class CGI::Session

Overview¶ ↑

This file provides the CGI::Session class, which provides session support for CGI scripts. A session is a sequence of HTTP requests and responses linked together and associated with a single client. Information associated with the session is stored on the server between requests. A session id is passed between client and server with every request and response, transparently to the user. This adds state information to the otherwise stateless HTTP request/response protocol.

Lifecycle¶ ↑

A CGI::Session instance is created from a CGI object. By default, this CGI::Session instance will start a new session if none currently exists, or continue the current session for this client if one does exist. The new_session option can be used to either always or never create a new session. See new() for more details.

delete() deletes a session from session storage. It does not however remove the session id from the client. If the client makes another request with the same id, the effect will be to start a new session with the old session’s id.

Setting and retrieving session data.¶ ↑

The Session class associates data with a session as key-value pairs. This data can be set and retrieved by indexing the Session instance using ‘[]’, much the same as hashes (although other hash methods are not supported).

When session processing has been completed for a request, the session should be closed using the close() method. This will store the session’s state to persistent storage. If you want to store the session’s state to persistent storage without finishing session processing for this request, call the update() method.

Storing session state¶ ↑

The caller can specify what form of storage to use for the session’s data with the database_manager option to CGI::Session::new. The following storage classes are provided as part of the standard library:

CGI::Session::FileStore: stores data as plain text in a flat file. Only works with String data. This is the default storage type.
CGI::Session::MemoryStore: stores data in an in-memory hash. The data only persists for as long as the current Ruby interpreter instance does.
CGI::Session::PStore: stores data in Marshalled format. Provided by cgi/session/pstore.rb. Supports data of any type, and provides file-locking and transaction support.

Custom storage types can also be created by defining a class with the following methods:

new(session, options)
restore  # returns hash of session data.
update
close
delete

Changing storage type mid-session does not work. Note in particular that by default the FileStore and PStore session data files have the same name. If your application switches from one to the other without making sure that filenames will be different and clients still have old sessions lying around in cookies, then things will break nastily!

Maintaining the session id.¶ ↑

Most session state is maintained on the server. However, a session id must be passed backwards and forwards between client and server to maintain a reference to this session state.

The simplest way to do this is via cookies. The CGI::Session class provides transparent support for session id communication via cookies if the client has cookies enabled.

If the client has cookies disabled, the session id must be included as a parameter of all requests sent by the client to the server. The CGI::Session class in conjunction with the CGI class will transparently add the session id as a hidden input field to all forms generated using the CGI#form() HTML generation method. No built-in support is provided for other mechanisms, such as URL re-writing. The caller is responsible for extracting the session id from the session_id attribute and manually encoding it in URLs and adding it as a hidden input to HTML forms created by other mechanisms. Also, session expiry is not automatically handled.

Examples of use¶ ↑

Setting the user’s name¶ ↑

require 'cgi'
require 'cgi/session'
require 'cgi/session/pstore'     # provides CGI::Session::PStore

cgi = CGI.new("html4")

session = CGI::Session.new(cgi,
    'database_manager' => CGI::Session::PStore,  # use PStore
    'session_key' => '_rb_sess_id',              # custom session key
    'session_expires' => Time.now + 30 * 60,     # 30 minute timeout
    'prefix' => 'pstore_sid_')                   # PStore option
if cgi.has_key?('user_name') and cgi['user_name'] != ''
    # coerce to String: cgi[] returns the
    # string-like CGI::QueryExtension::Value
    session['user_name'] = cgi['user_name'].to_s
elsif !session['user_name']
    session['user_name'] = "guest"
end
session.close

Creating a new session safely¶ ↑

require 'cgi'
require 'cgi/session'

cgi = CGI.new("html4")

# We make sure to delete an old session if one exists,
# not just to free resources, but to prevent the session
# from being maliciously hijacked later on.
begin
    session = CGI::Session.new(cgi, 'new_session' => false)
    session.delete
rescue ArgumentError  # if no old session
end
session = CGI::Session.new(cgi, 'new_session' => true)
session.close

Attributes

new_session[R]

The id of this session.

session_id[R]

The id of this session.

Public Class Methods

new(request, option={}) click to toggle source

Create a new CGI::Session object for request.

request is an instance of the CGI class (see cgi.rb). option is a hash of options for initialising this CGI::Session instance. The following options are recognised:

session_key: the parameter name used for the session id. Defaults to ‘_session_id’.
session_id: the session id to use. If not provided, then it is retrieved from the session_key parameter of the request, or automatically generated for a new session.
new_session: if true, force creation of a new session. If not set, a new session is only created if none currently exists. If false, a new session is never created, and if none currently exists and the session_id option is not set, an ArgumentError is raised.
database_manager: the name of the class providing storage facilities for session state persistence. Built-in support is provided for FileStore (the default), MemoryStore, and PStore (from cgi/session/pstore.rb). See the documentation for these classes for more details.

