From 137d862d9b275209b3d62a413396a15e9e14b4b4 Mon Sep 17 00:00:00 2001
From: Fernando Sahmkow <fsahmkow27@gmail.com>
Date: Thu, 27 Feb 2020 16:32:47 -0400
Subject: Common/Fiber: Implement Rewinding.

---
 src/common/fiber.cpp        | 32 +++++++++++++++++++++++++++++--
 src/common/fiber.h          |  8 ++++++++
 src/tests/common/fibers.cpp | 46 +++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 84 insertions(+), 2 deletions(-)

diff --git a/src/common/fiber.cpp b/src/common/fiber.cpp
index e4ecc73df..f61479e13 100644
--- a/src/common/fiber.cpp
+++ b/src/common/fiber.cpp
@@ -12,10 +12,13 @@
 
 namespace Common {
 
+constexpr std::size_t default_stack_size = 256 * 1024; // 256kb
+
 #if defined(_WIN32) || defined(WIN32)
 
 struct Fiber::FiberImpl {
     LPVOID handle = nullptr;
+    LPVOID rewind_handle = nullptr;
 };
 
 void Fiber::start() {
@@ -26,15 +29,29 @@ void Fiber::start() {
     UNREACHABLE();
 }
 
+void Fiber::onRewind() {
+    ASSERT(impl->handle != nullptr);
+    DeleteFiber(impl->handle);
+    impl->handle = impl->rewind_handle;
+    impl->rewind_handle = nullptr;
+    rewind_point(rewind_parameter);
+    UNREACHABLE();
+}
+
 void __stdcall Fiber::FiberStartFunc(void* fiber_parameter) {
     auto fiber = static_cast<Fiber*>(fiber_parameter);
     fiber->start();
 }
 
+void __stdcall Fiber::RewindStartFunc(void* fiber_parameter) {
+    auto fiber = static_cast<Fiber*>(fiber_parameter);
+    fiber->onRewind();
+}
+
 Fiber::Fiber(std::function<void(void*)>&& entry_point_func, void* start_parameter)
     : entry_point{std::move(entry_point_func)}, start_parameter{start_parameter} {
     impl = std::make_unique<FiberImpl>();
-    impl->handle = CreateFiber(0, &FiberStartFunc, this);
+    impl->handle = CreateFiber(default_stack_size, &FiberStartFunc, this);
 }
 
 Fiber::Fiber() {
@@ -60,6 +77,18 @@ void Fiber::Exit() {
     guard.unlock();
 }
 
+void Fiber::SetRewindPoint(std::function<void(void*)>&& rewind_func, void* start_parameter) {
+    rewind_point = std::move(rewind_func);
+    rewind_parameter = start_parameter;
+}
+
+void Fiber::Rewind() {
+    ASSERT(rewind_point);
+    ASSERT(impl->rewind_handle == nullptr);
+    impl->rewind_handle = CreateFiber(default_stack_size, &RewindStartFunc, this);
+    SwitchToFiber(impl->rewind_handle);
+}
+
 void Fiber::YieldTo(std::shared_ptr<Fiber> from, std::shared_ptr<Fiber> to) {
     ASSERT_MSG(from != nullptr, "Yielding fiber is null!");
     ASSERT_MSG(to != nullptr, "Next fiber is null!");
@@ -81,7 +110,6 @@ std::shared_ptr<Fiber> Fiber::ThreadToFiber() {
 }
 
 #else
-constexpr std::size_t default_stack_size = 1024 * 1024; // 1MB
 
 struct Fiber::FiberImpl {
     alignas(64) std::array<u8, default_stack_size> stack;
diff --git a/src/common/fiber.h b/src/common/fiber.h
index 7e3b130a4..a710df257 100644
--- a/src/common/fiber.h
+++ b/src/common/fiber.h
@@ -46,6 +46,10 @@ public:
     static void YieldTo(std::shared_ptr<Fiber> from, std::shared_ptr<Fiber> to);
     static std::shared_ptr<Fiber> ThreadToFiber();
 
+    void SetRewindPoint(std::function<void(void*)>&& rewind_func, void* start_parameter);
+
+    void Rewind();
+
     /// Only call from main thread's fiber
     void Exit();
 
@@ -58,8 +62,10 @@ private:
     Fiber();
 
 #if defined(_WIN32) || defined(WIN32)
+    void onRewind();
     void start();
     static void FiberStartFunc(void* fiber_parameter);
+    static void RewindStartFunc(void* fiber_parameter);
 #else
     void start(boost::context::detail::transfer_t& transfer);
     static void FiberStartFunc(boost::context::detail::transfer_t transfer);
@@ -69,6 +75,8 @@ private:
 
     SpinLock guard{};
     std::function<void(void*)> entry_point{};
+    std::function<void(void*)> rewind_point{};
+    void* rewind_parameter{};
     void* start_parameter{};
     std::shared_ptr<Fiber> previous_fiber{};
     std::unique_ptr<FiberImpl> impl;
diff --git a/src/tests/common/fibers.cpp b/src/tests/common/fibers.cpp
index 0d3d5153d..12536b6d8 100644
--- a/src/tests/common/fibers.cpp
+++ b/src/tests/common/fibers.cpp
@@ -309,4 +309,50 @@ TEST_CASE("Fibers::StartRace", "[common]") {
     REQUIRE(test_control.value3 == 1);
 }
 
+class TestControl4;
+
+static void WorkControl4(void* control);
+
+class TestControl4 {
+public:
+    TestControl4() {
+        fiber1 = std::make_shared<Fiber>(std::function<void(void*)>{WorkControl4}, this);
+        goal_reached = false;
+        rewinded = false;
+    }
+
+    void Execute() {
+        thread_fiber = Fiber::ThreadToFiber();
+        Fiber::YieldTo(thread_fiber, fiber1);
+        thread_fiber->Exit();
+    }
+
+    void DoWork() {
+        fiber1->SetRewindPoint(std::function<void(void*)>{WorkControl4}, this);
+        if (rewinded) {
+            goal_reached = true;
+            Fiber::YieldTo(fiber1, thread_fiber);
+        }
+        rewinded = true;
+        fiber1->Rewind();
+    }
+
+    std::shared_ptr<Common::Fiber> fiber1;
+    std::shared_ptr<Common::Fiber> thread_fiber;
+    bool goal_reached;
+    bool rewinded;
+};
+
+static void WorkControl4(void* control) {
+    auto* test_control = static_cast<TestControl4*>(control);
+    test_control->DoWork();
+}
+
+TEST_CASE("Fibers::Rewind", "[common]") {
+    TestControl4 test_control{};
+    test_control.Execute();
+    REQUIRE(test_control.goal_reached);
+    REQUIRE(test_control.rewinded);
+}
+
 } // namespace Common
-- 
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