() const;
+
+	iterator& operator++();
+	iterator& operator++(int);
+
+	[[nodiscard]] bool operator==(const iterator&);
+	[[nodiscard]] bool operator!=(const iterator&);
+
+private:
+	Result* backRef{nullptr};
+};
 } // namespace Script
 
 #endif // INCLUDED_SCRIPTMODULELOADER

source/scriptinterface/tests/test_Module.h

@@ -17,6 +17,8 @@
 
 #include "lib/self_test.h"
 
+#include "lib/file/vfs/vfs.h"
+#include "lib/sysdep/dir_watch.h"
 #include "ps/CLogger.h"
 #include "ps/Filesystem.h"
 #include "scriptinterface/FunctionWrapper.h"
@@ -26,11 +28,53 @@
 #include "scriptinterface/ScriptContext.h"
 #include "scriptinterface/ScriptInterface.h"
 
+#if OS_LINUX
+#include <cstdlib>
+#endif
+#if OS_WIN || OS_WIN64 || OS_MAC || OS_MACOSX
+#include <filesystem>
+#endif
+#include <fstream>
+
+Status wdir_watch_Init();
+Status wdir_watch_Shutdown();
+
+namespace
+{
+void ClearFromCache(const VfsPath& path)
+{
+#if OS_BSD
+	TS_SKIP("On BSD hotload isn't implemented.");
+#endif
+
+	OsPath file;
+	if (g_VFS->GetRealPath(path, file) != INFO::OK)
+		throw std::exception{};
+	PDirWatch dirWatch;
+	dir_watch_Add((file.Parent() / "").string8(), dirWatch);
+
+#if OS_WIN || OS_WIN64 || OS_MAC || OS_MACOSX
+	std::filesystem::last_write_time(file.string8(), std::filesystem::file_time_type::clock::now());
+#endif
+
+#if OS_LINUX
+	// On Linux only this aproach seems to trigger a file reload.
+	if (std::system(("touch " + file.string8()).c_str()) != 0)
+		throw std::runtime_error{"`touch` didn't work."};
+#endif
+
+	ReloadChangedFiles();
+}
+}
+
 class TestScriptModule : public CxxTest::TestSuite
 {
 public:
 	void setUp()
 	{
+		if constexpr (OS_WIN)
+			wdir_watch_Init();
+
 		g_VFS = CreateVfs();
 		TS_ASSERT_OK(g_VFS->Mount(L"", DataDir() / "mods" / "_test.scriptinterface" / "module" / "",
 			VFS_MOUNT_MUST_EXIST));
@@ -39,6 +83,9 @@ public:
 	void tearDown()
 	{
 		g_VFS.reset();
+
+		if constexpr (OS_WIN)
+			wdir_watch_Shutdown();
 	}
 
 	void test_StaticImport()
@@ -123,8 +170,9 @@ public:
 	{
 		ScriptInterface script{"Test", "Test", g_ScriptContext};
 		const ScriptRequest rq{script};
-		auto future = script.GetModuleLoader().LoadModule(rq, "top_level_await_finite.js");
+		auto result = script.GetModuleLoader().LoadModule(rq, "top_level_await_finite.js");
 
+		auto& future = *result.begin();
 		TS_ASSERT(!future.IsDone());
 		g_ScriptContext->RunJobs();
 		TS_ASSERT(future.IsDone());
@@ -136,8 +184,9 @@ public:
 		ScriptInterface script{"Test", "Test", g_ScriptContext};
 		const ScriptRequest rq{script};
 
-		auto future = script.GetModuleLoader().LoadModule(rq, "top_level_await_infinite.js");
+		auto result = script.GetModuleLoader().LoadModule(rq, "top_level_await_infinite.js");
 
+		auto& future = *result.begin();
 		g_ScriptContext->RunJobs();
 		TS_ASSERT(!future.IsDone());
 		TS_ASSERT_THROWS_ANYTHING(std::ignore = future.Get());
@@ -148,8 +197,8 @@ public:
 		ScriptInterface script{"Test", "Test", g_ScriptContext};
 		const ScriptRequest rq{script};
 
