RFC 2: Skeleton for ExecutionContext (crystal-lang#15350)

Integrates the skeleton as per crystal-lang/rfcs#2

- Add the `ExecutionContext` module;
- Add the `ExecutionContext::Scheduler` module;
- Add the `execution_context` compile-time flag.

When the `execution_context` flag is set:

- Don't load `Crystal::Scheduler`;
- Plug `ExecutionContext` instead of `Crystal::Scheduler` in `spawn`, `Fiber`, ...

This is only the skeleton: there are no implementations (yet). Trying to compile anything with `-Dexecution_context` will fail until the ST and/or MT context are implemented.

Co-authored-by: Johannes Müller <[email protected]>

Author: 2 people

src/concurrent.cr

@@ -1,8 +1,13 @@
 require "fiber"
 require "channel"
-require "crystal/scheduler"
 require "crystal/tracing"
 
+{% if flag?(:execution_context) %}
+  require "fiber/execution_context"
+{% else %}
+  require "crystal/scheduler"
+{% end %}
+
 # Blocks the current fiber for the specified number of seconds.
 #
 # While this fiber is waiting this time, other ready-to-execute
@@ -12,25 +17,36 @@ def sleep(seconds : Number) : Nil
   if seconds < 0
     raise ArgumentError.new "Sleep seconds must be positive"
   end
-
-  Crystal::Scheduler.sleep(seconds.seconds)
+  sleep(seconds.seconds)
 end
 
 # Blocks the current Fiber for the specified time span.
 #
 # While this fiber is waiting this time, other ready-to-execute
 # fibers might start their execution.
 def sleep(time : Time::Span) : Nil
-  Crystal::Scheduler.sleep(time)
+  Crystal.trace :sched, "sleep", for: time
+
+  {% if flag?(:execution_context) %}
+    Fiber.current.resume_event.add(time)
+    Fiber::ExecutionContext.reschedule
+  {% else %}
+    Crystal::Scheduler.sleep(time)
+  {% end %}
 end
 
 # Blocks the current fiber forever.
 #
 # Meanwhile, other ready-to-execute fibers might start their execution.
 def sleep : Nil
-  Crystal::Scheduler.reschedule
+  {% if flag?(:execution_context) %}
+    Fiber::ExecutionContext.reschedule
+  {% else %}
+    Crystal::Scheduler.reschedule
+  {% end %}
 end
 
+{% begin %}
 # Spawns a new fiber.
 #
 # NOTE: The newly created fiber doesn't run as soon as spawned.
@@ -64,12 +80,17 @@ end
 # wg.wait
 # ```
 def spawn(*, name : String? = nil, same_thread = false, &block)
-  fiber = Fiber.new(name, &block)
-  Crystal.trace :sched, "spawn", fiber: fiber
-  {% if flag?(:preview_mt) %} fiber.set_current_thread if same_thread {% end %}
-  fiber.enqueue
-  fiber
+  {% if flag?(:execution_context) %}
+    Fiber::ExecutionContext::Scheduler.current.spawn(name: name, same_thread: same_thread, &block)
+  {% else %}
+    fiber = Fiber.new(name, &block)
+    Crystal.trace :sched, "spawn", fiber: fiber
+    {% if flag?(:preview_mt) %} fiber.set_current_thread if same_thread {% end %}
+    fiber.enqueue
+    fiber
+  {% end %}
 end
+{% end %}
 
 # Spawns a fiber by first creating a `Proc`, passing the *call*'s
 # expressions to it, and letting the `Proc` finally invoke the *call*.

src/crystal/event_loop.cr

@@ -27,12 +27,20 @@ abstract class Crystal::EventLoop
 
   @[AlwaysInline]
   def self.current : self
-    Crystal::Scheduler.event_loop
+    {% if flag?(:execution_context) %}
+      Fiber::ExecutionContext.current.event_loop
+    {% else %}
+      Crystal::Scheduler.event_loop
+    {% end %}
   end
 
   @[AlwaysInline]
-  def self.current? : self?
-    Crystal::Scheduler.event_loop?
+  def self.current? : self | Nil
+    {% if flag?(:execution_context) %}
+      Fiber::ExecutionContext.current.event_loop
+    {% else %}
+      Crystal::Scheduler.event_loop?
+    {% end %}
   end
 
   # Runs the loop.
@@ -46,6 +54,13 @@ abstract class Crystal::EventLoop
   # events.
   abstract def run(blocking : Bool) : Bool
 
