I've investigated more. I think this is timeout problem. According to
following site,
http://www.oracle.com/technology/documentation/berkeley-
db/db/api_c/rep_timeout.html
timeout should be specified in microseconds. This means
self.dbenvMaster.rep_set_timeout(db.DB_REP_CONNECTION_RETRY,100123)
only waits 10msec. This is too small comparing to default timeout.
Probably 10sec is appropriate here.
After I applied an attached patch "fix_timeout.py", I ran test several
times and found no error.
# I changed time.time()+10 to time.time()+30 because my machine is too
slow, so 10sec was not enough. :-[