[Django] #22427: Session entry disappears before it can be saved

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Apr 12, 2014, 11:45:49 PM4/12/14

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#22427: Session entry disappears before it can be saved
---------------------------+-----------------------------------------------
Reporter: anonymous | Owner: nobody
Type: Bug | Status: new
Component: | Version: 1.6
contrib.sessions | Keywords: session dict disappear middleware
Severity: Normal | Has patch: 0
Triage Stage: | UI/UX: 0
Unreviewed |
Easy pickings: 0 |
---------------------------+-----------------------------------------------
I am running the exact same code on my local development machine and a
production server.

Everything important I can think of is the same: both Ubuntu 13.10, both
Python 2.7.5+, both Django 1.6.2, etc.

Yet the two machines are producing different behaviour!

I modified the file django/contrib/sessions/middleware.py, like so:

{{{
import sys
sys.stderr.write('\nBefore!\n')
for key, value in request.session.iteritems():
sys.stderr.write('%s=%s\n'%(key, value))
request.session.save()
sys.stderr.write('\nAfter!\n')
for key, value in request.session.iteritems():
sys.stderr.write('%s=%s\n'%(key, value))
}}}

So I am "printing" the session data before and after it is saved in the
middleware.

On my machine, the data in question is present before and after (e.g. key
= 'hello', value = '123'), but on the production server, the data in
question is neither present before or after the save.

I have no idea how this is possible, but it's happening and driving me
crazy.

Note: I am setting the session entry like so (in my view):
{{{
request.session['hello'] = '123'
}}}
So somewhere between that dict insertion and the middleware processing,
the entry is completely disappearing, yet both machines are running
identical code!

--
Ticket URL: <https://code.djangoproject.com/ticket/22427>
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Django

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Apr 13, 2014, 12:02:40 AM4/13/14

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#22427: Session entry disappears before it can be saved

-------------------------------------+-------------------------------------

Reporter: anonymous | Owner: nobody
Type: Bug | Status: new

Component: contrib.sessions | Version: 1.6
Severity: Normal | Resolution:
Keywords: session dict | Triage Stage:
disappear middleware | Unreviewed
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------
Changes (by anonymous):

* needs_better_patch: => 0
* needs_tests: => 0
* needs_docs: => 0

Comment:

Tried bringing the debug print statements forward, i.e. from the
middleware to my view code (right after I insert the session entry):

{{{
import random

request.session['hello'] = random.randint()

import sys
sys.stderr.write('\nBefore!\n')
for key, value in request.session.iteritems():
sys.stderr.write('%s=%s\n'%(key, value))
request.session.save()
sys.stderr.write('\nAfter!\n')
for key, value in request.session.iteritems():
sys.stderr.write('%s=%s\n'%(key, value))
}}}

The results on my local development machine are what a sane person would
expect, but the results on the production server are what an insane person
might expect.

I.e. On the production server, sometimes the results are OK, while other
times the "before" and "after" '''values''' don't match up!!!
The "before" value might be right, but then the "after" value displays the
previous value (from the last request).

I know it sounds like these two issues are not completely related, but the
code on both machines is exactly the same! What the heck is going on???

--
Ticket URL: <https://code.djangoproject.com/ticket/22427#comment:1>

Django

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Apr 13, 2014, 12:12:56 AM4/13/14

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#22427: Session entry disappears before it can be saved

-------------------------------------+-------------------------------------

Reporter: anonymous | Owner: nobody
Type: Bug | Status: new

Comment (by Matt <carsrule@…>):

Continuing on from the previous comment... or sometimes the "after" value
just disappears for that 'hello' key!

--
Ticket URL: <https://code.djangoproject.com/ticket/22427#comment:2>

Django

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Apr 13, 2014, 6:26:32 AM4/13/14

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#22427: Session entry disappears before it can be saved

-------------------------------------+-------------------------------------

Reporter: anonymous | Owner: nobody
Type: Bug | Status: new

Comment (by erikr):

This is certainly curious. What settings regarding sessions have you set,
and are you using exactly the same database server? And are the settings
between the two boxes the same? Calling it a "local development" machine
makes me think DEBUG might already be set differently.

--
Ticket URL: <https://code.djangoproject.com/ticket/22427#comment:3>

Django

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Apr 13, 2014, 6:44:29 AM4/13/14

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#22427: Session entry disappears before it can be saved

-------------------------------------+-------------------------------------

Reporter: anonymous | Owner: nobody
Type: Bug | Status: new

Comment (by Matt <carsrule@…>):

Using the default session engine, which I think is the database.

Yep, both running PostgreSQL 9.1.13

Settings are exactly the same (synchronisation managed by Git) and sorry
should have been more clear, I get sane/desirable results on my local
machine when DEBUG is True and False (i.e. when I'm in development and
production mode).

