Rails 2 session clobbered some time AFTER variable is stored

Question

I am working on an existing Rails 2 app. I have converted a few of the hash data structures into objects, and if I put one in the session store, it seems to clobber the session, clearing out the user_id, among other things, and forcing another login. I'm using dalli_store for the session.

The following code causes the session to be wiped out:

bug = MyClass.get_an_object()
session[:debug] = bug

It's not clear WHERE it is getting wiped out. I can step through in the debugger to the end of rendering the view, and the session is fine, but when I hit another link in the UI, session is empty (Hash[0]), and I am redirected to the login page.

If I change the code slightly, to this, the session is OK:

bug = MyClass.get_an_object()
session[:debug] = Marshal::dump(bug)

However, if I store the actual object (even a deep copy), the session is lost. I.e. even this does not work:

session[:debug] = Marshal::load( Marshal::dump(bug) )

bug.size is about 140K when marshalled, so it should not be overrunning memcached. At any rate, I would assume that session is serialized by Marshal::dump(), so the size should be identical. It doesn't matter if I access the object in the session after storing it. Just putting it into the session is enough to cause the problem, but as I said, the session is fine after storing the object, and all the way through the view rendering. It isn't until the start of the next request that I find out that it has been clobbered.

I'm stumped.

Do you have any recommendations on how to debug this? For the moment, I guess I can explicitly call Marshal to save the object, but I would really like to understand why the session is getting clobbered.

I know it's a Bad Thing to put big objects into the session, but fixing that part of the problem is out of scope for the current project ... maybe later. Plus, I am very curious about what is going on here.