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# `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)

### Design

The scheduler interface is designed to be a un-opinionated light-weight layer between user code and blocking operations. The scheduler hooks should avoid translating or converting arguments or return values. Ideally, the exact same arguments from the user code are provided directly to the scheduler hook with no changes.

### 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 io readiness 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

      # Read from the given io into the specified buffer.
      # WARNING: Experimental hook! Do not use in production code!
      # @parameter io [IO] The io to read from.
      # @parameter buffer [IO::Buffer] The buffer to read into.
      # @parameter length [Integer] The minimum amount to read.
      def io_read(io, buffer, length)
      end

      # Write from the given buffer into the specified IO.
      # WARNING: Experimental hook! Do not use in production code!
      # @parameter io [IO] The io to write to.
      # @parameter buffer [IO::Buffer] The buffer to write from.
      # @parameter length [Integer] The minimum amount to write.
      def io_write(io, buffer, length)
      end

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

      # Execute the given block. If the block execution exceeds the given timeout,
      # the specified exception `klass` will be raised. Typically, only non-blocking
      # methods which enter the scheduler will raise such exceptions.
      # @parameter duration [Integer] The amount of time to wait, after which an exception will be raised.
      # @parameter klass [Class] The exception class to raise.
      # @parameter *arguments [Array] The arguments to send to the constructor of the exception.
      # @yields {...} The user code to execute.
      def timeout_after(duration, klass, *arguments, &block)
      end

      # Resolve hostname to an array of IP addresses.
      # This hook is optional.
      # @parameter hostname [String] Example: "www.ruby-lang.org".
      # @returns [Array] An array of IPv4 and/or IPv6 address strings that the hostname resolves to.
      def address_resolve(hostname)
      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.

##### `IO#close`

Closing an `IO` interrupts all blocking operations on that `IO`. When a thread calls `IO#close`, it first attempts to interrupt any threads or fibers that are blocked on that `IO`. The closing thread waits until all blocked threads and fibers have been properly interrupted and removed from the IO’s blocking list. Each interrupted thread or fiber receives an `IOError` and is cleanly removed from the blocking operation. Only after all blocking operations have been interrupted and cleaned up will the actual file descriptor be closed, ensuring proper resource cleanup and preventing potential race conditions.

For fibers managed by a scheduler, the interruption process involves calling `rb_fiber_scheduler_fiber_interrupt` on the scheduler. This allows the scheduler to handle the interruption in a way that’s appropriate for its event loop implementation. The scheduler can then notify the fiber, which will receive an `IOError` and be removed from the blocking operation. This mechanism ensures that fiber-based concurrency works correctly with `IO` operations, even when those operations are interrupted by `IO#close`.

    sequenceDiagram
        participant ThreadB
        participant ThreadA
        participant Scheduler
        participant IO
        participant Fiber1
        participant Fiber2

        Note over ThreadA: Thread A has a fiber scheduler
        activate Scheduler
        ThreadA->>Fiber1: Schedule Fiber 1
        activate Fiber1
        Fiber1->>IO: IO.read
        IO->>Scheduler: rb_thread_io_blocking_region
        deactivate Fiber1

        ThreadA->>Fiber2: Schedule Fiber 2
        activate Fiber2
        Fiber2->>IO: IO.read
        IO->>Scheduler: rb_thread_io_blocking_region
        deactivate Fiber2

        Note over Fiber1,Fiber2: Both fibers blocked on same IO

        Note over ThreadB: IO.close
        activate ThreadB
        ThreadB->>IO: thread_io_close_notify_all
        Note over ThreadB: rb_mutex_sleep

        IO->>Scheduler: rb_fiber_scheduler_fiber_interrupt(Fiber1)
        Scheduler->>Fiber1: fiber_interrupt with IOError
        activate Fiber1
        Note over IO: fiber_interrupt causes removal from blocking list
        Fiber1->>IO: rb_io_blocking_operation_exit()
        IO-->>ThreadB: Wakeup thread
        deactivate Fiber1

        IO->>Scheduler: rb_fiber_scheduler_fiber_interrupt(Fiber2)
        Scheduler->>Fiber2: fiber_interrupt with IOError
        activate Fiber2
        Note over IO: fiber_interrupt causes removal from blocking list
        Fiber2->>IO: rb_io_blocking_operation_exit()
        IO-->>ThreadB: Wakeup thread
        deactivate Fiber2
        deactivate Scheduler

        Note over ThreadB: Blocking operations list empty
        ThreadB->>IO: close(fd)
        deactivate ThreadB

#### 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.