log chunking broken with large queries under load

Some of my PostgreSQL Experts colleagues have been complaining to me
that servers under load with very large queries cause CSV log files that
are corrupted, because lines are apparently multiplexed. The log
chunking protocol between the errlog routines and the syslogger is
supposed to prevent that, so I did a little work to try to reproduce it
in a controlled way. On my dual quad xeon setup, this script:

#!/bin/sh
par=$1
seq=$2

sed 2000q /usr/share/dict/words > words

psql -q -c 'drop table if exists foo'
psql -q -c 'create table foo (t text)'

echo '\set words `cat words`' > wordsin.sql
echo 'prepare fooplan (text) as insert into foo values ($1);' >>
wordsin.sql

for i in `seq 1 $seq`; do
echo "execute fooplan(:'words');" >> wordsin.sql
done

for i in `seq 1 $par`; do
psql -q -t -f wordsin.sql &
done
wait

called with parameters of 100 and 50 (i.e. 100 simultaneous clients each
doing 50 very large inserts) is enough to cause CSV log corruption quite
reliably on PostgreSQL 9.1.

This is a serious bug. I'm going to investigate, but it's causing major
pain, so anyone else who has any ideas is welcome to chime in.

cheers

andrew