-		Script::ModuleLoader::Future future0{
-			script.GetModuleLoader().LoadModule(rq, "empty.js")};
+		auto result{script.GetModuleLoader().LoadModule(rq, "empty.js")};
+		Script::ModuleLoader::Future& future0{*result.begin()};
 
 		g_ScriptContext->RunJobs();
 		TS_ASSERT(future0.IsDone());
@@ -168,8 +217,9 @@ public:
 		ScriptInterface script{"Test", "Test", g_ScriptContext};
 		const ScriptRequest rq{script};
 
-		Script::ModuleLoader::Future future0{
-			script.GetModuleLoader().LoadModule(rq, "top_level_await_finite.js")};
+		auto result{script.GetModuleLoader().LoadModule(rq, "top_level_await_finite.js")};
+		Script::ModuleLoader::Future& future0{*result.begin()};
+
 		Script::ModuleLoader::Future future1{std::move(future0)};
 		Script::ModuleLoader::Future future2;
 		future2 = std::move(future1);
@@ -189,11 +239,13 @@ public:
 		const ScriptRequest rq{script};
 
 		TestLogger logger;
-		auto future{script.GetModuleLoader().LoadModule(rq, "delayed_blabbermouth.js")};
+		auto blabbermouthResult{script.GetModuleLoader().LoadModule(rq, "delayed_blabbermouth.js")};
 		TS_ASSERT_STR_NOT_CONTAINS(logger.GetOutput(), "blah blah blah");
+		auto future = std::move(*blabbermouthResult.begin());
 		TS_ASSERT(!future.IsDone());
 
-		future = script.GetModuleLoader().LoadModule(rq, "empty.js");
+		auto emptyResult{script.GetModuleLoader().LoadModule(rq, "empty.js")};
+		future = std::move(*emptyResult.begin());
 		TS_ASSERT(!future.IsDone());
 
 		g_ScriptContext->RunJobs();
@@ -208,8 +260,9 @@ public:
 
 		// To silence the error.
 		const TestLogger _;
-		auto future = script.GetModuleLoader().LoadModule(rq, "top_level_throw.js");
+		auto result = script.GetModuleLoader().LoadModule(rq, "top_level_throw.js");
 
+		auto& future = *result.begin();
 		g_ScriptContext->RunJobs();
 		TS_ASSERT(future.IsDone());
 		TS_ASSERT_THROWS_EQUALS(std::ignore = future.Get(), const std::runtime_error& e, e.what(),
@@ -221,9 +274,10 @@ public:
 		ScriptInterface script{"Test", "Test", g_ScriptContext};
 		const ScriptRequest rq{script};
 
-		auto future = script.GetModuleLoader().LoadModule(rq, "export.js");
-
+		auto result = script.GetModuleLoader().LoadModule(rq, "export.js");
 		g_ScriptContext->RunJobs();
+
+		auto& future = *result.begin();
 		JS::RootedObject ns{rq.cx, future.Get()};
 		JS::RootedValue moduleValue{rq.cx, JS::ObjectValue(*ns)};
 
@@ -248,17 +302,17 @@ public:
 		const ScriptRequest rq{script};
 
 		{
-			auto future = script.GetModuleLoader().LoadModule(rq, "export.js");
+			auto result = script.GetModuleLoader().LoadModule(rq, "export.js");
 			g_ScriptContext->RunJobs();
-			JS::RootedObject ns{rq.cx, future.Get()};
+			JS::RootedObject ns{rq.cx, result.begin()->Get()};
 			JS::RootedValue moduleValue{rq.cx, JS::ObjectValue(*ns)};
 			TS_ASSERT(ScriptFunction::CallVoid(rq, moduleValue, "mutate", 12));
 		}
 
 		{
-			auto future = script.GetModuleLoader().LoadModule(rq, "include/../export.js");
+			auto result = script.GetModuleLoader().LoadModule(rq, "include/../export.js");
 			g_ScriptContext->RunJobs();
-			JS::RootedObject ns{rq.cx, future.Get()};
+			JS::RootedObject ns{rq.cx, result.begin()->Get()};
 			JS::RootedValue moduleValue{rq.cx, JS::ObjectValue(*ns)};
 			int value{0};
 			TS_ASSERT(Script::GetProperty(rq, moduleValue, "value", value));
@@ -272,17 +326,17 @@ public:
 		const ScriptRequest rq{script};
 
