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Description of problem:

When attempting to hot-add a new slave to a posgresql cluster, pg_basebackup will fail doing base sync from a remote node with either:

pg_basebackup: could not get transaction log end position from server: ERROR:  could not open file "./tmp/xlog_note.0": Permission denied

against another slave, or

pg_basebackup: could not get transaction log end position from server: ERROR:  could not open file "./tmp/PGSQL.lock": Permission denied

against the master.

Both of these files are root:root, 0640.

Granting this might not be the right way to sync up a new node, this seems an unreasonable restriction.

From the info I find you should stop the resource on the slave (e.g. standby the node) and run pg_basebackup on the slave node when postgresql isnt running on it.

When it isnt running the xlog and lock files are removed, which should solve the issue.

The xlog and lock files exist on the source of the backup, another slave or master, respectively. pg_basebackup runs against a running PG server.

It works fine for me when I run the following sequence:
# su - postgres -c "rm -rf /var/lib/pgsql/data/*"
# su - postgres -c "pg_basebackup -h 1.2.3.4 -U postgres -D /var/lib/pgsql/data -X stream -P"
# su - postgres -c "mkdir /var/lib/pgsql/pg_archive"

Did you get around to test by doing what I explained in the last comment?

Sorry. Yeah, please close as 1d10t