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Possible PdoSessionStorage issue #2591

Closed
mmucklo opened this Issue · 17 comments

4 participants

@mmucklo

We seem to have encountered a case where PdoSessionStorage, or a similar class may not save to DB / storage at the end of the request.

We've worked around it, however I'm posting this issue at the request of someone on the newsgroup.

We have a derivative of NativeSessionStorage who's start() method is identical (+/- whitespace) to PdoSessionStorage.php

    /**
     * Starts the session.
     */
    public function start()
    {
        if (self::$sessionStarted) {
            return;
        }

        // use this object as the session handler
        session_set_save_handler(
            array($this, 'sessionOpen'),
            array($this, 'sessionClose'),
            array($this, 'sessionRead'),
            array($this, 'sessionWrite'),
            array($this, 'sessionDestroy'),
            array($this, 'sessionGC')
        );

        parent::start();
    }

We then more or less implement sessionWrite, sessionRead, etc.

The problem is that sessionWrite in certain circumstances doesn't seem to be called at the end of the request, resulting in variables not written to our session storage.

What we see is :

1) PdoSessionStorage(or Equivalent)->sessionWrite() (called by PHP save_handler routine)
2) Session->__destruct
3) Calls Session->save()
4) Calls PdoSessionStorage(NativeSessionStorage)->write()
5) $_SESSION updated
6) end of request. (updated $_SESSION never saved)

What actually needs to happen:

1) Session->__destruct
2) Calls Session->save()
3) Calls PdoSessionStorage(NativeSessionStorage)->write()
4) $_SESSION updated
5) PdoSessionStorage(or Equivalent)->sessionWrite()
6) end of request.

We've hacked around it, but I think the problem is that PHP is calling the save handler before __destruct, and doesn't call it afterwards as perhaps it's cleared or something (don't know internally the order of things).

Our hack extends Session and implementing a __destruct such as below, and using this as the session handler instead by overriding it in a custom session.xml file similar to this:

namespace Us\OurBundle\Session;

class HackedSession
    extends \Symfony\Component\HttpFoundation\Session
{
    public function __destruct()
    {
        parent::__destruct();
        if (true === $this->started && !$this->closed) {
            if(method_exists($this->storage, 'sessionWrite'));
            $this->storage->sessionWrite($this->getId(), session_encode());
        }
    }
}

session.xml:

    <parameters>
                 <!-- ... -->
        <parameter key="session.class">Us\OurBundle\Session\HackedSession</parameter>
    </parameters>

Then load this session.xml under our DependencyInjection class using $loader->load...

Anyway, I'm not sure this is the best way to fix the problem but would appreciate input.

@mvrhov

Are you by any chance using APC?

@mvrhov

Addendum does adding session_write_close(); into sessions __destruct class solve the issue?
If it doesn't then we probably need a base Session class which all other session implementations should extend and it's destructor should contain call to that function.

This is coming from: http://si.php.net/manual/en/function.session-set-save-handler.php and this: https://bugs.php.net/bug.php?id=58763

@ghost

Somewhere in your application, you should register_shutdown_function('session_write_close'); and this should solve the issue. That will ensure the session is closed out properly.

@ghost Unknown referenced this issue from a commit
Commit has since been removed from the repository and is no longer available.
@ghost

Even better, try placing

register_shutdown_function('session_write_close');

In the constructor of NativeSessionStorage

@ghost Unknown referenced this issue from a commit
Commit has since been removed from the repository and is no longer available.
@mvrhov

drak it's not ok. We need to apply this to all storage implementations, so if it won't work putting it in save function of Session class then we should have a base class, because it will bite somebody else if done differently.
As I can't simulate that I wanted to have a confirmation before doing an actual PR.

@ghost

AFAIK, it's just a certain chicken and egg situation with using objects as the session handler because of the way PHP shuts down. The register_shutdown_function('session_write_close') calls whatever has been registered as the save handler when PHP exits. It does this before the session object is destroyed. In fact, doing it this way, there is no need to use __destruct() at all, as PHP will call the save() function.@fabpot

As I remember it, we actually removed our __destruct() entirely to solve this problem (and register the shutdown function as session_write_close).

@mmucklo

To answer the "Are you by any chance using APC?" question -

no it's actually Memcache, but it was originally supposed to be a MongoDB / Memcache hybrid, with MongoDB acting as the permanent store...However we've disabled the MongoDB part for now for performance reasons.

@ghost Unknown referenced this issue from a commit
Commit has since been removed from the repository and is no longer available.
@ghost

Try the change in cc07b00 - this is the solution we used to solve this issue in another framework. For reference, the session save handler will always be called when PHP shuts down. There is no need for a __destruct() method because of the we register shutdown function to call the session save handler. This is done outside of the storage driver, in the session class itself so should cover all storage driver cases.

