deflater: support "sync: false" option

Flushing after after very flush is great for real-time apps.
However, flushing is inefficient when apps use Rack::Response
to generate many small writes (e.g.  Rack::Lobster).

Allow users to disable the default "sync: true" behavior to
reduce bandwidth usage, otherwise using Rack::Deflater can lead
to using more bandwidth than without it.

2 files changed, 42 insertions, 3 deletions

diff --git a/lib/rack/deflater.rb b/lib/rack/deflater.rb
index d575adfe..821f708b 100644
--- a/lib/rack/deflater.rb
+++ b/lib/rack/deflater.rb
@@ -24,11 +24,15 @@ module Rack
     #           'if' - a lambda enabling / disabling deflation based on returned boolean value
     #                  e.g use Rack::Deflater, :if => lambda { |env, status, headers, body| body.map(&:bytesize).reduce(0, :+) > 512 }
     #           'include' - a list of content types that should be compressed
+    #           'sync' - Flushing after every chunk reduces latency for
+    #                    time-sensitive streaming applications, but hurts
+    #                    compression and throughput.  Defaults to `true'.
     def initialize(app, options = {})
       @app = app
 
       @condition = options[:if]
       @compressible_types = options[:include]
+      @sync = options[:sync] == false ? false : true
     end
 
     def call(env)
@@ -56,7 +60,7 @@ module Rack
         headers.delete('Content-Length')
         mtime = headers.key?("Last-Modified") ?
           Time.httpdate(headers["Last-Modified"]) : Time.now
-        [status, headers, GzipStream.new(body, mtime)]
+        [status, headers, GzipStream.new(body, mtime, @sync)]
       when "identity"
         [status, headers, body]
       when nil
@@ -67,7 +71,8 @@ module Rack
     end
 
     class GzipStream
-      def initialize(body, mtime)
+      def initialize(body, mtime, sync)
+        @sync = sync
         @body = body
         @mtime = mtime
       end
@@ -78,7 +83,7 @@ module Rack
         gzip.mtime = @mtime
         @body.each { |part|
           gzip.write(part)
-          gzip.flush
+          gzip.flush if @sync
         }
       ensure
         gzip.close
diff --git a/test/spec_deflater.rb b/test/spec_deflater.rb
index 0f27c859..410a1438 100644
--- a/test/spec_deflater.rb
+++ b/test/spec_deflater.rb
@@ -372,4 +372,38 @@ describe Rack::Deflater do
 
     verify(200, response, 'gzip', options)
   end
+
+  it 'will honor sync: false to avoid unnecessary flushing' do
+    app_body = Object.new
+    class << app_body
+      def each
+        (0..20).each { |i| yield "hello\n".freeze }
+      end
+    end
+
+    options = {
+      'deflater_options' => { :sync => false },
+      'app_body' => app_body,
+      'skip_body_verify' => true,
+    }
+    verify(200, app_body, deflate_or_gzip, options) do |status, headers, body|
+      headers.must_equal({
+        'Content-Encoding' => 'gzip',
+        'Vary' => 'Accept-Encoding',
+        'Content-Type' => 'text/plain'
+      })
+
+      buf = ''
+      raw_bytes = 0
+      inflater = auto_inflater
+      body.each do |part|
+        raw_bytes += part.bytesize
+        buf << inflater.inflate(part)
+      end
+      buf << inflater.finish
+      expect = "hello\n" * 21
+      buf.must_equal expect
+      raw_bytes.must_be(:<, expect.bytesize)
+    end
+  end
 end