# `Fiber`¶ ↑

Fibers provide a mechanism for cooperative concurrency.

## Context Switching¶ ↑

Fibers execute a user-provided block. During the execution, the block may call `Fiber.yield` or `Fiber.transfer` to switch to another fiber. `Fiber#resume` is used to continue execution from the point where `Fiber.yield` was called.

    #!/usr/bin/env ruby

    puts "1: Start program."

    f = Fiber.new do
      puts "3: Entered fiber."
      Fiber.yield
      puts "5: Resumed fiber."
    end

    puts "2: Resume fiber first time."
    f.resume

    puts "4: Resume fiber second time."
    f.resume

    puts "6: Finished."

This program demonstrates the flow control of fibers.

## Scheduler¶ ↑

The scheduler interface is used to intercept blocking operations. A typical implementation would be a wrapper for a gem like `EventMachine` or `Async`. This design provides separation of concerns between the event loop implementation and application code. It also allows for layered schedulers which can perform instrumentation.

To set the scheduler for the current thread:

    Fiber.set_scheduler(MyScheduler.new)

When the thread exits, there is an implicit call to `set_scheduler`:

    Fiber.set_scheduler(nil)

### Interface¶ ↑

This is the interface you need to implement.

    class Scheduler
      # Wait for the specified process ID to exit.
      # This hook is optional.
      # @parameter pid [Integer] The process ID to wait for.
      # @parameter flags [Integer] A bit-mask of flags suitable for `Process::Status.wait`.
      # @returns [Process::Status] A process status instance.
      def process_wait(pid, flags)
        Thread.new do
          Process::Status.wait(pid, flags)
        end.value
      end

      # Wait for the given file descriptor to match the specified events within
      # the specified timeout.
      # @parameter event [Integer] A bit mask of `IO::READABLE`,
      #   `IO::WRITABLE` and `IO::PRIORITY`.
      # @parameter timeout [Numeric] The amount of time to wait for the event in seconds.
      # @returns [Integer] The subset of events that are ready.
      def io_wait(io, events, timeout)
      end

      # Sleep the current task for the specified duration, or forever if not
      # specified.
      # @param duration [Numeric] The amount of time to sleep in seconds.
      def kernel_sleep(duration = nil)
      end

      # Block the calling fiber.
      # @parameter blocker [Object] What we are waiting on, informational only.
      # @parameter timeout [Numeric | Nil] The amount of time to wait for in seconds.
      # @returns [Boolean] Whether the blocking operation was successful or not.
      def block(blocker, timeout = nil)
      end

      # Unblock the specified fiber.
      # @parameter blocker [Object] What we are waiting on, informational only.
      # @parameter fiber [Fiber] The fiber to unblock.
      # @reentrant Thread safe.
      def unblock(blocker, fiber)
      end

      # Intercept the creation of a non-blocking fiber.
      # @returns [Fiber]
      def fiber(&block)
        Fiber.new(blocking: false, &block)
      end

      # Invoked when the thread exits.
      def close
        self.run
      end

      def run
        # Implement event loop here.
      end
    end

Additional hooks may be introduced in the future, we will use feature detection in order to enable these hooks.

### Non-blocking Execution¶ ↑

The scheduler hooks will only be used in special non-blocking execution contexts. Non-blocking execution contexts introduce non-determinism because the execution of scheduler hooks may introduce context switching points into your program.

#### Fibers¶ ↑

Fibers can be used to create non-blocking execution contexts.

    Fiber.new do
      puts Fiber.current.blocking? # false

      # May invoke `Fiber.scheduler&.io_wait`.
      io.read(...)

      # May invoke `Fiber.scheduler&.io_wait`.
      io.write(...)

      # Will invoke `Fiber.scheduler&.kernel_sleep`.
      sleep(n)
    end.resume

We also introduce a new method which simplifies the creation of these non-blocking fibers:

    Fiber.schedule do
      puts Fiber.current.blocking? # false
    end

The purpose of this method is to allow the scheduler to internally decide the policy for when to start the fiber, and whether to use symmetric or asymmetric fibers.

You can also create blocking execution contexts:

    Fiber.new(blocking: true) do
      # Won't use the scheduler:
      sleep(n)
    end

However you should generally avoid this unless you are implementing a scheduler.

#### `IO`¶ ↑

By default, I/O is non-blocking. Not all operating systems support non-blocking I/O. Windows is a notable example where socket I/O can be non-blocking but pipe I/O is blocking. Provided that there is a scheduler and the current thread is non-blocking, the operation will invoke the scheduler.

#### `Mutex`¶ ↑

The `Mutex` class can be used in a non-blocking context and is fiber specific.

#### `ConditionVariable`¶ ↑

The `ConditionVariable` class can be used in a non-blocking context and is fiber-specific.

#### `Queue` / `SizedQueue`¶ ↑

The `Queue` and `SizedQueue` classes can be used in a non-blocking context and are fiber-specific.

#### `Thread`¶ ↑

The `Thread#join` operation can be used in a non-blocking context and is fiber-specific.