@ghost

I think I see a problem. Symfony's session save handler is in the Session class, however, when the session save handler is called, it will call the storage driver, not the session since the storage driver's methods are registered. The problem is before the session can be saved, Symfony does some work in Session->save(); which really should be in the storage driver itself...

@mmucklo

I tried the patch that was suggested, and it didn't work. Basically it ends up saving blank sessions, it seems.

So far we have now something similar to the following:

    $kernel->handle(Request::createFromGlobals())->send();
    // ....
    $container = $kernel->getContainer();
    if (($session = $container->get('session')) && $session->all())
    {
        $session->save();
        $session->close();
        session_write_close();
    }

That and we've put some extra checks in our NativeSessionStorage derivative's sessionWrite function to make sure we don't write the same sessionData twice.

However I did find, pertaining to the original issue that we had a bug in our code

It turns out we had a session_write_close() handler that was being called outside of symfony (you might consider as if it was part of app.php after $kernel->handle).

Removing that made the original __destruct sequence work, with caveats. Basically we found we could no longer reference certain class static variables as the Class objects themselves had been destructed, someone here said it's basically a bad idea to be doing things in a destructor (such as saving to an external session store). Hence we went with the above solution for now...

@ghost

Yes, the problem is the that the save handlers call code that doesn't do the Session->save() tasks, it calls the storage drivers. I've opened a ticket because the entire Session management is flawed. The only reason things sort of work at the moment is because when PHP shutdown, Session->__destruct() mostly saves the session data, but this is not how sessions are supposed to work. I've explained a lot in #2607 and am working on a PR as there is a lot wrong with the current state of things.

@ghost

As a temporary work around, you could try adding this into the session class constructor:

register_shutdown_function(array($this, 'save'));

This is not the fix, there are much deeper issues, but it could make things usable for you in the meantime.

@fabpot fabpot referenced this issue from a commit
@fabpot fabpot merged branch drak/session_refactor (PR #2853)
Commits
-------

cb6fdb1 [HttpFoundation] removed Session::close()
c59d880 Docblocks.
b8df162 Correct instanceof condition.
8a01dd5 renamed getFlashes() to getFlashBag() to avoid clashes
282d3ae updated CHANGELOG for 2.1
0f6c50a [HttpFoundation] added some method for a better BC
146a502 [FrameworkBundle] added some service aliases to avoid some BC breaks
93d81a1 [HttpFoundation] removed configuration for session storages in session.xml as we cannot provide a way to configure them (like before this PR anyway)
74ccf70 reverted 5b7ef11 (Simplify session storage class names now we have a separate namespace for sessions)
91f4f8a [HttpFoundation] changed default flash bag to auto-expires to keep BC
0494250 removed unused use statements
7878a0a [HttpFoundation] renamed pop() to all() and getAll() to all()
0d2745f [HttpFoundation] Remove constants from FlashBagInterface
dad60ef [HttpFoundation] Add back get defaults and small clean-up.
5b7ef11 [HttpFoundation] Simplify session storage class names now we have a separate namespace for sessions.
27530cb [HttpFoundation] Moved session related classes to own sub-namespace.
4683915 [HttpFoundation] Free bags from session storage and move classes to their own namespaces.
d64939a [DoctrineBridge] Refactored driver for changed interface.
f9951a3 Fixed formatting.
398acc9 [HttpFoundation] Reworked flashes to maintain same behaviour as in Symfony 2.0
f98f9ae [HttpFoundation] Refactor for DRY code.
9dd4dbe Documentation, changelogs and coding standards.
1ed6ee3 [DoctribeBridge][SecurityBundle][WebProfiler] Refactor code for HttpFoundation changes.
7aaf024 [FrameworkBundle] Refactored code for changes to HttpFoundation component.
669bc96 [HttpFoundation] Added pure Memcache, Memcached and Null storage drivers.
e185c8d [HttpFoundation] Refactored component for session workflow.
85b5c43 [HttpFoundation] Added drivers for PHP native session save handlers, files, sqlite, memcache and memcached.
57ef984 [HttpFoundation] Added unit and functional testing session storage objects.
3a263dc [HttpFoundation] Introduced session storage base class and interfaces.
c969423 [HttpFoundation] Added FlashBagInterface and concrete implementation.
39288bc [HttpFoundation] Added AttributesInterface and AttributesBagInterface and concrete implementations.

