BUG #6602: concurrent psql session clobbers history

The following bug has been logged on the website:

Bug reference:      6602
Logged by:          Adrian Dries
Email address:      i+pgbugs@avdd.tk
PostgreSQL version: 9.1.3
Operating system:   Ubuntu ppa:pitti/postgresql
Description:=20=20=20=20=20=20=20=20

A concurrent psql session will overwrite the history of the first.  This
does not look good: a database tool causing me to lose my data!

# session 1

$ psql -q
postgres=3D# \echo first session exits first
first session exits first
postgres=3D# \q
$ cat .psql_history=20
\echo first session exits first
\q
$ psql -q
postgres=3D#=20
$ cat .psql_history=20
\echo second session exits second
\q
$ psql -q
postgres=3D# \echo first session exits second
first session exits second
postgres=3D# \q
$ cat .psql_history=20
\echo second session exits second
\q
\echo first session exits second
\q

# session 2

$ psql -q
postgres=3D# \echo second session exits second
second session exits second
postgres=3D# \q
$ cat .psql_history=20
\echo second session exits second
\q
$ psql -q
postgres=3D# \echo second session exits first
second session exits first
postgres=3D# \q
$ cat .psql_history=20
\echo second session exits second
\q
\echo second session exits first
\q
=20