Reference#exec_recursive[_clone] methods aren't fiber aware

Recursion of the `#inspect` and `#pretty_print` methods is handled by a
global hash, which is a per-thread global, but if any of these methods
yield —which will happen because they're writing to an IO— another
fiber running on the same thread will falsely detect a recursion if it
inspects the same object.

This patch moves `Reference#exec_recursive` methods as instances of
`Fiber`, using a per-fiber hash instead of per-thread/process hash.

References #15088

src/fiber.cr

@@ -162,6 +162,10 @@ class Fiber
     @timeout_event.try &.free
     @timeout_select_action = nil
 
+    # Additional cleanup (avoid stale references)
+    @exec_recursive = nil
+    @exec_recursive_clone = nil
+
     @alive = false
     {% unless flag?(:interpreted) %}
       Crystal::Scheduler.stack_pool.release(@stack)
@@ -331,4 +335,41 @@ class Fiber
       @current_thread.lazy_get
     end
   {% end %}
+
+  # :nodoc:
+  #
+  # Protects `Reference#inspect` and `Reference#pretty_print` against recursion.
+  def exec_recursive(object_id : UInt64, method : Symbol, &) : Bool
+    LibC.dprintf 2, "#{Fiber.current.name}: #{self.object_id}#exec_recursive(#{object_id}, #{method})\n"
+
+    # NOTE: can't use `Set` because of prelude require order
+    hash = (@exec_recursive ||= Hash({UInt64, Symbol}, Nil).new)
+
+    key = {object_id, method}
+    hash.put(key, nil) do
+      LibC.dprintf 2, "#{Fiber.current.name}: #{self.object_id}#exec_recursive(#{object_id}, #{method}): yield\n"
+      yield
+      hash.delete(key)
+      LibC.dprintf 2, "#{Fiber.current.name}: #{self.object_id}#exec_recursive(#{object_id}, #{method}): done\n"
+      return true
+    end
+
+    LibC.dprintf 2, "#{Fiber.current.name}: #{self.object_id}#exec_recursive(#{object_id}, #{method}): done\n"
+    false
+  end
+
+  # :nodoc:
+  #
+  # Protects `Reference#clone` implementations against recursion. See
+  # `Reference#exec_recursive_clone` for more details.
+  def exec_recursive_clone(object_id : UInt64, &) : Bool
+    # NOTE: can't use `Set` because of prelude require order
+    hash = (@exec_recursive_clone ||= Hash(UInt64, Nil).new)
+    hash.put(object_id, nil) do
+      yield
+      hash.delete(object_id)
+      return true
+    end
+    false
+  end
 end

src/reference.cr

@@ -1,7 +1,3 @@
-{% if flag?(:preview_mt) %}
-  require "crystal/thread_local_value"
-{% end %}
-
 # `Reference` is the base class of classes you define in your program.
 # It is set as a class' superclass when you don't specify one:
 #
@@ -180,54 +176,8 @@ class Reference
     io << '>'
   end
 
-  # :nodoc:
-  module ExecRecursive
-    # NOTE: can't use `Set` here because of prelude require order
-    alias Registry = Hash({UInt64, Symbol}, Nil)
-
-    {% if flag?(:preview_mt) %}
-      @@exec_recursive = Crystal::ThreadLocalValue(Registry).new
-    {% else %}
-      @@exec_recursive = Registry.new
-    {% end %}
-
-    def self.hash
-      {% if flag?(:preview_mt) %}
-        @@exec_recursive.get { Registry.new }
-      {% else %}
-        @@exec_recursive
-      {% end %}
-    end
-  end
-
   private def exec_recursive(method, &)
-    hash = ExecRecursive.hash
-    key = {object_id, method}
-    hash.put(key, nil) do
-      yield
-      hash.delete(key)
-      return true
-    end
-    false
-  end
-
-  # :nodoc:
-  module ExecRecursiveClone
-    alias Registry = Hash(UInt64, UInt64)
-
-    {% if flag?(:preview_mt) %}
-      @@exec_recursive = Crystal::ThreadLocalValue(Registry).new
-    {% else %}
-      @@exec_recursive = Registry.new
-    {% end %}
-
-    def self.hash
-      {% if flag?(:preview_mt) %}
-        @@exec_recursive.get { Registry.new }
-      {% else %}
-        @@exec_recursive
-      {% end %}
-    end
+    Fiber.current.exec_recursive(object_id, method) { yield }
   end
 
   # Helper method to perform clone by also checking recursiveness.
@@ -249,12 +199,6 @@ class Reference
   # end
   # ```
   private def exec_recursive_clone(&)
-    hash = ExecRecursiveClone.hash
-    clone_object_id = hash[object_id]?
-    unless clone_object_id
-      clone_object_id = yield(hash).object_id
-      hash.delete(object_id)
-    end
-    Pointer(Void).new(clone_object_id).as(self)
+    Fiber.current.exec_recursive_clone(object_id) { yield }
   end
 end