Discussion
----------

[2.1][HttpFoundation] Refactor session handling and flash messages

Bug fix: yes
Feature addition: yes
Backwards compatibility break: yes
Symfony2 tests pass: yes
Fixes the following tickets: #2607, #2591, #2717, #2773
References the following tickets: #2592, #2543, #2541, #2510, #2714, #2684
Todo: -

__Introduction__

This extensive PR is a refactor with minimal BC breaks of the `[HttpFoundation]` component's session management which fixes several issues in the current implementation.  This PR includes all necessary changes to other bundles and components is documented in the `CHANGELOG-2.1` and `UPGRADING-2.1`.

__Summary of Changes__

__Session:__
  - Session object now implements `SessionInterface`

__Attributes:__
  - Attributes now handled by `AttributeBagInterface`
  - Added two AttributeBag implementations: `AttributeBag` replicates the current Symfony2 attributes behaviour, and the second, `NamespacedAttributeBag` introduces structured namespaced representation using '/' in the key.  Both are BC.  `FrameworkBundle` defaults to the old behaviour.

__Flash messages:__
  - Flash messages now handled by `FlashBagInterface`
  - Introduced `FlashBag` which changes the way flash messages expire, they now expire on use rather than automatically, useful for ESI.
  - Introduced `AutoExpireFlashBag` (default) which replicates the old automatic expiry behaviour of flash messages.

__Session Storage:__
  - Introduced a base object, `AbstractSessionStorage` for session storage drivers
  - Introduced a `SessionSaveHandlerInterface` when using custom session save handlers
  - Introduced a `NullSessionStorage` driver which allows for unsaved sessions
  - Introduced new session storage drivers for Memcache and Memcached
  - Introduced new session storage drivers for PHP's native SQLite, Memcache and Memcached support

__General:__
  - Fixed bugs where session attributes are not saved and all cases where flash messages would get lost
  - Wrote new tests, refactored related existing tests and increased test coverage extensively.

__Rationale/Details__

I'll explain more detail in the following sections.

__Unit Tests__

All unit and functional tests pass.

__Note on Functional Testing__

I've introduced `MockFileSessionStorage` which replaces `FilesystemSessionStorage` to emulate a PHP session for functional testing.  Essentially the same methodology of functional testing has been maintained but without interrupting the other session storage drivers interaction with real PHP sessions.  The service is now called `session.storage.mock_file`.

__Session Workflow__

PHP sessions follow a specific workflow which is not being followed by the current session management implementation and is responsible for some unpredictable bugs and behaviours.

Basically, PHP session workflow is as follows: `open`, `read`, `write`, `close`.  In between these can occur, `destroy` and `garbage collection`.  These actions are handled by `session save handlers` and one is always registered in all cases.  By default, the `files` save handler (internally to PHP) is registered by PHP at the start of code execution.

PHP offers the possibility to change the save handler to another internal type, for example one provided by a PHP extension (think SQLite, Memcache etc), or you can register a user type and set your own handlers.  However __in all cases__ PHP requires the handlers.

The handlers are called when the following things occur:

  - `open` and `read` when `session_start()` or the session autostarts when PHP outputs some display
  - `destroy` when `session_regenerate_id(true)` is called
  - `write` and `close` when PHP shuts down or when `session_write_close()` is called
  - `garbage collection` is called randomly according to configurable probability

The next very important aspect of this PR is that `$_SESSION` plays an important part in this workflow because the contents of the $_SESSION is populated __after__ the `read` handler returns the previously saved serialised session data.  The `write` handler is sent the serialised `$_SESSION` contents for save.  Please note the serialisation is a different format to `serialize()`.

For this reason, any session implementation cannot get rid of using `$_SESSION`.

I wrote more details on this issue [here](#2607 (comment))

In order to make writing session storage drivers simple, I created a light base class `AbstractSessionStorage` and the `SessionSaveHandlerInterface` which allows you to quickly write native and custom save handler drivers.

__Flash Messages [BC BREAK]__

Flash messages currently allow representation of a single message per `$name`.  Fabien designed the original system so that `$name` was equivalent to flash message type.  The current PR changes the fact that Flash messages expire explicitly when retrieved for display to the user as opposed to immediately on the next page load.

The last issue fixes potential cases when flash messages are lost due to an unexpected intervening page-load (an error for example).  The API `get()` has a flag which allows you to override the `clear()` action.

__Flash message translation__

This PR does not cover translation of flash messages because  messages should be translated before calling the flash message API.  This is because flash messages are used to present messages to the user after a specific action, and in any case, immediately on the next page load.  Since we know the locale of the request in every case we can translate the message before storing.  Secondly, translation is simply a string manipulation.  Translation API calls should always have the raw untranslated string present because it allows for extraction of translation catalogs.  For a complete answer see my answer [here](#2543 (comment))

__Session attribute and structured namespacing__

__This has been implemented without changing the current default behaviour__ but details are below for the alternative:

Attributes are currently stored in a flat array which limits the potential of session attributes:

Here are some examples to see why this 'structured namespace' methodology is extremely convenient over using a flat system.  Let's look at an example with csrf tokens.  Let's say we have multiple csrftokens stored by form ID (allowing multiple forms on the page and tabbed browsing).

