Add session escape functionality

lib/mongo/client.rb

@@ -1317,7 +1317,7 @@ def do_close
     #
     # @api private
     def get_session!(options = {})
-      if options[:session]
+      if options[:session] && !Session.escaped?
         return options[:session].validate!(self)
       end
 

lib/mongo/session.rb

@@ -17,6 +17,7 @@
 
 require 'mongo/session/session_pool'
 require 'mongo/session/server_session'
+require 'mongo/session/session_escapable'
 
 module Mongo
 

lib/mongo/session/session_escapable.rb

@@ -0,0 +1,26 @@
+
+module Mongo
+
+  class Session
+
+    class << self
+      def escape
+        old_escaped = self.escaped?
+        self.escaped = true
+        yield
+      ensure
+        self.escaped = old_escaped
+      end
+
+      def escaped?
+        !!Thread.current["[mongo]:session:escaped"]
+      end
+
+      private
+
+      def escaped=(value)
+        Thread.current["[mongo]:session:escaped"] = !!value
+      end
+    end
+  end
+end