POSKeyError when getting Zope SESSION

Hi,

Full details are at:
https://community.plone.org/t/sessions-and-poskeyerrors/5939

Summary:
- Using Python 2.7.14, ZODB 5.3.0, Zope2 2.13.26, tempstorage 4.0.1, 
BTrees 4.4.1, Plone 5.1.
- Zope stores sessions in temporary storage, based on a session in in a 
cookie.
- My authentication plugin gets the session and may set some values in it.
- Problem: getting the session sometimes gives a POSKeyError.
- This is for a new site that is in development, so I only tested it 
locally on my Mac.  One Zope instance, no ZEO setup.

Sample traceback:

2018-03-02T16:07:51 ERROR client.myplugin KeyError getting SESSION for 
request http://127.0.0.1:8080/Plone/++resource++protect.js
Traceback (most recent call last):
   File ".../myplugin/plugins.py", line 31, in get_session
     return request.SESSION
   File 
"/Users/maurits/shared-eggs/cp27m/Zope2-2.13.26-py2.7.egg/ZPublisher/HTTPRequest.py", 
line 1380, in __getattr__
     v = self.get(key, default, returnTaints=returnTaints)
   File 
"/Users/maurits/shared-eggs/cp27m/Zope2-2.13.26-py2.7.egg/ZPublisher/HTTPRequest.py", 
line 1337, in get
     v = v()
   File 
"/Users/maurits/shared-eggs/cp27m/Zope2-2.13.26-py2.7.egg/Products/Sessions/SessionDataManager.py", 
line 101, in getSessionData
     return self._getSessionDataObject(key)
   File 
"/Users/maurits/shared-eggs/cp27m/Zope2-2.13.26-py2.7.egg/Products/Sessions/SessionDataManager.py", 
line 188, in _getSessionDataObject
     ob = container.new_or_existing(key)
   File 
"/Users/maurits/shared-eggs/cp27m/Zope2-2.13.26-py2.7.egg/Products/Transience/Transience.py", 
line 839, in new_or_existing
     item = self.get(key, _marker)
   File 
"/Users/maurits/shared-eggs/cp27m/Zope2-2.13.26-py2.7.egg/Products/Transience/Transience.py", 
line 495, in get
     item = self._move_item(k, current_ts, default)
   File 
"/Users/maurits/shared-eggs/cp27m/Zope2-2.13.26-py2.7.egg/Products/Transience/Transience.py", 
line 314, in _move_item
     if abucket.get(k, None) is not None:
   File 
"/Users/maurits/shared-eggs/cp27m/ZODB-5.3.0-py2.7.egg/ZODB/Connection.py", 
line 796, in setstate
     p, serial = self._storage.load(oid)
   File 
"/Users/maurits/shared-eggs/cp27m/ZODB-5.3.0-py2.7.egg/ZODB/mvccadapter.py", 
line 143, in load
     r = self._storage.loadBefore(oid, self._start)
   File 
"/Users/maurits/shared-eggs/cp27m/tempstorage-4.0.1-py2.7.egg/tempstorage/TemporaryStorage.py", 
line 187, in loadBefore
     raise POSException.POSKeyError(oid)
POSKeyError: 0x13

This error then keeps happening (at least for the user with this 
session) until I restart the Zope instance.

I tried creating a simple script to reproduce it, but I failed.
Today I did not see the error at all.
I enabled a debugging environment variable Z_TOC_DEBUG from 
Products.Transience, but since the error did not happen since then, I 
have no debug data from it...
Maybe it only fails when a BTree in the session needs to be reshuffled 
or something.

My current plan is to avoid using a session for this, and instead store 
what I need in a cookie.  That should also avoid possible problems in a 
setup with multiple zeoclients.

But does anyone have an idea what is going on here and how to avoid this?

Again, full details are at:
https://community.plone.org/t/sessions-and-poskeyerrors/5939

Thanks,

-- 
Maurits van Rees: http://maurits.vanrees.org/
Zest Software: http://zestsoftware.nl

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