If we're using a flat system, you might have

    'tokens' => array('a' => 'a6c1e0b6',
                      'b' => 'f4a7b1f3')

With a flat system when you get the key `tokens`, you will get back an array, so now you have to analyse the array.  So if you simply want to add another token, you have to follow three steps: get the session attribute `tokens`, have to add to the array, and lastly set the entire array back to the session.

    $tokens = $session->get('tokens');
    $tokens['c'] = $value;
    $session->set('tokens', $tokens);

Doable, but you can see it's pretty long winded.

With structured namespacing you can simply do:

    $session->set('c', $value, '/tokens');

There are several ways to implement this, either with an additional `$namespace` argument, or by treating a character in the `$key` as a namespacer.  `NamespacedAttributeBag` treats `/` as a namespacer so you can represent `user.tokens/a` for example.  The namespace character is configurable in `NamespacedAttributeBag`.

---------------------------------------------------------------------------

by marijn at 2011-12-18T15:43:17Z

I haven't read the code yet but the description from this PR and your line of thought seem very well structured.
Seems like a big +1 for me.

---------------------------------------------------------------------------

by lsmith77 at 2011-12-19T16:01:19Z

@deviantintegral could you look over this to see if it really addresses everything you wanted with PR #2510 ?

---------------------------------------------------------------------------

by deviantintegral at 2011-12-24T20:12:03Z

I've read through the documentation and upgrade notes, and I can't see anything that's obviously missing from #2510. Being able to support multiple flashes per type is the most important, and the API looks reasonable to me. Drupal does support supressing repeat messages, but that can easily be implemented in our code unless there's a compelling case for it to be a part of Symfony.

I wonder if PHP memcache support is required in Symfony given the availability of memcached. I'm not familiar with how other parts of Symfony handle it, but there is often quite a bit of confusion between the two PHP extensions. It could be simpler to remove one, or add a bit of info describing or linking to why there are two nearly identical classes.

Is it possible to make one class inherit from the other (memcached is a child of memcache)?

---------------------------------------------------------------------------

by Fristi at 2011-12-24T20:29:46Z

Interesting, maybe add: session events as I did with the current impl: https://github.com/Fristi/SessionBundle

---------------------------------------------------------------------------

by drak at 2011-12-25T00:50:03Z

@deviantintegral - I agree about the confusion between memcache and memcached but actually, it is necessary to support both because `memcached` is not available everywhere.  For example on Debian Lenny and RHEL/CentOS 5, only memcache is available by default.  This would preclude a massive amount of shared hosting environments.  Also, it is not possible to inherit one from the other, they are completely different drivers.

@Fristi - I also thought about the events, but they do not belong as part of the standalone component as this would create a coupling to the event dispatcher.  The way you have done it, ie, in a bundle is the right way to achieve it.

---------------------------------------------------------------------------

by matheo at 2011-12-25T01:12:00Z

Impressive work, looks like a big improvement and deserves a big +1

---------------------------------------------------------------------------

by datiecher at 2011-12-26T11:57:12Z

Took some time to grok all the changes in this PR but all in all it is a keeper. Specially the new flash message API, it's really nicer to work with it then the previous one.

Nicely done @drak!

---------------------------------------------------------------------------

by lsmith77 at 2012-01-02T15:00:00Z

@fabpot did you have time to review this yet? with all the work @drak has done its important that he gets some feedback soon. its clear this PR breaks BC in ways we never wanted to allow. but i think this PR also clearly explains why its necessary none the less.

---------------------------------------------------------------------------

by drak at 2012-01-02T15:41:53Z

@fabpot - I have removed the WIP status from this PR now and rebased against the current master branch.

---------------------------------------------------------------------------

by Tobion at 2012-01-07T07:13:38Z

From what I read from the IRC chat logs, the main concern of @fabpot is whether we really need multiple flash messages per type. I'm in favor of this PR and just want to add one point to this discussion.
At the moment you can add multiple flash messages of different type/category/identifier. For example you can specify one error message and one info message after an operation. I think most agree that this can be usefull.
But then it makes semantically no sense that you currently cannot add 2 info messages. This approach feels a bit half-done.
So I think this PR eliminates this paradox.

---------------------------------------------------------------------------

by drak at 2012-01-07T09:11:07Z

For reference there is also a discussion started by @lsmith77 on the mailing list at https://groups.google.com/forum/#!topic/symfony-devs/cy4wokD0mQI

---------------------------------------------------------------------------

by dlsniper at 2012-01-07T16:02:15Z

@drak I could also add the next scenario that I currently have to live with, in addition to @lsmith77 ones.

