Fix concurrency issues with file caches

Fixes #300

lib/i18n/tasks/concurrent/cache.rb

@@ -0,0 +1,22 @@
+# frozen_string_literal: true
+
+require 'i18n/tasks/concurrent/cached_value'
+
+module I18n::Tasks::Concurrent
+  # A thread-safe cache.
+  # @since 0.9.25
+  class Cache
+    def initialize
+      @mutex = Mutex.new
+      @map = {}
+    end
+
+    # @param [Object] key
+    # @return [Object] Cached or computed value.
+    def fetch(key, &block)
+      @mutex.synchronize do
+        @map[key] ||= CachedValue.new(&block)
+      end.get
+    end
+  end
+end

lib/i18n/tasks/concurrent/cached_value.rb

@@ -0,0 +1,27 @@
+# frozen_string_literal: true
+
+require 'i18n/tasks/concurrent/cached_value'
+
+module I18n::Tasks::Concurrent
+  # A thread-safe memoized value.
+  # The given computation is guaranteed to be invoked at most once.
+  # @since 0.9.25
+  class CachedValue
+    NULL = Object.new
+
+    # @param [Proc] computation The computation that returns the value to cache.
+    def initialize(&computation)
+      @computation = computation
+      @mutex = Mutex.new
+      @result = NULL
+    end
+
+    # @return [Object] Result of the computation.
+    def get
+      return @result unless @result == NULL
+      @mutex.synchronize do
+        @result = @computation.call
+      end
+    end
+  end
+end

lib/i18n/tasks/logging.rb

@@ -23,10 +23,8 @@ def log_error(message)
   end
 
   def log_stderr(*args)
+    # We don't want output from different threads to get intermixed.
     MUTEX.synchronize do
-      # 1. We don't want output from different threads to get intermixed.
-      # 2. StringIO is currently not thread-safe (blows up) on JRuby:
-      # https://github.com/jruby/jruby/issues/4417
       $stderr.puts(*args)
     end
   end

lib/i18n/tasks/scanners/files/caching_file_finder.rb

@@ -1,6 +1,8 @@
 # frozen_string_literal: true
 
+require 'i18n/tasks/concurrent/cached_value'
 require 'i18n/tasks/scanners/files/file_finder'
+
 module I18n::Tasks::Scanners::Files
   # Finds the files in the specified search paths with support for exclusion / inclusion patterns.
   # Wraps a {FileFinder} and caches the results.
@@ -11,8 +13,7 @@ class CachingFileFinder < FileFinder
     # @param (see FileFinder#initialize)
     def initialize(**args)
       super
-      @mutex = Mutex.new
-      @cached_paths = nil
+      @cached_value = ::I18n::Tasks::Concurrent::CachedValue.new { uncached_find_files }
     end
 
     # Traverse the paths and yield the matching ones.
@@ -25,10 +26,13 @@ def traverse_files
       super
     end
 
+    alias uncached_find_files find_files
+    private :uncached_find_files
+
     # @note This method is cached, it will only access the filesystem on the first invocation.
     # @return (see FileFinder#find_files)
     def find_files
-      @cached_paths || @mutex.synchronize { @cached_paths ||= super }
+      @cached_value.get
     end
   end
 end

lib/i18n/tasks/scanners/files/caching_file_finder_provider.rb

@@ -1,5 +1,6 @@
 # frozen_string_literal: true
 
+require 'i18n/tasks/concurrent/cache'
 require 'i18n/tasks/scanners/files/caching_file_finder'
 
 module I18n::Tasks::Scanners::Files
@@ -10,8 +11,7 @@ module I18n::Tasks::Scanners::Files
   class CachingFileFinderProvider
     # @param exclude [Array<String>]
     def initialize(exclude: [])
-      @cache = {}
-      @mutex = Mutex.new
+      @cache = ::I18n::Tasks::Concurrent::Cache.new
       @defaults = { exclude: exclude }
     end
 
@@ -20,13 +20,11 @@ def initialize(exclude: [])
     # @param (see FileFinder#initialize)
     # @return [CachingFileFinder]
     def get(**file_finder_args)
-      @cache[file_finder_args] || @mutex.synchronize do
-        @cache[file_finder_args] ||= begin
-          args = file_finder_args.dup
-          args[:exclude] = @defaults[:exclude] + (args[:exclude] || [])
-          args[:exclude].uniq!
-          CachingFileFinder.new(**args)
-        end
+      @cache.fetch(file_finder_args) do
+        args = file_finder_args.dup
+        args[:exclude] = @defaults[:exclude] + (args[:exclude] || [])
+        args[:exclude].uniq!
+        CachingFileFinder.new(**args)
       end
     end
   end

lib/i18n/tasks/scanners/files/caching_file_reader.rb

@@ -1,6 +1,8 @@
 # frozen_string_literal: true
 
+require 'i18n/tasks/concurrent/cache'
 require 'i18n/tasks/scanners/files/file_reader'
+
 module I18n::Tasks::Scanners::Files
   # Reads the files in 'rb' mode and UTF-8 encoding.
   # Wraps a {FileReader} and caches the results.
@@ -10,8 +12,7 @@ module I18n::Tasks::Scanners::Files
   class CachingFileReader < FileReader
     def initialize
       super
-      @mutex = Mutex.new
-      @cache = {}
+      @cache = ::I18n::Tasks::Concurrent::Cache.new
     end
 
     # Return the contents of the file at the given path.
@@ -21,8 +22,7 @@ def initialize
     # @return (see FileReader#read_file)
     # @note This method is cached, it will only access the filesystem on the first invocation.
     def read_file(path)
-      absolute_path = File.expand_path(path)
-      @cache[absolute_path] || @mutex.synchronize { @cache[absolute_path] ||= super }
+      @cache.fetch(File.expand_path(path)) { super }
     end
   end
 end