Reduce memory usage when reading response body

Currently for each chunk read from the socket a new string is being
allocated. This is not necessary, because we're feeding that string
object into the HTTP parser and never using it again.

So we initialize a buffer string and pass it in when reading chunks.
This will make each chunk be read into the same buffer object, no new
string will be allocated in that operation.

In my benchmark this halves the allocated memory when downloading
large response bodies.

Author: janko

lib/http/connection.rb

@@ -35,6 +35,7 @@ def initialize(req, options)
       @pending_request      = false
       @pending_response     = false
       @failed_proxy_connect = false
+      @buffer               = "".b
 
       @parser = Response::Parser.new
 
@@ -210,7 +211,7 @@ def set_keep_alive
     def read_more(size)
       return if @parser.finished?
 
-      value = @socket.readpartial(size)
+      value = @socket.readpartial(size, @buffer)
       if value == :eof
         @parser << ""
         :eof

lib/http/timeout/global.rb

@@ -46,8 +46,8 @@ def connect_ssl
       end
 
       # Read from the socket
-      def readpartial(size)
-        perform_io { read_nonblock(size) }
+      def readpartial(size, buffer = nil)
+        perform_io { read_nonblock(size, buffer) }
       end
 
       # Write to the socket
@@ -60,16 +60,16 @@ def write(data)
       private
 
       if RUBY_VERSION < "2.1.0"
-        def read_nonblock(size)
-          @socket.read_nonblock(size)
+        def read_nonblock(size, buffer = nil)
+          @socket.read_nonblock(size, buffer)
         end
 
         def write_nonblock(data)
           @socket.write_nonblock(data)
         end
       else
-        def read_nonblock(size)
-          @socket.read_nonblock(size, :exception => false)
+        def read_nonblock(size, buffer = nil)
+          @socket.read_nonblock(size, buffer, :exception => false)
         end
 
         def write_nonblock(data)

lib/http/timeout/null.rb

@@ -41,8 +41,8 @@ def start_tls(host, ssl_socket_class, ssl_context)
       end
 
       # Read from the socket
-      def readpartial(size)
-        @socket.readpartial(size)
+      def readpartial(size, buffer = nil)
+        @socket.readpartial(size, buffer)
       rescue EOFError
         :eof
       end

lib/http/timeout/per_operation.rb

@@ -37,9 +37,9 @@ def connect_ssl
       # NIO with exceptions
       if RUBY_VERSION < "2.1.0"
         # Read data from the socket
-        def readpartial(size)
+        def readpartial(size, buffer = nil)
           rescue_readable do
-            @socket.read_nonblock(size)
+            @socket.read_nonblock(size, buffer)
           end
         rescue EOFError
           :eof
@@ -57,10 +57,10 @@ def write(data)
       # NIO without exceptions
       else
         # Read data from the socket
-        def readpartial(size)
+        def readpartial(size, buffer = nil)
           timeout = false
           loop do
-            result = @socket.read_nonblock(size, :exception => false)
+            result = @socket.read_nonblock(size, buffer, :exception => false)
 
             return :eof   if result.nil?
             return result if result != :wait_readable