I had this issue while working on our shopping cart implementation for a customer where the customer wanted to show the unavailability of the items as different lines in the 'flash-error' section of the cart. We had to set an array as the 'flash message' in order to display that information.

So in this case for example having the flash messages types as array would actually make more sense that sending an array to the flasher. Plus the the other issue we had was that we also wanted to add another error in the message but we had to do a check to see if the flash message is an array already or we need to make it an array.

I think it's better not to impose a limit of this sort and let the users be able to handle every scenario, even if some are rare, rather that forcing users to overcome limitations such as these.

I really hope this PR gets approved faster and thanks everyone for their hard work :)

---------------------------------------------------------------------------

by Tobion at 2012-01-07T21:01:07Z

@dlsniper I think you misinterpreted my point.

---------------------------------------------------------------------------

by dlsniper at 2012-01-07T21:04:04Z

@Tobion I'm sorry I did that, I'll edit the message asap. Seems no sleep in 26 hours can cause brain not to function as intended :)

---------------------------------------------------------------------------

by lsmith77 at 2012-02-01T14:38:52Z

FYI the drupal guys are liking this PR (including the flash changes):
http://drupal.org/node/335411

---------------------------------------------------------------------------

by drak at 2012-02-01T14:51:33Z

@lsmith77 Fabien asked me to remove the changes to the flash messages so that they are as before - i.e. only one flash per name/type /cc @fabpot

---------------------------------------------------------------------------

by fabpot at 2012-02-01T14:58:23Z

To be clear, I've asked to split this PR in two parts:

 * one about the session refactoring (which is non-controversial and should be merged ASAP)
 * this one with only the flash refactoring

---------------------------------------------------------------------------

by drak at 2012-02-02T11:29:26Z

@fabpot this is ready to be merged now.  I will open a separate PR later today for the flash messages as a bucket.

---------------------------------------------------------------------------

by fabpot at 2012-02-02T11:34:39Z

I must have missed something, but I still see a lot of changes related to the flash messages.

---------------------------------------------------------------------------

by drak at 2012-02-02T11:39:10Z

When I spoke to you you said you wanted to make the commit with flash messages with one message per name/type rather than multiple.  The old flash messages behaviour is 100% maintained in `AutoExpireFlashBag` which can be the default in the framework if you wish.  The `FlashBag` implementation makes Symfony2 ESI compatible.

---------------------------------------------------------------------------

by stof at 2012-02-02T11:47:38Z

@drak splitting into 2 PRs means you should not refactor the flash messages in this one but in the dedicated one.

---------------------------------------------------------------------------

by drak at 2012-02-02T12:29:43Z

@stof Yes. I discussed with Fabien over chat there are basically no changes
to flashes in `FlashBag` and `AutoExpireFlashBag` maintains the exact
behaviour as before.  The FlashBag just introduces ESI compatible flashes.
 There is no way to refactor the sessions without moving the flash messages
to their own bag.  The next PR will propose the changes to flashes that
allow multiple messages per name/type.  I can size the PR down a little
more removing the new storage drivers and so on to make the PR smaller but
that's really as far as I can go.  To be clear, while the API has changed a
little for flashes, the behaviour is the same.
4841400
@fabpot
Owner

Does the recent PRs merges solve this issue?

@mmucklo

I apologize for the delay. I did start to take a look at this, but please give me a few more days as I need more time

@ghost

@mmucklo please remember this is not fixed in Symfony 2.0 but only in the master branch which will become Symfony 2.1

@mmucklo

drak,

I started reviewing the changes, they seem to make sense. It seems that now you'd now create a SessionHandler extending from NativeSessionHandler if you wanted to do something custom (such as a hybrid memcache + something else approach).

Do you then now override / define a session.handler service in your session.xml (or whatever xml file is being used for dependency injection definitions in your bundle)?

@stof stof closed this
@mmucklo mmucklo referenced this issue from a commit
@fabpot fabpot merged branch drak/session_refactor (PR #2853)
Commits
-------

