Merge pull request rails#25302 from schneems/schneems/evented-file-bo…

…ot-at-check-time-master

EventedFileUpdateChecker boots once per process

activesupport/lib/active_support/evented_file_update_checker.rb

@@ -3,6 +3,33 @@
 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 @@ def initialize(files, dirs = {}, &block)
         @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 @@ def initialize(files, dirs = {}, &block)
             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 @@ def execute_if_updated
     end
 
     private
+      def boot!
+        Listen.to(*@dtw, &method(:changed)).start
+      end
 
       def changed(modified, added, removed)
         unless updated?

activesupport/test/evented_file_update_checker_test.rb

@@ -11,7 +11,7 @@ def setup
   end
 
   def new_checker(files = [], dirs = {}, &block)
-    ActiveSupport::EventedFileUpdateChecker.new(files, dirs, &block).tap do
+    ActiveSupport::EventedFileUpdateChecker.new(files, dirs, &block).tap do |c|
       wait
     end
   end
@@ -34,6 +34,48 @@ def rm_f(files)
     super
     wait
   end
+
+  test 'notifies forked processes' do
+    FileUtils.touch(tmpfiles)
+
+    checker = new_checker(tmpfiles) { }
+    assert !checker.updated?
+
+    # Pipes used for flow controll across fork.
+    boot_reader,  boot_writer  = IO.pipe
+    touch_reader, touch_writer = IO.pipe
+
+    pid = fork do
+      assert checker.updated?
+
+      # Clear previous check value.
+      checker.execute
+      assert !checker.updated?
+
+      # Fork is booted, ready for file to be touched
+      # notify parent process.
+      boot_writer.write("booted")
+
+      # Wait for parent process to signal that file
+      # has been touched.
+      IO.select([touch_reader])
+
+      assert checker.updated?
+    end
+
+    assert pid
+
+    # Wait for fork to be booted before touching files.
+    IO.select([boot_reader])
+    touch(tmpfiles)
+
+    # Notify fork that files have been touched.
+    touch_writer.write("touched")
+
+    assert checker.updated?
+
+    Process.wait(pid)
+  end
 end
 
 class EventedFileUpdateCheckerPathHelperTest < ActiveSupport::TestCase