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Description of problem:
While reproducing bug 139834, my bayes learning was migrated to
spamassassin 3 format. However, my home directory is shared by a
machine still running spamassassin 2.x. The updated bayes database
drove spamassassin 2.x mad, sending it to an infinitely loop and
filling up the logs. The only recourse was to "throw away" the updated
bayes database (because there was no way to migrate it back into a 2.x
format) thus destroying 2 years of email training and causing spam to
get through.

Version-Release number of selected component (if applicable):
spamassassin-3.0.0-3

How reproducible:
Always

Steps to Reproduce:
(with an existing spamassassin 2.x bayes database / settings)
1. Run sa-learn --sync on a machinen with spamassassin 3.x

Actual Results:
The bayes database is irreversibly changed to 3.x format.

Expected Results:
2.x style bayes to be backed up or 3.x to migrate settings into a
different place and default to using the new settings.

Additional info:
The 2.x spamd wound up printing the following over and over in the
logs until it was killed when using the new format bayes database:

spamd[18068]: Use of uninitialized value in numeric eq (==) at
/usr/lib/perl5/vendor_perl/5.8.1/Mail/SpamAssassin/BayesStore.pm line
1459, <GEN25343> line 56.