tool/downloader.rb: added Downloader.download_if_modified_since

to reduce downloads of large files that change only rarely.
[ruby-core:65164] [CommonRuby - Feature #10084]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47691 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

ChangeLog

@@ -1,3 +1,9 @@
+Tue Sep 23 16:07:07 2014  Martin Duerst <duerst@it.aoyama.ac.jp>
+
+	* tool/downloader.rb: added Downloader.download_if_modified_since
+	  to reduce downloads of large files that change only rarely.
+	  [ruby-core:65164] [CommonRuby - Feature #10084]
+
 Tue Sep 23 11:55:09 2014  SHIBATA Hiroshi  <shibata.hiroshi@gmail.com>
 
 	* .travis.yml: added rubyspec into travis tasks and eliminate to stdout.

tool/downloader.rb

@@ -8,4 +8,26 @@ class Downloader
   rescue => e
     raise "failed to download #{name}\n#{e.message}: #{url}"
   end
+  
+  # Update a file from url if newer version is available.
+  # Creates the file if the file doesn't yet exist; however, the
+  # directory where the file is being created has to exist already.
+  # Example usage:
+  #   download_if_modified_since 'http://www.unicode.org/Public/UCD/latest/ucd/UnicodeData.txt',
+  #           'enc/unicode/data/UnicodeData.txt'
+  def self.download_if_modified_since(url, name, dir=nil, since=nil)
+    file = dir ? File.join(dir, name) : name
+    since = Date.new(1980,1,1) unless File.exist? file            # use very old date to assure file creation
+    since = File.mtime file    unless since                       # get last modification time for file
+    since = since.to_datetime   if since.respond_to? :to_datetime # convert Time/Date to DateTime
+    since = since.httpdate      if since.respond_to? :httpdate    # convert DateTime to String
+    open(url, 'If-Modified-Since' => since) do |r|
+      body = r.read
+      open(file, 'wb', 0755) { |f| f.write(body) }
+    end
+  rescue OpenURI::HTTPError => http_error
+    unless http_error.message =~ /^304 / # 304 Not Modified
+      raise "Failed to (check for) downloading #{url}: #{http_error.message}"
+    end
+  end
 end