Reuse cached session in MRU order, not in LRU

MRU is more server friendly than LRU because it reduces number of cached
sessions when a number of requests drops after an usaage spike.
Closes #68.

With reusing sessions in LRU order, all sessions are equally checked if
it's closed or not, as far as there's a request to the same site.  With
reusing sessions in MRU order, old cold sessions are kept in cache long
time even if there's a request to the same site.  To avoid this leakage,
this commit adds keep_alive_timeout property and let SessionManager
scrub all sessions with checking the timeout for each session.  When the
session expires against the last used time, it's closed and collected.

keep_alive_timeout is 15[sec] by default. The value is from the default
value for KeepAliveTimeout of Apache httpd 2.  This change is related
to #56.

lib/httpclient.rb

@@ -335,6 +335,8 @@ def attr_proxy(symbol, assignable = false)
   attr_proxy(:send_timeout, true)
   # Response receiving timeout in sec.
   attr_proxy(:receive_timeout, true)
+  # Reuse the same connection within this timeout in sec. from last used.
+  attr_proxy(:keep_alive_timeout, true)
   # Negotiation retry count for authentication.  5 by default.
   attr_proxy(:protocol_retry_count, true)
   # if your ruby is older than 2005-09-06, do not set socket_sync = false to

lib/httpclient/session.rb

@@ -102,6 +102,7 @@ class SessionManager
     attr_accessor :connect_retry
     attr_accessor :send_timeout
     attr_accessor :receive_timeout
+    attr_accessor :keep_alive_timeout
     attr_accessor :read_block_size
     attr_accessor :protocol_retry_count
 
@@ -129,7 +130,8 @@ def initialize(client)
       @connect_timeout = 60
       @connect_retry = 1
       @send_timeout = 120
-      @receive_timeout = 60       # For each read_block_size bytes
+      @receive_timeout = 60        # For each read_block_size bytes
+      @keep_alive_timeout = 15     # '15' is from Apache 2 default
       @read_block_size = 1024 * 16 # follows net/http change in 1.8.7
       @protocol_retry_count = 5
 
@@ -172,6 +174,7 @@ def reset_all
       close_all
     end
 
+    # assert: sess.last_used must not be nil
     def keep(sess)
       add_cached_session(sess)
     end
@@ -233,10 +236,11 @@ def close(dest)
 
     def get_cached_session(uri)
       cached = nil
+      now = Time.now
       @sess_pool_mutex.synchronize do
         new_pool = []
         @sess_pool.each do |s|
-          if s.invalidated?
+          if s.invalidated? or now > s.last_used + @keep_alive_timeout
             s.close # close & remove from the pool
           elsif !cached && s.dest.match(uri)
             cached = s
@@ -251,7 +255,7 @@ def get_cached_session(uri)
 
     def add_cached_session(sess)
       @sess_pool_mutex.synchronize do
-        @sess_pool << sess
+        @sess_pool.unshift(sess)
       end
     end
   end
@@ -523,6 +527,7 @@ class Session
     attr_accessor :test_loopback_http_response
 
     attr_accessor :transparent_gzip_decompression
+    attr_reader :last_used
 
     def initialize(client, dest, agent_name, from)
       @client = client
@@ -561,6 +566,7 @@ def initialize(client, dest, agent_name, from)
       @readbuf = nil
 
       @transparent_gzip_decompression = false
+      @last_used = nil
     end
 
     # Send a request to the server
@@ -594,6 +600,7 @@ def query(req)
       @state = :META if @state == :WAIT
       @next_connection = nil
       @requests.push(req)
+      @last_used = Time.now
     end
 
     def close