+  {% if flag?(:execution_context) %}
+    # Same as `#run` but collects runnable fibers into *queue* instead of
+    # enqueueing in parallel, so the caller is responsible and in control for
+    # when and how the fibers will be enqueued.
+    abstract def run(queue : Fiber::List*, blocking : Bool) : Nil
+  {% end %}
+
   # Tells a blocking run loop to no longer wait for events to activate. It may
   # for example enqueue a NOOP event with an immediate (or past) timeout. Having
   # activated an event, the loop shall return, allowing the blocked thread to

src/crystal/event_loop/iocp.cr

@@ -55,6 +55,7 @@ class Crystal::EventLoop::IOCP < Crystal::EventLoop
     iocp
   end
 
+  # thread unsafe
   def run(blocking : Bool) : Bool
     enqueued = false
 
@@ -66,6 +67,13 @@ class Crystal::EventLoop::IOCP < Crystal::EventLoop
     enqueued
   end
 
+  {% if flag?(:execution_context) %}
+    # thread unsafe
+    def run(queue : Fiber::List*, blocking : Bool) : Nil
+      run_impl(blocking) { |fiber| queue.value.push(fiber) }
+    end
+  {% end %}
+
   # Runs the event loop and enqueues the fiber for the next upcoming event or
   # completion.
   private def run_impl(blocking : Bool, &) : Nil

src/crystal/event_loop/libevent.cr

@@ -20,26 +20,55 @@ class Crystal::EventLoop::LibEvent < Crystal::EventLoop
     event_base.loop(flags)
   end
 
+  {% if flag?(:execution_context) %}
+    def run(queue : Fiber::List*, blocking : Bool) : Nil
+      Crystal.trace :evloop, "run", fiber: fiber, blocking: blocking
+      @runnables = queue
+      run(blocking)
+    ensure
+      @runnables = nil
+    end
+
+    def callback_enqueue(fiber : Fiber) : Nil
+      if queue = @runnables
+        queue.value.push(fiber)
+      else
+        raise "BUG: libevent callback executed outside of #run(queue*, blocking) call"
+      end
+    end
+  {% end %}
+
   def interrupt : Nil
     event_base.loop_exit
   end
 
-  # Create a new resume event for a fiber.
+  # Create a new resume event for a fiber (sleep).
   def create_resume_event(fiber : Fiber) : Crystal::EventLoop::LibEvent::Event
     event_base.new_event(-1, LibEvent2::EventFlags::None, fiber) do |s, flags, data|
-      data.as(Fiber).enqueue
+      f = data.as(Fiber)
+      {% if flag?(:execution_context) %}
+        event_loop = Crystal::EventLoop.current.as(Crystal::EventLoop::LibEvent)
+        event_loop.callback_enqueue(f)
+      {% else %}
+        f.enqueue
+      {% end %}
     end
   end
 
-  # Creates a timeout_event.
+  # Creates a timeout event (timeout action of select expression).
   def create_timeout_event(fiber) : Crystal::EventLoop::LibEvent::Event
     event_base.new_event(-1, LibEvent2::EventFlags::None, fiber) do |s, flags, data|
       f = data.as(Fiber)
-      if (select_action = f.timeout_select_action)
+      if select_action = f.timeout_select_action
         f.timeout_select_action = nil
-        select_action.time_expired(f)
-      else
-        f.enqueue
+        if select_action.time_expired?
+          {% if flag?(:execution_context) %}
+            event_loop = Crystal::EventLoop.current.as(Crystal::EventLoop::LibEvent)
+            event_loop.callback_enqueue(f)
+          {% else %}
+            f.enqueue
+          {% end %}
+        end
       end
     end
   end

src/crystal/event_loop/polling.cr

@@ -112,14 +112,25 @@ abstract class Crystal::EventLoop::Polling < Crystal::EventLoop
     end
   {% end %}
 
-  # NOTE: thread unsafe
+  # thread unsafe
   def run(blocking : Bool) : Bool
     system_run(blocking) do |fiber|
-      Crystal::Scheduler.enqueue(fiber)
+      {% if flag?(:execution_context) %}
+        fiber.execution_context.enqueue(fiber)
+      {% else %}
+        Crystal::Scheduler.enqueue(fiber)
+      {% end %}
     end
     true
   end
 
