Sychronize cache sweeper action to prevent race condition (#189)

lib/propshaft/load_path.rb

@@ -44,9 +44,13 @@ def cache_sweeper
     @cache_sweeper ||= begin
       exts_to_watch  = Mime::EXTENSION_LOOKUP.map(&:first)
       files_to_watch = Array(paths).collect { |dir| [ dir.to_s, exts_to_watch ] }.to_h
+      mutex = Mutex.new
 
       Rails.application.config.file_watcher.new([], files_to_watch) do
-        clear_cache
+        mutex.synchronize do
+          clear_cache
+          seed_cache
+        end
       end
     end
   end
@@ -81,6 +85,10 @@ def clear_cache
       @cached_asset_paths_by_glob = nil
     end
 
+    def seed_cache
+      assets_by_path
+    end
+
     def dedup(paths)
       paths   = Array(paths).map { |path| Pathname.new(path) }
       deduped = [].tap do |deduped|