Cache contents of included file read into memory

lib/jekyll/site.rb

@@ -83,7 +83,6 @@ def print_stats
       Jekyll.logger.info @liquid_renderer.stats_table
     end
 
-    # rubocop:disable Metrics/MethodLength
     #
     # Reset Site details.
     #
@@ -103,17 +102,14 @@ def reset
       @collections = nil
       @documents = nil
       @docs_to_write = nil
-      @regenerator.clear_cache
-      @liquid_renderer.reset
       @site_cleaner = nil
-      frontmatter_defaults.reset
 
+      reset_caches
       raise ArgumentError, "limit_posts must be a non-negative number" if limit_posts.negative?
 
       Jekyll::Cache.clear_if_config_changed config
       Jekyll::Hooks.trigger :site, :after_reset, self
     end
-    # rubocop:enable Metrics/MethodLength
 
     # Load necessary libraries, plugins, converters, and generators.
     #
@@ -467,6 +463,13 @@ def site_cleaner
       @site_cleaner ||= Cleaner.new(self)
     end
 
+    def reset_caches
+      @regenerator.clear_cache
+      @liquid_renderer.reset
+      frontmatter_defaults.reset
+      Jekyll::Tags::IncludeTag.reset_contents_cache
+    end
+
     # Disable Marshaling cache to disk in Safe Mode
     def configure_cache
       Jekyll::Cache.cache_dir = in_source_dir(config["cache_dir"], "Jekyll/Cache")

lib/jekyll/tags/include.rb

@@ -176,7 +176,18 @@ def realpath_prefixed_with?(path, dir)
 
       # This method allows to modify the file content by inheriting from the class.
       def read_file(file, context)
-        File.read(file, **file_read_opts(context))
+        self.class.contents_cache[file] ||= begin
+          Jekyll.logger.debug "Reading Include:", file.cyan
+          File.read(file, **file_read_opts(context))
+        end
+      end
+
+      def self.contents_cache
+        @contents_cache ||= {}
+      end
+
+      def self.reset_contents_cache
+        @contents_cache = nil
       end
 
       private