Do not add parent directory to file system monitoring

`EventedFileUpdateChecker` will search the parent directory if the
specified directory does not exist.

Since `test/mailers/previews` is included in the watch target by default,
if there is no test directory (e.g. using `rspec`), the Rails root directory
will be included in the watch target.

```
$ rails new app
$ cd app
$ ./bin/rails r "p Rails.application.reloaders.last.send(:directories_to_watch).include?(Rails.root)"
false
$ rm -rf test
$ ./bin/rails r "p Rails.application.reloaders.last.send(:directories_to_watch).include?(Rails.root)"
true
```

This causes `node_modules` to be included in watch target. Adding parent
directories to watch target may include unexpected directories.

In order to avoid this, fixed that parents of nonexistent directories are
not added to the watch targets, instead checking that the directory
exists when checking changes.

Related to #32700.

[Matthew Draper & Yuji Yaginuma]

2 files changed, 50 insertions, 2 deletions

diff --git a/activesupport/lib/active_support/evented_file_update_checker.rb b/activesupport/lib/active_support/evented_file_update_checker.rb
index 97e982eb05..1ef8584c52 100644
--- a/activesupport/lib/active_support/evented_file_update_checker.rb
+++ b/activesupport/lib/active_support/evented_file_update_checker.rb
@@ -52,7 +52,10 @@ module ActiveSupport
       @pid        = Process.pid
       @boot_mutex = Mutex.new
 
-      if (@dtw = directories_to_watch).any?
+      dtw = directories_to_watch
+      @dtw, @missing = dtw.partition(&:exist?)
+
+      if @dtw.any?
         # Loading listen triggers warnings. These are originated by a legit
         # usage of attr_* macros for private attributes, but adds a lot of noise
         # to our test suite. Thus, we lazy load it and disable warnings locally.
@@ -75,6 +78,19 @@ module ActiveSupport
           @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
+      end
+
       @updated.true?
     end
 
@@ -96,6 +112,10 @@ module ActiveSupport
         Listen.to(*@dtw, &method(:changed)).start
       end
 
+      def shutdown!
+        Listen.stop
+      end
+
       def changed(modified, added, removed)
         unless updated?
           @updated.make_true if (modified + added + removed).any? { |f| watching?(f) }
@@ -123,7 +143,7 @@ module ActiveSupport
       end
 
       def directories_to_watch
-        dtw = (@files + @dirs.keys).map { |f| @ph.existing_parent(f) }
+        dtw = @files.map(&:dirname) + @dirs.keys
         dtw.compact!
         dtw.uniq!
 
diff --git a/activesupport/test/evented_file_update_checker_test.rb b/activesupport/test/evented_file_update_checker_test.rb
index a557608986..b2d5eb94c2 100644
--- a/activesupport/test/evented_file_update_checker_test.rb
+++ b/activesupport/test/evented_file_update_checker_test.rb
@@ -76,6 +76,34 @@ class EventedFileUpdateCheckerTest < ActiveSupport::TestCase
 
     Process.wait(pid)
   end
+
+  test "updated should become true when nonexistent directory is added later" do
+    Dir.mktmpdir do |dir|
+      watched_dir = File.join(dir, "app")
+      unwatched_dir = File.join(dir, "node_modules")
+      not_exist_watched_dir = File.join(dir, "test")
+
+      Dir.mkdir(watched_dir)
+      Dir.mkdir(unwatched_dir)
+
+      checker = new_checker([], watched_dir => ".rb", not_exist_watched_dir => ".rb") { }
+
+      FileUtils.touch(File.join(watched_dir, "a.rb"))
+      wait
+      assert_predicate checker, :updated?
+      assert checker.execute_if_updated
+
+      Dir.mkdir(not_exist_watched_dir)
+      wait
+      assert_predicate checker, :updated?
+      assert checker.execute_if_updated
+
+      FileUtils.touch(File.join(unwatched_dir, "a.rb"))
+      wait
+      assert_not_predicate checker, :updated?
+      assert_not checker.execute_if_updated
+    end
+  end
 end
 
 class EventedFileUpdateCheckerPathHelperTest < ActiveSupport::TestCase