Make EventedFileUpdateChecker garbage collectable

The Listen gem dispatches "file changed" events from a separate thread.
This thread holds a reference to the `changed` callback, and runs until
the listener is stopped.  Thus, objects that are part of the `changed`
callback's scope cannot be garbage collected until the listener is
stopped.

This commit isolates the `changed` callback and associated scope in an
`EventedFileUpdateChecker::Core` instance.  This ensures that the
`EventedFileUpdateChecker` instance can be garbage collected.
Additionally, this commit adds a finalizer to the
`EventedFileUpdateChecker` instance, which stops the listener and
ensures that the `EventedFileUpdateChecker::Core` instance can be
garbage collected.

activesupport/lib/active_support/evented_file_update_checker.rb

@@ -39,52 +39,31 @@ module ActiveSupport
         raise ArgumentError, "A block is required to initialize an EventedFileUpdateChecker"
       end
 
-      @ph    = PathHelper.new
-      @files = files.map { |f| @ph.xpath(f) }.to_set
-
-      @dirs = {}
-      dirs.each do |dir, exts|
-        @dirs[@ph.xpath(dir)] = Array(exts).map { |ext| @ph.normalize_extension(ext) }
-      end
-
       @block = block
-      @updated    = Concurrent::AtomicBoolean.new(false)
-      @lcsp       = @ph.longest_common_subpath(@dirs.keys)
       @pid = Process.pid
-      @boot_mutex = Mutex.new
-
-      dtw = directories_to_watch
-      @dtw, @missing = dtw.partition(&:exist?)
-
-      boot!
+      @core = Core.new(files, dirs)
+      ObjectSpace.define_finalizer(self, @core.finalizer)
     end
 
     def updated?
-      @boot_mutex.synchronize do
+      @core.mutex.synchronize do
         if @pid != Process.pid
-          boot!
+          @core.start
           @pid = Process.pid
-          @updated.make_true
+          @core.updated.make_true
         end
       end
 
-      if @missing.any?(&:exist?)
-        @boot_mutex.synchronize do
-          appeared, @missing = @missing.partition(&:exist?)
-          shutdown!
-
-          @dtw += appeared
-          boot!
-
-          @updated.make_true
-        end
+      if @core.restart?
+        @core.thread_safely(&:restart)
+        @core.updated.make_true
       end
 
-      @updated.true?
+      @core.updated.true?
     end
 
     def execute
-      @updated.make_false
+      @core.updated.make_false
       @block.call
     end
 
@@ -96,18 +75,58 @@ module ActiveSupport
       end
     end
 
-    private
-      def boot!
-        normalize_dirs!
+    class Core
+      attr_reader :updated, :mutex
 
+      def initialize(files, dirs)
+        @ph = PathHelper.new
+        @files = files.map { |file| @ph.xpath(file) }.to_set
+
+        @dirs = dirs.each_with_object({}) do |(dir, exts), hash|
+          hash[@ph.xpath(dir)] = Array(exts).map { |ext| @ph.normalize_extension(ext) }.to_set
+        end
+
+        @common_path = @ph.longest_common_subpath(@dirs.keys)
+
+        @dtw = directories_to_watch
+        @missing = []
+
+        @updated = Concurrent::AtomicBoolean.new(false)
+        @mutex = Mutex.new
+
+        start
+      end
+
+      def finalizer
+        proc { stop }
+      end
+
+      def thread_safely
+        @mutex.synchronize do
+          yield self
+        end
+      end
+
+      def start
+        normalize_dirs!
+        @dtw, @missing = [*@dtw, *@missing].partition(&:exist?)
         @listener = @dtw.any? ? Listen.to(*@dtw, &method(:changed)) : nil
         @listener&.start
       end
 
-      def shutdown!
+      def stop
         @listener&.stop
       end
 
+      def restart
+        stop
+        start
+      end
+
+      def restart?
+        @missing.any?(&:exist?)
+      end
+
       def normalize_dirs!
         @dirs.transform_keys! do |dir|
           dir.exist? ? dir.realpath : dir
@@ -115,7 +134,7 @@ module ActiveSupport
       end
 
       def changed(modified, added, removed)
-        unless updated?
+        unless @updated.true?
           @updated.make_true if (modified + added + removed).any? { |f| watching?(f) }
         end
       end
@@ -135,7 +154,7 @@ module ActiveSupport
 
             if matching && (matching.empty? || matching.include?(ext))
               break true
-            elsif dir == @lcsp || dir.root?
+            elsif dir == @common_path || dir.root?
               break false
             end
           end
@@ -154,6 +173,7 @@ module ActiveSupport
 
         @ph.filter_out_descendants(dtw)
       end
+    end
 
       class PathHelper
         def xpath(path)

activesupport/test/evented_file_update_checker_test.rb

@@ -2,6 +2,7 @@
 
 require_relative "abstract_unit"
 require "pathname"
+require "weakref"
 require_relative "file_update_checker_shared_tests"
 
 class EventedFileUpdateCheckerTest < ActiveSupport::TestCase
@@ -77,6 +78,18 @@ class EventedFileUpdateCheckerTest < ActiveSupport::TestCase
     Process.wait(pid)
   end
 
+  test "can be garbage collected" do
+    previous_threads = Thread.list
+    checker_ref = WeakRef.new(ActiveSupport::EventedFileUpdateChecker.new([], tmpdir => ".rb") { })
+    listener_threads = Thread.list - previous_threads
+
+    wait # Wait for listener thread to start processing events.
+    GC.start
+
+    assert_not_predicate checker_ref, :weakref_alive?
+    assert_empty Thread.list & listener_threads
+  end
+
   test "should detect changes through symlink" do
     actual_dir = File.join(tmpdir, "actual")
     linked_dir = File.join(tmpdir, "linked")