Listen on fork in EventedFileUpdateChecker

The Listen gem does not notify across process forks, and so forked
processes must re-create their listeners.  `EventedFileUpdateChecker`
handled this in `updated?` by checking if `Process.pid` was different.
Because file updates may have occurred after the start of the fork but
before the call to `updated?`, `updated?` would always return true in
this case.

However, the above approach can result in unnecessary application
reloading, particularly when using Spring.  That, in turn, can cause
unexpected and problematic behavior: #39431, #37591.  This commit
changes listener re-creation to occur immediately after fork, and
changes `updated?` to return true only when necessary.

activesupport/lib/active_support/evented_file_update_checker.rb

@@ -4,6 +4,7 @@ require "set"
 require "pathname"
 require "concurrent/atomic/atomic_boolean"
 require "listen"
+require "active_support/fork_tracker"
 
 module ActiveSupport
   # Allows you to "listen" to changes in a file system.
@@ -40,20 +41,11 @@ module ActiveSupport
       end
 
       @block = block
-      @pid = Process.pid
       @core = Core.new(files, dirs)
       ObjectSpace.define_finalizer(self, @core.finalizer)
     end
 
     def updated?
-      @core.mutex.synchronize do
-        if @pid != Process.pid
-          @core.start
-          @pid = Process.pid
-          @core.updated.make_true
-        end
-      end
-
       if @core.restart?
         @core.thread_safely(&:restart)
         @core.updated.make_true
@@ -76,7 +68,7 @@ module ActiveSupport
     end
 
     class Core
-      attr_reader :updated, :mutex
+      attr_reader :updated
 
       def initialize(files, dirs)
         @files = files.map { |file| Pathname(file).expand_path }.to_set
@@ -94,10 +86,14 @@ module ActiveSupport
         @mutex = Mutex.new
 
         start
+        @after_fork = ActiveSupport::ForkTracker.after_fork { start }
       end
 
       def finalizer
-        proc { stop }
+        proc do
+          stop
+          ActiveSupport::ForkTracker.unregister(@after_fork)
+        end
       end
 
       def thread_safely

activesupport/test/evented_file_update_checker_test.rb

@@ -47,10 +47,6 @@ class EventedFileUpdateCheckerTest < ActiveSupport::TestCase
     touch_reader, touch_writer = IO.pipe
 
     pid = fork do
-      assert_predicate checker, :updated?
-
-      # Clear previous check value.
-      checker.execute
       assert_not_predicate checker, :updated?
 
       # Fork is booted, ready for file to be touched