--
Ticket URL: <https://code.djangoproject.com/ticket/22427#comment:4>

Django

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Apr 23, 2014, 7:38:44 PM4/23/14

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#22427: Session entry disappears before it can be saved

-------------------------------------+-------------------------------------

Reporter: anonymous | Owner: nobody
Type: Bug | Status: new

Comment (by Matt <carsrule@…>):

Upgraded both machines to Ubuntu 14.04, which included an upgrade from
PostgreSQL 9.1.13 to PostgreSQL 9.3

The problem still occurred.

I just tried switching from the default database session engine to a file
system session engine and the problem seems to disappear.

Will try and do some investigation on that (not sure how helpful I will
be), but it definitely looks like the database session engine is at fault
here.

--
Ticket URL: <https://code.djangoproject.com/ticket/22427#comment:5>

Django

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Apr 23, 2014, 8:33:38 PM4/23/14

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#22427: Session entry disappears before it can be saved

-------------------------------------+-------------------------------------

Reporter: anonymous | Owner: nobody
Type: Bug | Status: new

Comment (by Matt <carsrule@…>):

I'm pretty sure this is what's going on with the database session engine:

My manual call to save the session (request.session.save()) is working
properly (have confirmed this), but then another copy of a request (and
hence it's copy of a session) is overwriting my manual save.

I.e. I put debug prints in the Django database session engine code and
noticed my manual save (looking as it should), but then there are two more
saves after my manual one which sometimes (randomly) have the session
data/values before my manual save and thus overwrite my desired
data/values.

I'm guessing these overwriting saves are happening in the session
middleware, but will confirm this.

So the problem seems to lie with the fact that copies of a request (and
hence session) are being made, as opposed to creating references/pointers
to the one canonical request (and session).

--
Ticket URL: <https://code.djangoproject.com/ticket/22427#comment:6>

Django

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Apr 23, 2014, 9:20:48 PM4/23/14

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#22427: Session entry disappears before it can be saved

-------------------------------------+-------------------------------------

Reporter: anonymous | Owner: nobody
Type: Bug | Status: new

Comment (by Matt <carsrule@…>):

I have pinpointed the issue to as close as I can get without spending an
enormous amount of time on it.

My latest find is that the "_session_cache" attribute of the SessionBase
class (parent of the database SessionStore class) is changing when the
database Session object/record gets saved.

I.e. I debug printed the value and id() of the _session_cache attribute
before and after the transaction/atomic database save and sometimes
(randomly) there are differences (which there shouldn't be).

Since session lookups are based off the _session_cache attribute, this is
the most likely cause of the problem, but I have no idea why it is
occurring on my production server machine and not my development machine.

Seems like some crazy Python bug to me, because I can't see why a database
save would modify a different object, but who knows.

I'm just going to use the file system session engine for now (because I
can't tolerate this absurd behaviour anymore), but I do hope someone is
able to shed some light.

--
Ticket URL: <https://code.djangoproject.com/ticket/22427#comment:7>

Django

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Apr 23, 2014, 9:26:26 PM4/23/14

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#22427: _session_cache of SessionBase affected by database session engine save
-------------------------------------+-------------------------------------

Reporter: anonymous | Owner: nobody
Type: Bug | Status: new

--
Ticket URL: <https://code.djangoproject.com/ticket/22427#comment:8>

Django

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Apr 23, 2014, 9:52:01 PM4/23/14

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#22427: _session_cache of SessionBase affected by database session engine save
-------------------------------------+-------------------------------------

Reporter: anonymous | Owner: nobody
Type: Bug | Status: new

Changes (by freshquiz):

* cc: carsrule@… (added)

--
Ticket URL: <https://code.djangoproject.com/ticket/22427#comment:9>

Django

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Apr 23, 2014, 9:53:11 PM4/23/14

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#22427: _session_cache of SessionBase affected by database session engine save
-------------------------------------+-------------------------------------

Reporter: anonymous | Owner: nobody
Type: Bug | Status: new

Component: contrib.sessions | Version: 1.6
Severity: Normal | Resolution:
Keywords: session dict | Triage Stage:

* keywords: session dict disappear middleware => session dict disappear
middleware save database

--
Ticket URL: <https://code.djangoproject.com/ticket/22427#comment:10>

Django

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Apr 29, 2014, 7:07:13 PM4/29/14

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#22427: _session_cache of SessionBase affected by database session engine save
-------------------------------------+-------------------------------------

Reporter: anonymous | Owner: nobody
Type: Bug | Status: closed
Component: contrib.sessions | Version: 1.6
Severity: Normal | Resolution: needsinfo

Changes (by timo):

* status: new => closed
* resolution: => needsinfo

Comment:

I am going to close this as needsinfo for now as I am not sure we'll be
able to reproduce this given the details so far. If you can provide a
minimal project that reproduces the issue that would be helpful.

--
Ticket URL: <https://code.djangoproject.com/ticket/22427#comment:11>

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