cb6fdb1 [HttpFoundation] removed Session::close()
c59d880 Docblocks.
b8df162 Correct instanceof condition.
8a01dd5 renamed getFlashes() to getFlashBag() to avoid clashes
282d3ae updated CHANGELOG for 2.1
0f6c50a [HttpFoundation] added some method for a better BC
146a502 [FrameworkBundle] added some service aliases to avoid some BC breaks
93d81a1 [HttpFoundation] removed configuration for session storages in session.xml as we cannot provide a way to configure them (like before this PR anyway)
74ccf70 reverted 5b7ef11 (Simplify session storage class names now we have a separate namespace for sessions)
91f4f8a [HttpFoundation] changed default flash bag to auto-expires to keep BC
0494250 removed unused use statements
7878a0a [HttpFoundation] renamed pop() to all() and getAll() to all()
0d2745f [HttpFoundation] Remove constants from FlashBagInterface
dad60ef [HttpFoundation] Add back get defaults and small clean-up.
5b7ef11 [HttpFoundation] Simplify session storage class names now we have a separate namespace for sessions.
27530cb [HttpFoundation] Moved session related classes to own sub-namespace.
4683915 [HttpFoundation] Free bags from session storage and move classes to their own namespaces.
d64939a [DoctrineBridge] Refactored driver for changed interface.
f9951a3 Fixed formatting.
398acc9 [HttpFoundation] Reworked flashes to maintain same behaviour as in Symfony 2.0
f98f9ae [HttpFoundation] Refactor for DRY code.
9dd4dbe Documentation, changelogs and coding standards.
1ed6ee3 [DoctribeBridge][SecurityBundle][WebProfiler] Refactor code for HttpFoundation changes.
7aaf024 [FrameworkBundle] Refactored code for changes to HttpFoundation component.
669bc96 [HttpFoundation] Added pure Memcache, Memcached and Null storage drivers.
e185c8d [HttpFoundation] Refactored component for session workflow.
85b5c43 [HttpFoundation] Added drivers for PHP native session save handlers, files, sqlite, memcache and memcached.
57ef984 [HttpFoundation] Added unit and functional testing session storage objects.
3a263dc [HttpFoundation] Introduced session storage base class and interfaces.
c969423 [HttpFoundation] Added FlashBagInterface and concrete implementation.
39288bc [HttpFoundation] Added AttributesInterface and AttributesBagInterface and concrete implementations.

Discussion
----------

[2.1][HttpFoundation] Refactor session handling and flash messages

Bug fix: yes
Feature addition: yes
Backwards compatibility break: yes
Symfony2 tests pass: yes
Fixes the following tickets: #2607, #2591, #2717, #2773
References the following tickets: #2592, #2543, #2541, #2510, #2714, #2684
Todo: -

__Introduction__

This extensive PR is a refactor with minimal BC breaks of the `[HttpFoundation]` component's session management which fixes several issues in the current implementation.  This PR includes all necessary changes to other bundles and components is documented in the `CHANGELOG-2.1` and `UPGRADING-2.1`.

__Summary of Changes__

__Session:__
  - Session object now implements `SessionInterface`

__Attributes:__
  - Attributes now handled by `AttributeBagInterface`
  - Added two AttributeBag implementations: `AttributeBag` replicates the current Symfony2 attributes behaviour, and the second, `NamespacedAttributeBag` introduces structured namespaced representation using '/' in the key.  Both are BC.  `FrameworkBundle` defaults to the old behaviour.

__Flash messages:__
  - Flash messages now handled by `FlashBagInterface`
  - Introduced `FlashBag` which changes the way flash messages expire, they now expire on use rather than automatically, useful for ESI.
  - Introduced `AutoExpireFlashBag` (default) which replicates the old automatic expiry behaviour of flash messages.

__Session Storage:__
  - Introduced a base object, `AbstractSessionStorage` for session storage drivers
  - Introduced a `SessionSaveHandlerInterface` when using custom session save handlers
  - Introduced a `NullSessionStorage` driver which allows for unsaved sessions
  - Introduced new session storage drivers for Memcache and Memcached
  - Introduced new session storage drivers for PHP's native SQLite, Memcache and Memcached support

__General:__
  - Fixed bugs where session attributes are not saved and all cases where flash messages would get lost
  - Wrote new tests, refactored related existing tests and increased test coverage extensively.

__Rationale/Details__

I'll explain more detail in the following sections.

__Unit Tests__

All unit and functional tests pass.

__Note on Functional Testing__

I've introduced `MockFileSessionStorage` which replaces `FilesystemSessionStorage` to emulate a PHP session for functional testing.  Essentially the same methodology of functional testing has been maintained but without interrupting the other session storage drivers interaction with real PHP sessions.  The service is now called `session.storage.mock_file`.

__Session Workflow__

PHP sessions follow a specific workflow which is not being followed by the current session management implementation and is responsible for some unpredictable bugs and behaviours.

Basically, PHP session workflow is as follows: `open`, `read`, `write`, `close`.  In between these can occur, `destroy` and `garbage collection`.  These actions are handled by `session save handlers` and one is always registered in all cases.  By default, the `files` save handler (internally to PHP) is registered by PHP at the start of code execution.