 		{
-			auto future = script.GetModuleLoader().LoadModule(rq, "export.js");
+			auto result = script.GetModuleLoader().LoadModule(rq, "export.js");
 			g_ScriptContext->RunJobs();
-			JS::RootedObject ns{rq.cx, future.Get()};
+			JS::RootedObject ns{rq.cx, result.begin()->Get()};
 			JS::RootedValue moduleValue{rq.cx, JS::ObjectValue(*ns)};
 			TS_ASSERT(ScriptFunction::CallVoid(rq, moduleValue, "mutate", 12));
 		}
 
 		{
-			auto future = script.GetModuleLoader().LoadModule(rq, "indirect.js");
+			auto result = script.GetModuleLoader().LoadModule(rq, "indirect.js");
 			g_ScriptContext->RunJobs();
-			JS::RootedObject ns{rq.cx, future.Get()};
+			JS::RootedObject ns{rq.cx, result.begin()->Get()};
 			JS::RootedValue moduleValue{rq.cx, JS::ObjectValue(*ns)};
 			int value{0};
 			TS_ASSERT(Script::GetProperty(rq, moduleValue, "value", value));
@@ -295,10 +349,10 @@ public:
 		ScriptInterface script{"Test", "Test", g_ScriptContext};
 		const ScriptRequest rq{script};
 
-		auto future = script.GetModuleLoader().LoadModule(rq, "export_default/immutable.js");
-
+		auto result = script.GetModuleLoader().LoadModule(rq, "export_default/immutable.js");
 		g_ScriptContext->RunJobs();
 
+		auto& future = *result.begin();
 		JS::RootedObject ns{rq.cx, future.Get()};
 		JS::RootedValue moduleValue{rq.cx, JS::ObjectValue(*ns)};
 
@@ -324,10 +378,11 @@ public:
 		ScriptInterface script{"Test", "Test", g_ScriptContext};
 		const ScriptRequest rq{script};
 
-		auto future = script.GetModuleLoader().LoadModule(rq, "export_default/does_not_work_around.js");
+		auto result = script.GetModuleLoader().LoadModule(rq, "export_default/does_not_work_around.js");
 
 		g_ScriptContext->RunJobs();
 
+		auto& future = *result.begin();
 		JS::RootedObject ns{rq.cx, future.Get()};
 		JS::RootedValue moduleValue{rq.cx, JS::ObjectValue(*ns)};
 
@@ -342,10 +397,10 @@ public:
 		ScriptInterface script{"Test", "Test", g_ScriptContext};
 		const ScriptRequest rq{script};
 
-		auto future = script.GetModuleLoader().LoadModule(rq, "export_default/works_around.js");
-
+		auto result = script.GetModuleLoader().LoadModule(rq, "export_default/works_around.js");
 		g_ScriptContext->RunJobs();
 
+		auto& future = *result.begin();
 		JS::RootedObject ns{rq.cx, future.Get()};
 		JS::RootedValue moduleValue{rq.cx, JS::ObjectValue(*ns)};
 
@@ -359,8 +414,10 @@ public:
 		ScriptInterface script{"Test", "Test", g_ScriptContext};
 		const ScriptRequest rq{script};
 
-		auto future = script.GetModuleLoader().LoadModule(rq, "top_level_await_finite.js");
-		future = script.GetModuleLoader().LoadModule(rq, "export.js");
+		auto awaitResult = script.GetModuleLoader().LoadModule(rq, "top_level_await_finite.js");
+		auto future = std::move(*awaitResult.begin());
+		auto exportResult = script.GetModuleLoader().LoadModule(rq, "export.js");
+		future = std::move(*exportResult.begin());
 
 		g_ScriptContext->RunJobs();
 		JS::RootedObject ns{rq.cx, future.Get()};
@@ -376,10 +433,10 @@ public:
 		ScriptInterface script{"Test", "Test", g_ScriptContext};
 		const ScriptRequest rq{script};
 
