src/osfiber_ucontext.h

// Copyright 2019 The Marl Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     https://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

#if !defined(_XOPEN_SOURCE)
// This must come before other #includes, otherwise we'll end up with ucontext_t
// definition mismatches, leading to memory corruption hilarity.
#define _XOPEN_SOURCE
#endif  //  !defined(_XOPEN_SOURCE)

#include "marl/debug.h"

#include <functional>
#include <memory>

#include <ucontext.h>

#if defined(__clang__)
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wdeprecated-declarations"
#endif  // defined(__clang__)

namespace marl {

class OSFiber {
 public:
  // createFiberFromCurrentThread() returns a fiber created from the current
  // thread.
  static inline OSFiber* createFiberFromCurrentThread();

  // createFiber() returns a new fiber with the given stack size that will
  // call func when switched to. func() must end by switching back to another
  // fiber, and must not return.
  static inline OSFiber* createFiber(size_t stackSize,
                                     const std::function<void()>& func);

  // switchTo() immediately switches execution to the given fiber.
  // switchTo() must be called on the currently executing fiber.
  inline void switchTo(OSFiber*);

 private:
  std::unique_ptr<uint8_t[]> stack;
  ucontext_t context;
  std::function<void()> target;
};

OSFiber* OSFiber::createFiberFromCurrentThread() {
  auto out = new OSFiber();
  out->context = {};
  getcontext(&out->context);
  return out;
}

OSFiber* OSFiber::createFiber(size_t stackSize,
                              const std::function<void()>& func) {
  union Args {
    OSFiber* self;
    struct {
      int a;
      int b;
    };
  };

  struct Target {
    static void Main(int a, int b) {
      Args u;
      u.a = a;
      u.b = b;
      std::function<void()> func;
      std::swap(func, u.self->target);
      func();
    }
  };

  auto out = new OSFiber();
  out->context = {};
  out->stack = std::unique_ptr<uint8_t[]>(new uint8_t[stackSize]);
  out->target = func;

  auto alignmentOffset =
      15 - (reinterpret_cast<uintptr_t>(out->stack.get() + 15) & 15);
  auto res = getcontext(&out->context);
  MARL_ASSERT(res == 0, "getcontext() returned %d", int(res));
  out->context.uc_stack.ss_sp = out->stack.get() + alignmentOffset;
  out->context.uc_stack.ss_size = stackSize - alignmentOffset;
  out->context.uc_link = nullptr;

  Args args;
  args.self = out;
  makecontext(&out->context, reinterpret_cast<void (*)()>(&Target::Main), 2,
              args.a, args.b);

  return out;
}

void OSFiber::switchTo(OSFiber* fiber) {
  auto res = swapcontext(&context, &fiber->context);
  MARL_ASSERT(res == 0, "swapcontext() returned %d", int(res));
}

}  // namespace marl

#if defined(__clang__)
#pragma clang diagnostic pop
#endif  // defined(__clang__)