1 files changed, 44 insertions, 5 deletions

diff --git a/activesupport/lib/active_support/evented_file_update_checker.rb b/activesupport/lib/active_support/evented_file_update_checker.rb
index 21fdf7bb80..a2dcf31132 100644
--- a/activesupport/lib/active_support/evented_file_update_checker.rb
+++ b/activesupport/lib/active_support/evented_file_update_checker.rb
@@ -3,6 +3,33 @@ require 'pathname'
 require 'concurrent/atomic/atomic_boolean'
 
 module ActiveSupport
+  # Allows you to "listen" to changes in a file system.
+  # The evented file updater does not hit disk when checking for updates
+  # instead it uses platform specific file system events to trigger a change
+  # in state.
+  #
+  # The file checker takes an array of files to watch or a hash specifying directories
+  # and file extensions to watch. It also takes a block that is called when
+  # EventedFileUpdateChecker#execute is run or when EventedFileUpdateChecker#execute_if_updated
+  # is run and there have been changes to the file system.
+  #
+  # Note: Forking will cause the first call to `updated?` to return `true`.
+  #
+  # Example:
+  #
+  #     checker = EventedFileUpdateChecker.new(["/tmp/foo"], -> { puts "changed" })
+  #     checker.updated?
+  #     # => false
+  #     checker.execute_if_updated
+  #     # => nil
+  #
+  #     FileUtils.touch("/tmp/foo")
+  #
+  #     checker.updated?
+  #     # => true
+  #     checker.execute_if_updated
+  #     # => "changed"
+  #
   class EventedFileUpdateChecker #:nodoc: all
     def initialize(files, dirs = {}, &block)
       @ph    = PathHelper.new
@@ -13,11 +40,13 @@ module ActiveSupport
         @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)
+      @block      = block
+      @updated    = Concurrent::AtomicBoolean.new(false)
+      @lcsp       = @ph.longest_common_subpath(@dirs.keys)
+      @pid        = Process.pid
+      @boot_mutex = Mutex.new
 
-      if (dtw = directories_to_watch).any?
+      if (@dtw = directories_to_watch).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.
@@ -28,11 +57,18 @@ module ActiveSupport
             raise LoadError, "Could not load the 'listen' gem. Add `gem 'listen'` to the development group of your Gemfile", e.backtrace
           end
         end
-        Listen.to(*dtw, &method(:changed)).start
       end
+      boot!
     end
 
     def updated?
+      @boot_mutex.synchronize do
+        if @pid != Process.pid
+          boot!
+          @pid = Process.pid
+          @updated.make_true
+        end
+      end
       @updated.true?
     end
 
@@ -50,6 +86,9 @@ module ActiveSupport
     end
 
     private
+      def boot!
+        Listen.to(*@dtw, &method(:changed)).start
+      end
 
       def changed(modified, added, removed)
         unless updated?