PHP offers the possibility to change the save handler to another internal type, for example one provided by a PHP extension (think SQLite, Memcache etc), or you can register a user type and set your own handlers.  However __in all cases__ PHP requires the handlers.

The handlers are called when the following things occur:

  - `open` and `read` when `session_start()` or the session autostarts when PHP outputs some display
  - `destroy` when `session_regenerate_id(true)` is called
  - `write` and `close` when PHP shuts down or when `session_write_close()` is called
  - `garbage collection` is called randomly according to configurable probability

The next very important aspect of this PR is that `$_SESSION` plays an important part in this workflow because the contents of the $_SESSION is populated __after__ the `read` handler returns the previously saved serialised session data.  The `write` handler is sent the serialised `$_SESSION` contents for save.  Please note the serialisation is a different format to `serialize()`.

For this reason, any session implementation cannot get rid of using `$_SESSION`.

I wrote more details on this issue [here](#2607 (comment))

In order to make writing session storage drivers simple, I created a light base class `AbstractSessionStorage` and the `SessionSaveHandlerInterface` which allows you to quickly write native and custom save handler drivers.

__Flash Messages [BC BREAK]__

Flash messages currently allow representation of a single message per `$name`.  Fabien designed the original system so that `$name` was equivalent to flash message type.  The current PR changes the fact that Flash messages expire explicitly when retrieved for display to the user as opposed to immediately on the next page load.

The last issue fixes potential cases when flash messages are lost due to an unexpected intervening page-load (an error for example).  The API `get()` has a flag which allows you to override the `clear()` action.

__Flash message translation__

This PR does not cover translation of flash messages because  messages should be translated before calling the flash message API.  This is because flash messages are used to present messages to the user after a specific action, and in any case, immediately on the next page load.  Since we know the locale of the request in every case we can translate the message before storing.  Secondly, translation is simply a string manipulation.  Translation API calls should always have the raw untranslated string present because it allows for extraction of translation catalogs.  For a complete answer see my answer [here](#2543 (comment))

__Session attribute and structured namespacing__

__This has been implemented without changing the current default behaviour__ but details are below for the alternative:

Attributes are currently stored in a flat array which limits the potential of session attributes:

Here are some examples to see why this 'structured namespace' methodology is extremely convenient over using a flat system.  Let's look at an example with csrf tokens.  Let's say we have multiple csrftokens stored by form ID (allowing multiple forms on the page and tabbed browsing).

If we're using a flat system, you might have

    'tokens' => array('a' => 'a6c1e0b6',
                      'b' => 'f4a7b1f3')

With a flat system when you get the key `tokens`, you will get back an array, so now you have to analyse the array.  So if you simply want to add another token, you have to follow three steps: get the session attribute `tokens`, have to add to the array, and lastly set the entire array back to the session.

    $tokens = $session->get('tokens');
    $tokens['c'] = $value;
    $session->set('tokens', $tokens);

Doable, but you can see it's pretty long winded.

With structured namespacing you can simply do:

    $session->set('c', $value, '/tokens');

There are several ways to implement this, either with an additional `$namespace` argument, or by treating a character in the `$key` as a namespacer.  `NamespacedAttributeBag` treats `/` as a namespacer so you can represent `user.tokens/a` for example.  The namespace character is configurable in `NamespacedAttributeBag`.

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by marijn at 2011-12-18T15:43:17Z

I haven't read the code yet but the description from this PR and your line of thought seem very well structured.
Seems like a big +1 for me.

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by lsmith77 at 2011-12-19T16:01:19Z

@deviantintegral could you look over this to see if it really addresses everything you wanted with PR #2510 ?

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by deviantintegral at 2011-12-24T20:12:03Z

I've read through the documentation and upgrade notes, and I can't see anything that's obviously missing from #2510. Being able to support multiple flashes per type is the most important, and the API looks reasonable to me. Drupal does support supressing repeat messages, but that can easily be implemented in our code unless there's a compelling case for it to be a part of Symfony.

I wonder if PHP memcache support is required in Symfony given the availability of memcached. I'm not familiar with how other parts of Symfony handle it, but there is often quite a bit of confusion between the two PHP extensions. It could be simpler to remove one, or add a bit of info describing or linking to why there are two nearly identical classes.

Is it possible to make one class inherit from the other (memcached is a child of memcache)?

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by Fristi at 2011-12-24T20:29:46Z

Interesting, maybe add: session events as I did with the current impl: https://github.com/Fristi/SessionBundle

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by drak at 2011-12-25T00:50:03Z

@deviantintegral - I agree about the confusion between memcache and memcached but actually, it is necessary to support both because `memcached` is not available everywhere.  For example on Debian Lenny and RHEL/CentOS 5, only memcache is available by default.  This would preclude a massive amount of shared hosting environments.  Also, it is not possible to inherit one from the other, they are completely different drivers.