The following options are also recognised, but only apply if the session id is stored in a cookie.

session_expires: the time the current session expires, as a Time object. If not set, the session will terminate when the user’s browser is closed.
session_domain: the hostname domain for which this session is valid. If not set, defaults to the hostname of the server.
session_secure: if true, this session will only work over HTTPS.
session_path: the path for which this session applies. Defaults to the directory of the CGI script.

option is also passed on to the session storage class initializer; see the documentation for each session storage class for the options they support.

The retrieved or created session is automatically added to request as a cookie, and also to its output_hidden table, which is used to add hidden input elements to forms.

WARNING the output_hidden fields are surrounded by a <fieldset> tag in HTML 4 generation, which is not invisible on many browsers; you may wish to disable the use of fieldsets with code similar to the following (see blade.ruby-lang.org/ruby-list/37805)

cgi = CGI.new("html4")
class << cgi
    undef_method :fieldset
end

# File cgi/session.rb, line 289
def initialize(request, option={})
  @new_session = false
  session_key = option['session_key'] || '_session_id'
  session_id = option['session_id']
  unless session_id
    if option['new_session']
      session_id = create_new_id
      @new_session = true
    end
  end
  unless session_id
    if request.key?(session_key)
      session_id = request[session_key]
      session_id = session_id.read if session_id.respond_to?(:read)
    end
    unless session_id
      session_id, = request.cookies[session_key]
    end
    unless session_id
      unless option.fetch('new_session', true)
        raise ArgumentError, "session_key `%s' should be supplied"%session_key
      end
      session_id = create_new_id
      @new_session = true
    end
  end
  @session_id = session_id
  dbman = option['database_manager'] || FileStore
  begin
    @dbman = dbman::new(self, option)
  rescue NoSession
    unless option.fetch('new_session', true)
      raise ArgumentError, "invalid session_id `%s'"%session_id
    end
    session_id = @session_id = create_new_id unless session_id
    @new_session=true
    retry
  end
  request.instance_eval do
    @output_hidden = {session_key => session_id} unless option['no_hidden']
    @output_cookies =  [
      Cookie::new("name" => session_key,
      "value" => session_id,
      "expires" => option['session_expires'],
      "domain" => option['session_domain'],
      "secure" => option['session_secure'],
      "path" =>
      if option['session_path']
        option['session_path']
      elsif ENV["SCRIPT_NAME"]
        File::dirname(ENV["SCRIPT_NAME"])
      else
      ""
      end)
    ] unless option['no_cookies']
  end
  @dbprot = [@dbman]
  ObjectSpace::define_finalizer(self, Session::callback(@dbprot))
end

Public Instance Methods

[](key) click to toggle source

Retrieve the session data for key key.

# File cgi/session.rb, line 350
def [](key)
  @data ||= @dbman.restore
  @data[key]
end

[]=(key, val) click to toggle source

Set the session data for key key.

# File cgi/session.rb, line 356
def []=(key, val)
  @write_lock ||= true
  @data ||= @dbman.restore
  @data[key] = val
end

close() click to toggle source

Store session data on the server and close the session storage. For some session storage types, this is a no-op.

# File cgi/session.rb, line 370
def close
  @dbman.close
  @dbprot.clear
end

delete() click to toggle source

Delete the session from storage. Also closes the storage.

Note that the session’s data is not automatically deleted upon the session expiring.

# File cgi/session.rb, line 379
def delete
  @dbman.delete
  @dbprot.clear
end

update() click to toggle source

Store session data on the server. For some session storage types, this is a no-op.

# File cgi/session.rb, line 364
def update
  @dbman.update
end

Private Instance Methods

create_new_id() click to toggle source

Create a new session id.

The session id is a secure random number by SecureRandom if possible, otherwise an SHA512 hash based upon the time, a random number, and a constant string. This routine is used internally for automatically generated session ids.

# File cgi/session.rb, line 171
def create_new_id
  require 'securerandom'
  begin
    # by OpenSSL, or system provided entropy pool
    session_id = SecureRandom.hex(16)
  rescue NotImplementedError
    # never happens on modern systems
    require 'digest'
    d = Digest('SHA512').new
    now = Time::now
    d.update(now.to_s)
    d.update(String(now.usec))
    d.update(String(rand(0)))
    d.update(String($$))
    d.update('foobar')
    session_id = d.hexdigest[0, 32]
  end
  session_id
end