1 files changed, 130 insertions, 0 deletions
diff --git a/activesupport/lib/active_support/reloader.rb b/activesupport/lib/active_support/reloader.rb
new file mode 100644
index 0000000000..2f81cd4f80
--- /dev/null
+++ b/activesupport/lib/active_support/reloader.rb
@@ -0,0 +1,130 @@
+# frozen_string_literal: true
+
+require "active_support/execution_wrapper"
+require "active_support/executor"
+
+module ActiveSupport
+  #--
+  # This class defines several callbacks:
+  #
+  #   to_prepare -- Run once at application startup, and also from
+  #   +to_run+.
+  #
+  #   to_run -- Run before a work run that is reloading. If
+  #   +reload_classes_only_on_change+ is true (the default), the class
+  #   unload will have already occurred.
+  #
+  #   to_complete -- Run after a work run that has reloaded. If
+  #   +reload_classes_only_on_change+ is false, the class unload will
+  #   have occurred after the work run, but before this callback.
+  #
+  #   before_class_unload -- Run immediately before the classes are
+  #   unloaded.
+  #
+  #   after_class_unload -- Run immediately after the classes are
+  #   unloaded.
+  #
+  class Reloader < ExecutionWrapper
+    define_callbacks :prepare
+
+    define_callbacks :class_unload
+
+    # Registers a callback that will run once at application startup and every time the code is reloaded.
+    def self.to_prepare(*args, &block)
+      set_callback(:prepare, *args, &block)
+    end
+
+    # Registers a callback that will run immediately before the classes are unloaded.
+    def self.before_class_unload(*args, &block)
+      set_callback(:class_unload, *args, &block)
+    end
+
+    # Registers a callback that will run immediately after the classes are unloaded.
+    def self.after_class_unload(*args, &block)
+      set_callback(:class_unload, :after, *args, &block)
+    end
+
+    to_run(:after) { self.class.prepare! }
+
+    # Initiate a manual reload
+    def self.reload!
+      executor.wrap do
+        new.tap do |instance|
+          instance.run!
+        ensure
+          instance.complete!
+        end
+      end
+      prepare!
+    end
+
+    def self.run! # :nodoc:
+      if check!
+        super
+      else
+        Null
+      end
+    end
+
+    # Run the supplied block as a work unit, reloading code as needed
+    def self.wrap
+      executor.wrap do
+        super
+      end
+    end
+
+    class_attribute :executor, default: Executor
+    class_attribute :check, default: lambda { false }
+
+    def self.check! # :nodoc:
+      @should_reload ||= check.call
+    end
+
+    def self.reloaded! # :nodoc:
+      @should_reload = false
+    end
+
+    def self.prepare! # :nodoc:
+      new.run_callbacks(:prepare)
+    end
+
+    def initialize
+      super
+      @locked = false
+    end
+
+    # Acquire the ActiveSupport::Dependencies::Interlock unload lock,
+    # ensuring it will be released automatically
+    def require_unload_lock!
+      unless @locked
+        ActiveSupport::Dependencies.interlock.start_unloading
+        @locked = true
+      end
+    end
+
+    # Release the unload lock if it has been previously obtained
+    def release_unload_lock!
+      if @locked
+        @locked = false
+        ActiveSupport::Dependencies.interlock.done_unloading
+      end
+    end
+
+    def run! # :nodoc:
+      super
+      release_unload_lock!
+    end
+
+    def class_unload!(&block) # :nodoc:
+      require_unload_lock!
+      run_callbacks(:class_unload, &block)
+    end
+
+    def complete! # :nodoc:
+      super
+      self.class.reloaded!
+    ensure
+      release_unload_lock!
+    end
+  end
+end