src/base/fiber.hh

/*
 * Copyright 2018 Google, Inc.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are
 * met: redistributions of source code must retain the above copyright
 * notice, this list of conditions and the following disclaimer;
 * redistributions in binary form must reproduce the above copyright
 * notice, this list of conditions and the following disclaimer in the
 * documentation and/or other materials provided with the distribution;
 * neither the name of the copyright holders nor the names of its
 * contributors may be used to endorse or promote products derived from
 * this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
 * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
 * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
 * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
 * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 *
 * Authors: Gabe Black
 */

#ifndef __BASE_FIBER_HH__
#define __BASE_FIBER_HH__

#include <ucontext.h>

#include <cstddef>
#include <cstdint>

/**
 * This class represents a fiber, which is a light weight sort of thread which
 * is cooperatively scheduled and runs sequentially with other fibers, swapping
 * in and out of a single actual thread of execution.
 *
 * To define your own threads, create a subclass of Fiber and override its
 * main() function to do what you want your fiber to do. You can start it by
 * calling its run() method which will stop your execution and start the other
 * fiber in your place.
 *
 * If your main() function ends, that fiber will automatically switch to either
 * the primary fiber, or to a particular fiber you specified at construction
 * time, and your fiber is considered finished.
 */

class Fiber
{
  public:
    const static size_t DefaultStackSize = 0x50000;

    /// stack_size is the size of the stack available to this fiber.
    /// link points to another fiber which will start executing when this
    /// fiber's main function returns.
    Fiber(size_t stack_size=DefaultStackSize);
    Fiber(Fiber *link, size_t stack_size=DefaultStackSize);

    virtual ~Fiber();

    /// Start executing the fiber represented by this object. This function
    /// will "return" when the current fiber is switched back to later on.
    void run();

    /// Returns whether the "main" function of this fiber has finished.
    ///
    bool finished() const { return _finished; };

    /// Get a pointer to the current running Fiber.
    ///
    static Fiber *currentFiber();
    /// Get a pointer to the primary Fiber.
    /// This Fiber represents the thread of execution started by the OS, and
    /// which has a Fiber attached to it after the fact.
    static Fiber *primaryFiber();

  protected:
    /// This method is called when this fiber is first run. Override it to
    /// give your fiber something to do. When main returns, the fiber will
    /// mark itself as finished and switch to its link fiber.
    virtual void main() = 0;

    void setStarted() { started = true; }

  private:
    static void entryTrampoline();
    void start();

    ucontext_t ctx;
    Fiber *link;

    // The stack for this context, or a nullptr if allocated elsewhere.
    uint8_t *stack;
    size_t stackSize;

    bool started;
    bool _finished;
    void createContext();
};

#endif // __BASE_FIBER_HH__