Refactor initialization of main Fiber/Thread

Remove ThreadLocal usage for current thread. Initialize a main thread globally. Let that initialize the main fiber. Initialize main fiber *of each new thread* when thread starts. Clarify intent different of constructors.

Let fiber know if it's currently running in any thread by Fiber#thread.

src/fiber.cr

@@ -22,9 +22,11 @@ class Fiber
   protected property stack_bottom : Void*
   protected property next_fiber : Fiber?
   protected property prev_fiber : Fiber?
+  protected property thread : Thread?
   property name : String?
 
   def initialize(@name : String? = nil, &@proc : ->)
+    @thread = nil
     @stack = Fiber.allocate_stack
     @stack_bottom = @stack + STACK_SIZE
     fiber_main = ->(f : Fiber) { f.run }
@@ -82,15 +84,23 @@ class Fiber
     end
   end
 
-  def initialize
+  # Used to initialize the crystal object of the
+  # existing main fiber in the *thread*.
+  def initialize(@thread : Thread)
     @proc = Proc(Void).new { }
     @stack = Pointer(Void).null
     @stack_top = _fiber_get_stack_top
     @stack_bottom = GC.stack_bottom
     @name = "main"
 
     @@list_mutex.synchronize do
-      @@first_fiber = @@last_fiber = self
+      @prev_fiber = nil
+      if last_fiber = @@last_fiber
+        @prev_fiber = last_fiber
+        last_fiber.next_fiber = @@last_fiber = self
+      else
+        @@first_fiber = @@last_fiber = self
+      end
     end
   end
 
@@ -267,8 +277,12 @@ class Fiber
   end
 
   def resume : Nil
+    # The purpose of this method is to suspend a fiber (current) and give control back
+    # to another one (self).
     current, Thread.current.current_fiber = Thread.current.current_fiber, self
-    GC.stack_bottom = @stack_bottom
+    # current will be suspended, therefore it won't be assigned to any execution thread.
+    current.thread = nil
+    self.thread = Thread.current
     {% if flag?(:aarch64) %}
       Fiber.switch_stacks(pointerof(current.@stack_top), @stack_top)
     {% else %}
@@ -316,14 +330,6 @@ class Fiber
     GC.push_stack @stack_top, @stack_bottom
   end
 
-  @@root = new
-
-  def self.root : self
-    @@root
-  end
-
-  Thread.current.current_fiber = root
-
   def self.current : self
     Thread.current.current_fiber
   end
@@ -332,7 +338,7 @@ class Fiber
   GC.before_collect do
     fiber = @@first_fiber
     while fiber
-      fiber.push_gc_roots unless fiber == Thread.current.current_fiber
+      fiber.push_gc_roots if fiber.thread.nil?
       fiber = fiber.next_fiber
     end
   end

src/lib_c/x86_64-macosx-darwin/c/pthread.cr

@@ -8,7 +8,9 @@ lib LibC
   fun pthread_cond_wait(x0 : PthreadCondT*, x1 : PthreadMutexT*) : Int
   fun pthread_create(x0 : PthreadT*, x1 : PthreadAttrT*, x2 : Void* -> Void*, x3 : Void*) : Int
   fun pthread_detach(x0 : PthreadT) : Int
+  fun pthread_equal(thread1 : PthreadT, thread2 : PthreadT) : Int
   fun pthread_join(x0 : PthreadT, x1 : Void**) : Int
+  fun pthread_main_np : Int
   fun pthread_mutex_destroy(x0 : PthreadMutexT*) : Int
   fun pthread_mutex_init(x0 : PthreadMutexT*, x1 : PthreadMutexattrT*) : Int
   fun pthread_mutex_lock(x0 : PthreadMutexT*) : Int

src/thread.cr

@@ -11,11 +11,9 @@ class Thread
   @th : LibC::PthreadT?
   @exception : Exception?
   @detached = false
-
-  property current_fiber
+  property! current_fiber : Fiber?
 
   def initialize(&@func : ->)
-    @current_fiber = uninitialized Fiber
     @@mutex.synchronize do
       @@threads << self
       @th = th = uninitialized LibC::PthreadT
@@ -32,13 +30,17 @@ class Thread
     end
   end
 
+  # Used to initialize the crystal object of the
+  # existing main thread.
+  # Note *the* thread initialized with this constructor
+  # will not call `Thread#start`.
   def initialize
-    @current_fiber = uninitialized Fiber
     @func = ->{}
 
     @@mutex.synchronize do
       @@threads << self
       @th = LibC.pthread_self
+      @current_fiber = Fiber.new(self)
     end
   end
 
@@ -60,42 +62,21 @@ class Thread
   # thread local storage (eg: OpenBSD)
   @@threads = Set(Thread).new
 
-  {% if flag?(:openbsd) %}
-    @@main = new
-
-    def self.current : Thread
-      if LibC.pthread_main_np == 1
-        return @@main
-      end
-
-      current_thread_id = LibC.pthread_self
-
-      @@mutex.synchronize do
-        current_thread = @@threads.find do |thread|
-          LibC.pthread_equal(thread.id, current_thread_id) != 0
-        end
-      end
+  @@main = new
 
-      raise "Error: failed to find current thread" unless current_thread
-      current_thread
-    end
-
-    protected def id
-      @th.not_nil!
+  def self.current : Thread
+    if main?
+      return @@main
+    else
+      find_current_by_id
     end
-  {% else %}
-    @[ThreadLocal]
-    @@current = new
-
-    def self.current
-      @@current
-    end
-  {% end %}
+  end
 
   protected def start
-    {% unless flag?(:openbsd) %}
-    @@current = self
-    {% end %}
+    # Initialize main fiber of thread once the thread has started.
+    # Before the thread actually starts there is no fiber information
+    @current_fiber ||= Fiber.new(self)
+
     begin
       @func.call
     rescue ex
@@ -104,4 +85,27 @@ class Thread
       @@threads.delete(self)
     end
   end
+
+  # Checks if the current thread is the main thread
+  def self.main?
+    LibC.pthread_main_np == 1
+  end
+
+  # Find the current thread object with a linear search among all threads
+  protected def self.find_current_by_id : Thread
+    @@mutex.synchronize do
+      current_thread_id = LibC.pthread_self
+
+      current_thread = @@threads.find do |thread|
+        LibC.pthread_equal(thread.id, current_thread_id) != 0
+      end
+
+      raise "Error: failed to find current thread" unless current_thread
+      current_thread
+    end
+  end
+
+  protected def id
+    @th.not_nil!
+  end
 end