-		auto future = script.GetModuleLoader().LoadModule(rq, "dynamic_import.js");
-
+		auto result = script.GetModuleLoader().LoadModule(rq, "dynamic_import.js");
 		g_ScriptContext->RunJobs();
 
+		auto& future = *result.begin();
 		JS::RootedObject ns{rq.cx, future.Get()};
 		JS::RootedValue moduleValue{rq.cx, JS::ObjectValue(*ns)};
 
@@ -407,10 +464,10 @@ public:
 		ScriptInterface script{"Test", "Test", g_ScriptContext};
 		const ScriptRequest rq{script};
 
-		auto future = script.GetModuleLoader().LoadModule(rq, "meta.js");
-
+		auto result = script.GetModuleLoader().LoadModule(rq, "meta.js");
 		g_ScriptContext->RunJobs();
 
+		auto& future = *result.begin();
 		JS::RootedObject ns{rq.cx, future.Get()};
 		const JS::RootedValue modNamespace{rq.cx, JS::ObjectValue(*ns)};
 
@@ -427,15 +484,186 @@ public:
 		ScriptInterface script{"Test", "Test", g_ScriptContext};
 		const ScriptRequest rq{script};
 
-		auto future = script.GetModuleLoader().LoadModule(rq, "modified/base.js");
-
+		auto result = script.GetModuleLoader().LoadModule(rq, "modified/base.js");
 		g_ScriptContext->RunJobs();
 
+		auto& future = *result.begin();
 		JS::RootedObject ns{rq.cx, future.Get()};
 		JS::RootedValue moduleValue{rq.cx, JS::ObjectValue(*ns)};
 