@Fristi - I also thought about the events, but they do not belong as part of the standalone component as this would create a coupling to the event dispatcher.  The way you have done it, ie, in a bundle is the right way to achieve it.

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by matheo at 2011-12-25T01:12:00Z

Impressive work, looks like a big improvement and deserves a big +1

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by datiecher at 2011-12-26T11:57:12Z

Took some time to grok all the changes in this PR but all in all it is a keeper. Specially the new flash message API, it's really nicer to work with it then the previous one.

Nicely done @drak!

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by lsmith77 at 2012-01-02T15:00:00Z

@fabpot did you have time to review this yet? with all the work @drak has done its important that he gets some feedback soon. its clear this PR breaks BC in ways we never wanted to allow. but i think this PR also clearly explains why its necessary none the less.

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by drak at 2012-01-02T15:41:53Z

@fabpot - I have removed the WIP status from this PR now and rebased against the current master branch.

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by Tobion at 2012-01-07T07:13:38Z

From what I read from the IRC chat logs, the main concern of @fabpot is whether we really need multiple flash messages per type. I'm in favor of this PR and just want to add one point to this discussion.
At the moment you can add multiple flash messages of different type/category/identifier. For example you can specify one error message and one info message after an operation. I think most agree that this can be usefull.
But then it makes semantically no sense that you currently cannot add 2 info messages. This approach feels a bit half-done.
So I think this PR eliminates this paradox.

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by drak at 2012-01-07T09:11:07Z

For reference there is also a discussion started by @lsmith77 on the mailing list at https://groups.google.com/forum/#!topic/symfony-devs/cy4wokD0mQI

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by dlsniper at 2012-01-07T16:02:15Z

@drak I could also add the next scenario that I currently have to live with, in addition to @lsmith77 ones.

I had this issue while working on our shopping cart implementation for a customer where the customer wanted to show the unavailability of the items as different lines in the 'flash-error' section of the cart. We had to set an array as the 'flash message' in order to display that information.

So in this case for example having the flash messages types as array would actually make more sense that sending an array to the flasher. Plus the the other issue we had was that we also wanted to add another error in the message but we had to do a check to see if the flash message is an array already or we need to make it an array.

I think it's better not to impose a limit of this sort and let the users be able to handle every scenario, even if some are rare, rather that forcing users to overcome limitations such as these.

I really hope this PR gets approved faster and thanks everyone for their hard work :)

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by Tobion at 2012-01-07T21:01:07Z

@dlsniper I think you misinterpreted my point.

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by dlsniper at 2012-01-07T21:04:04Z

@Tobion I'm sorry I did that, I'll edit the message asap. Seems no sleep in 26 hours can cause brain not to function as intended :)

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by lsmith77 at 2012-02-01T14:38:52Z

FYI the drupal guys are liking this PR (including the flash changes):
http://drupal.org/node/335411

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by drak at 2012-02-01T14:51:33Z

@lsmith77 Fabien asked me to remove the changes to the flash messages so that they are as before - i.e. only one flash per name/type /cc @fabpot

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by fabpot at 2012-02-01T14:58:23Z

To be clear, I've asked to split this PR in two parts:

 * one about the session refactoring (which is non-controversial and should be merged ASAP)
 * this one with only the flash refactoring

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by drak at 2012-02-02T11:29:26Z

@fabpot this is ready to be merged now.  I will open a separate PR later today for the flash messages as a bucket.

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by fabpot at 2012-02-02T11:34:39Z

I must have missed something, but I still see a lot of changes related to the flash messages.

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by drak at 2012-02-02T11:39:10Z

When I spoke to you you said you wanted to make the commit with flash messages with one message per name/type rather than multiple.  The old flash messages behaviour is 100% maintained in `AutoExpireFlashBag` which can be the default in the framework if you wish.  The `FlashBag` implementation makes Symfony2 ESI compatible.

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by stof at 2012-02-02T11:47:38Z

@drak splitting into 2 PRs means you should not refactor the flash messages in this one but in the dedicated one.

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by drak at 2012-02-02T12:29:43Z

@stof Yes. I discussed with Fabien over chat there are basically no changes
to flashes in `FlashBag` and `AutoExpireFlashBag` maintains the exact
behaviour as before.  The FlashBag just introduces ESI compatible flashes.
 There is no way to refactor the sessions without moving the flash messages
to their own bag.  The next PR will propose the changes to flashes that
allow multiple messages per name/type.  I can size the PR down a little
more removing the new storage drivers and so on to make the PR smaller but
that's really as far as I can go.  To be clear, while the API has changed a
little for flashes, the behaviour is the same.
24dc082
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