EventedFileUpdateChecker boots once per process

We need one file checker booted per process as talked about in rails#24990. Before we do a check to see if any updates have been registered by the listener we first check to make sure that the current process has booted a listener.

We are intentionally not starting a listener when the checker is created. This way we can avoid rails#25259 in which puma warns of multiple threads created before fork. As written the listener for each process will be invoked by the `ActionDispatch::Executor` middleware when the `updated?` method is called. This is the first middleware on the stack and will be invoked before application code is read into memory.

The downside of this approach is that the API is a little less obvious. I.e. that you have to call `updated?` to get the listener to start is not intuitive. We could make `boot!` not private if we want to make the API a little nicer. Alternatively we could boot when the checker is initialized however this reintroduces the puma threads warning, and also means that in cases of `rails server` or when using `preload!` that we have extra threads notifying of changes on a process that we don't care about.

[close rails#24990] [close rails#25259]

activesupport/lib/active_support/evented_file_update_checker.rb

@@ -13,11 +13,12 @@ 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_hash = Concurrent::Hash.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 +29,11 @@ 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
     end
 
     def updated?
+      boot! unless @pid_hash[Process.pid]
       @updated.true?
     end
 
@@ -50,6 +51,10 @@ def execute_if_updated
     end
 
     private
+      def boot!
+        Listen.to(*@dtw, &method(:changed)).start
+        @pid_hash[Process.pid] = true
+      end
 
       def changed(modified, added, removed)
         unless updated?

activesupport/test/evented_file_update_checker_test.rb

@@ -11,7 +11,8 @@ 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|
+      c.updated?
       wait
     end
   end