BUG #7546: Backups on hot standby cancelled despite hot_standby=on

The following bug has been logged on the website:

Bug reference:      7546
Logged by:          Stuart Bishop
Email address:      stuart@stuartbishop.net
PostgreSQL version: 9.1.5
Operating system:   Ubuntu 12.10
Description:        =


I have a primary and a hot standby using streaming replication. The hot
standby specifies 'hot_standby_feedback=3Don' with other replication settin=
gs
set to default.

If a vacuum occurs on the primary while pg_dump is dumping a large table,
the pg_dump is cancelled, usually with the following error:

ERROR:  canceling statement due to conflict with recovery
DETAIL:  User was holding shared buffer pin for too long.

I can excercise this problem with the following script:

#!/bin/sh

dbname=3D"repl_test"
master_port=3D5432
slave_port=3D5434

rows=3D2000000

slow=3D"pv --rate-limit 20k"

createdb -p $master_port $dbname

psql -p $master_port -d $dbname -f - <<EOM
CREATE TABLE IF NOT EXISTS BigStuff (
    a serial primary key, b integer, c text, d text)
WITH (autovacuum_enabled =3D FALSE);

INSERT INTO BigStuff (b, c, d)
    SELECT i,md5(i::text),reverse(md5(i::text))
    FROM generate_series(1,${rows}) AS i;

DELETE FROM BigStuff WHERE random() < 0.15;
EOM

synced=3D0
while [ $synced -ne 1 ]
do sleep 5
synced=3D`psql -qtA -p $master_port -d $dbname -c "SELECT
(pg_current_xlog_location() =3D min(replay_location))::integer FROM
pg_stat_replication;"`
echo synced $synced synced
done

(pg_dump -p $slave_port --format=3Dc $dbname | $slow > /dev/null) &

sleep 5;

psql -p $master_port -d $dbname -c "vacuum verbose BigStuff;"