+  {% if flag?(:execution_context) %}
+    # thread unsafe
+    def run(queue : Fiber::List*, blocking : Bool) : Nil
+      system_run(blocking) { |fiber| queue.value.push(fiber) }
+    end
+  {% end %}
+
   # fiber interface, see Crystal::EventLoop
 
   def create_resume_event(fiber : Fiber) : FiberEvent
@@ -327,13 +338,21 @@ abstract class Crystal::EventLoop::Polling < Crystal::EventLoop
     Polling.arena.free(index) do |pd|
       pd.value.@readers.ready_all do |event|
         pd.value.@event_loop.try(&.unsafe_resume_io(event) do |fiber|
-          Crystal::Scheduler.enqueue(fiber)
+          {% if flag?(:execution_context) %}
+            fiber.execution_context.enqueue(fiber)
+          {% else %}
+            Crystal::Scheduler.enqueue(fiber)
+          {% end %}
         end)
       end
 
       pd.value.@writers.ready_all do |event|
         pd.value.@event_loop.try(&.unsafe_resume_io(event) do |fiber|
-          Crystal::Scheduler.enqueue(fiber)
+          {% if flag?(:execution_context) %}
+            fiber.execution_context.enqueue(fiber)
+          {% else %}
+            Crystal::Scheduler.enqueue(fiber)
+          {% end %}
         end)
       end
 

src/crystal/scheduler.cr

@@ -1,3 +1,5 @@
+{% skip_file if flag?(:execution_context) %}
+
 require "crystal/event_loop"
 require "crystal/system/print_error"
 require "fiber"
@@ -66,7 +68,6 @@ class Crystal::Scheduler
   end
 
   def self.sleep(time : Time::Span) : Nil
-    Crystal.trace :sched, "sleep", for: time
     Thread.current.scheduler.sleep(time)
   end
 

src/crystal/system/thread.cr

@@ -79,6 +79,47 @@ class Thread
 
   getter name : String?
 
+  {% if flag?(:execution_context) %}
+    # :nodoc:
+    getter! execution_context : Fiber::ExecutionContext
+
+    # :nodoc:
+    property! scheduler : Fiber::ExecutionContext::Scheduler
+
+    # :nodoc:
+    def execution_context=(@execution_context : Fiber::ExecutionContext) : Fiber::ExecutionContext
+      main_fiber.execution_context = execution_context
+    end
+
+    # When a fiber terminates we can't release its stack until we swap context
+    # to another fiber. We can't free/unmap nor push it to a shared stack pool,
+    # that would result in a segfault.
+    @dead_fiber_stack : Fiber::Stack?
+
+    # :nodoc:
+    def dying_fiber(fiber : Fiber) : Fiber::Stack?
+      stack = @dead_fiber_stack
+      @dead_fiber_stack = fiber.@stack
+      stack
+    end
+
+    # :nodoc:
+    def dead_fiber_stack? : Fiber::Stack?
+      if stack = @dead_fiber_stack
+        @dead_fiber_stack = nil
+        stack
+      end
+    end
+  {% else %}
+    # :nodoc:
+    getter scheduler : Crystal::Scheduler { Crystal::Scheduler.new(self) }
+
+    # :nodoc:
+    def scheduler? : ::Crystal::Scheduler?
+      @scheduler
+    end
+  {% end %}
+
   def self.unsafe_each(&)
     # nothing to iterate when @@threads is nil + don't lazily allocate in a
     # method called from a GC collection callback!
@@ -165,14 +206,6 @@ class Thread
     thread.name = name
   end
 
-  # :nodoc:
-  getter scheduler : Crystal::Scheduler { Crystal::Scheduler.new(self) }
-
-  # :nodoc:
-  def scheduler? : ::Crystal::Scheduler?
-    @scheduler
-  end
-
   protected def start
     Thread.threads.push(self)
     Thread.current = self

src/crystal/tracing.cr

@@ -82,6 +82,16 @@ module Crystal
           write value.name || '?'
         end
 
+        {% if flag?(:execution_context) %}
+          def write(value : Fiber::ExecutionContext) : Nil
+            write value.name
+          end
+
+          def write(value : Fiber::ExecutionContext::Scheduler) : Nil
+            write value.name
+          end
+        {% end %}
+
         def write(value : Pointer) : Nil
           write "0x"
           write System.to_int_slice(@int_buf.to_slice, value.address, 16, true, 2)