How to recover a broken python "cPickle" dump?

Question

I am using rss2email for converting a number of RSS feeds into mail for easier consumption. That is, I was using it because it broke in a horrible way today: On every run, it only gives me this backtrace:

Traceback (most recent call last):
  File "/usr/share/rss2email/rss2email.py", line 740, in <module>
    elif action == "list": list()
  File "/usr/share/rss2email/rss2email.py", line 681, in list
    feeds, feedfileObject = load(lock=0)
  File "/usr/share/rss2email/rss2email.py", line 422, in load
    feeds = pickle.load(feedfileObject)
TypeError: ("'str' object is not callable", 'sxOYAAuyzSx0WqN3BVPjE+6pgPU', ((2009, 3, 19, 1, 19, 31, 3, 78, 0), {}))

The only helpful fact that I have been able to construct from this backtrace is that the file ~/.rss2email/feeds.dat in which rss2email keeps all its configuration and runtime state is somehow broken. Apparently, rss2email reads its state and dumps it back using cPickle on every run.

I have even found the line containing that 'sxOYAAuyzSx0WqN3BVPjE+6pgPU'string mentioned above in the giant (>12MB) feeds.dat file. To my untrained eye, the dump does not appear to be truncated or otherwise damaged.

What approaches could I try in order to reconstruct the file?

The Python version is 2.5.4 on a Debian/unstable system.

EDIT

Peter Gibson and J.F. Sebastian have suggested directly loading from the pickle file and I had tried that before. Apparently, a Feed class that is defined in rss2email.py is needed, so here's my script:

#!/usr/bin/python

import sys
# import pickle
import cPickle as pickle
sys.path.insert(0,"/usr/share/rss2email")
from rss2email import Feed

feedfile = open("feeds.dat", 'rb')
feeds = pickle.load(feedfile)

The "plain" pickle variant produces the following traceback:

Traceback (most recent call last):
  File "./r2e-rescue.py", line 8, in <module>
    feeds = pickle.load(feedfile)
  File "/usr/lib/python2.5/pickle.py", line 1370, in load
    return Unpickler(file).load()
  File "/usr/lib/python2.5/pickle.py", line 858, in load
    dispatch[key](self)
  File "/usr/lib/python2.5/pickle.py", line 1133, in load_reduce
    value = func(*args)
TypeError: 'str' object is not callable

The cPickle variant produces essentially the same thing as calling r2e itself:

Traceback (most recent call last):
  File "./r2e-rescue.py", line 10, in <module>
    feeds = pickle.load(feedfile)
TypeError: ("'str' object is not callable", 'sxOYAAuyzSx0WqN3BVPjE+6pgPU', ((2009, 3, 19, 1, 19, 31, 3, 78, 0), {}))

EDIT 2

Following J.F. Sebastian's suggestion around putting "printf debugging" into Feed.__setstate__ into my test script, these are the last few lines before Python bails out.

          u'http:/com/news.ars/post/20080924-everyone-declares-victory-in-smutfree-wireless-broadband-test.html': u'http:/com/news.ars/post/20080924-everyone-declares-victory-in-smutfree-wireless-broadband-test.html'},
 'to': None,
 'url': 'http://arstechnica.com/'}
Traceback (most recent call last):
  File "./r2e-rescue.py", line 23, in ?
    feeds = pickle.load(feedfile)
TypeError: ("'str' object is not callable", 'sxOYAAuyzSx0WqN3BVPjE+6pgPU', ((2009, 3, 19, 1, 19, 31, 3, 78, 0), {}))

The same thing happens on a Debian/etch box using python 2.4.4-2.

Answer 1

How I solved my problem

A Perl port of pickle.py

Following J.F. Sebastian's comment about how simple the pickle format is, I went out to port parts of pickle.py to Perl. A couple of quick regular expressions would have been a faster way to access my data, but I felt that the hack value and an opportunity to learn more about Python would be be worth it. Plus, I still feel much more comfortable using (and debugging code in) Perl than Python.

Most of the porting effort (simple types, tuples, lists, dictionaries) went very straightforward. Perl's and Python's different notions of classes and objects has been the only issue so far where a bit more than simple translation of idioms was needed. The result is a module called Pickle::Parse which after a bit of polishing will be published on CPAN.

A module called Python::Serialise::Pickle existed on CPAN, but I found its parsing capabilities lacking: It spews debugging output all over the place and doesn't seem to support classes/objects.

Parsing, transforming data, detecting actual errors in the stream

Based upon Pickle::Parse, I tried to parse the feeds.dat file. After a few iteration of fixing trivial bugs in my parsing code, I got an error message that was strikingly similar to pickle.py's original object not callable error message:

Can't use string ("sxOYAAuyzSx0WqN3BVPjE+6pgPU") as a subroutine
ref while "strict refs" in use at lib/Pickle/Parse.pm line 489,
<STDIN> line 187102.

Ha! Now we're at a point where it's quite likely that the actual data stream is broken. Plus, we get an idea where it is broken.

It turned out that the first line of the following sequence was wrong:

g7724
((I2009
I3
I19
I1
I19
I31
I3
I78
I0
t(dtRp62457

Position 7724 in the "memo" pointed to that string "sxOYAAuyzSx0WqN3BVPjE+6pgPU". From similar records earlier in the stream, it was clear that a time.struct_time object was needed instead. All later records shared this wrong pointer. With a simple search/replace operation, it was trivial to fix this.

I find it ironic that I found the source of the error by accident through Perl's feature that tells the user its position in the input data stream when it dies.

Conclusion

1. I will move away from rss2email as soon as I find time to automatically transform its pickled configuration/state mess to another tool's format.
2. pickle.py needs more meaningful error messages that tell the user about the position of the data stream (not the poision in its own code) where things go wrong.
3. Porting parts pickle.py to Perl was fun and, in the end, rewarding.