-		std::string result;
-		TS_ASSERT(ScriptFunction::Call(rq, moduleValue, "fn", result));
-		TS_ASSERT_STR_EQUALS(result, "Base01");
+		std::string returnValue;
+		TS_ASSERT(ScriptFunction::Call(rq, moduleValue, "fn", returnValue));
+		TS_ASSERT_STR_EQUALS(returnValue, "Base01");
+	}
+
+	void test_Hotload()
+	{
+		constexpr int goal{2};
+
+		ScriptInterface script{"Test", "Test", g_ScriptContext};
+		const ScriptRequest rq{script};
+
+		int counter{0};
+		for (auto& future : script.GetModuleLoader().LoadModule(rq, "empty.js"))
+		{
+			TS_ASSERT(!future.IsDone());
+
+			if (counter != 0)
+				ClearFromCache("empty.js");
+
+			g_ScriptContext->RunJobs();
+			TS_ASSERT(future.IsDone());
+
+			if (counter == goal)
+				break;
+
+			++counter;
+		}
+
+		TS_ASSERT_EQUALS(counter, goal);
+	}
+
+	void test_HotloadWithoutIncrement()
+	{
+		ScriptInterface script{"Test", "Test", g_ScriptContext};
+		const ScriptRequest rq{script};
+
+		auto result = script.GetModuleLoader().LoadModule(rq, "top_level_await_finite.js");
+		g_ScriptContext->RunJobs();
+		TS_ASSERT(result.begin()->IsDone());
+		ClearFromCache("top_level_await_finite.js");
+		TS_ASSERT(result.begin()->IsDone());
+	}
+
+	void test_HotloadIndipendence()
+	{
+		ScriptInterface script{"Test", "Test", g_ScriptContext};
+		const ScriptRequest rq{script};
+
+		// It's intended to be used as in the test above but it's easier to test when it's unrolled.
+		auto result = script.GetModuleLoader().LoadModule(rq, "export.js");
+		auto iter = result.begin();
+		{
+			auto& future = *iter;
+			g_ScriptContext->RunJobs();
+			JS::RootedObject ns{rq.cx, future.Get()};
+			const JS::RootedValue moduleValue{rq.cx, JS::ObjectValue(*ns)};
+
+			TS_ASSERT(ScriptFunction::CallVoid(rq, moduleValue, "mutate", 12));
+
+			int value{0};
+			TS_ASSERT(Script::GetProperty(rq, moduleValue, "value", value));
+			TS_ASSERT_EQUALS(value, 12);
+		}
+		++iter;
+		{
+			auto& future = *iter;
+			g_ScriptContext->RunJobs();
+			TS_ASSERT(!future.IsDone());
+			ClearFromCache("export.js");
+			g_ScriptContext->RunJobs();
+			TS_ASSERT(future.IsDone());
+
+			JS::RootedObject ns{rq.cx, future.Get()};
+			JS::RootedValue moduleValue{rq.cx, JS::ObjectValue(*ns)};
+
+			int value{0};
+			TS_ASSERT(Script::GetProperty(rq, moduleValue, "value", value));
+			TS_ASSERT_DIFFERS(value, 12);
+			TS_ASSERT_EQUALS(value, 6);
+		}
+	}
+
+	void test_HotloadModified()
+	{
+		ScriptInterface script{"Test", "Test", g_ScriptContext};
+		const ScriptRequest rq{script};
+
+		auto result = script.GetModuleLoader().LoadModule(rq, "empty.js");
+		auto iter = result.begin();
+		g_ScriptContext->RunJobs();
+		TS_ASSERT(iter->IsDone());
+
+		++iter;
+		TS_ASSERT(!iter->IsDone());
+
+		ClearFromCache("empty~trigger.append.js");
+
+		g_ScriptContext->RunJobs();
+		TS_ASSERT(iter->IsDone());
+	}
+
+	void test_HotloadIndirect()
+	{
+		ScriptInterface script{"Test", "Test", g_ScriptContext};
+		const ScriptRequest rq{script};
+
+		auto result = script.GetModuleLoader().LoadModule(rq, "indirect.js");
+		auto iter = result.begin();
+		g_ScriptContext->RunJobs();
+		TS_ASSERT(iter->IsDone());
+
+		++iter;
+		ClearFromCache("export.js");
+
+		g_ScriptContext->RunJobs();
+		TS_ASSERT(iter->IsDone());
+	}
+
+	void test_HotloadUnobserved()
+	{
+		ScriptInterface script{"Test", "Test", g_ScriptContext};
+		{
+			const ScriptRequest rq{script};
+
+			TestLogger logger;
+			auto result = script.GetModuleLoader().LoadModule(rq, "blabbermouth.js");
+			g_ScriptContext->RunJobs();
+			TS_ASSERT_STR_CONTAINS(logger.GetOutput(), "blah blah blah");
+		}
+
+		{
+			TestLogger logger;
+			ClearFromCache("blabbermouth.js");
+			g_ScriptContext->RunJobs();
+			TS_ASSERT_STR_NOT_CONTAINS(logger.GetOutput(), "blah blah blah");
+		}
+
+		{
+			const ScriptRequest rq{script};
+
+			TestLogger logger;
+			auto result = script.GetModuleLoader().LoadModule(rq, "blabbermouth.js");
+			g_ScriptContext->RunJobs();
+			TS_ASSERT_STR_CONTAINS(logger.GetOutput(), "blah blah blah");
+		}
+	}
+
+	void test_HotloadAfterResultDestruction()
+	{
+		ScriptInterface script{"Test", "Test", g_ScriptContext};
+		{
+			const ScriptRequest rq{script};
+
+			TestLogger logger;
+			auto result = script.GetModuleLoader().LoadModule(rq, "blabbermouth.js");
+			g_ScriptContext->RunJobs();
+			TS_ASSERT_STR_CONTAINS(logger.GetOutput(), "blah blah blah");
+
+			auto iter = result.begin();
+			TS_ASSERT(iter->IsDone());
+			++iter;
+		}
+
+		TestLogger logger;
+		ClearFromCache("blabbermouth.js");
+		g_ScriptContext->RunJobs();
+		TS_ASSERT_STR_NOT_CONTAINS(logger.GetOutput(), "blah blah blah");
+	}
+
+	void test_ResultDestructionAfterScriptRequestDestruction()
+	{
+		ScriptInterface script{"Test", "Test", g_ScriptContext};
+		auto _ = script.GetModuleLoader().LoadModule(ScriptRequest{script}, "empty.js");
 	}
 };