could not create directory "...": File exists

Greetings,

We've come across this rather annoying error happening during our
builds:

ERROR: could not create directory "pg_tblspc/25120/PG_9.3_201212081/231253": File exists

It turns out that this is coming from copydir() when called by
createdb() during a CREATE DATABASE .. FROM TEMPLATE where the
template has tables in tablespaces.

It turns out that createdb() currently only takes an AccessShareLock
on pg_tablespace when scanning it with SnapshotNow, making it possible
for a concurrent process to make some uninteresting modification to a
tablespace (such as an ACL change) which will cause the heap scan in
createdb() to see a given tablespace multiple times. copydir() will
then, rightfully, complain that it's being asked to create a directory
which already exists.

Given that this is during createdb(), I'm guessing we don't have any
option but to switch the scan to using ShareLock, since there isn't a
snapshot available to do an MVCC scan with (I'm guessing that there
could be other issues trying to do that anyway).

Attached is a patch which does this and corrects the problem for us
(of course, we're now going to go rework our build system to not
modify tablespace ACLs, since with this patch concurrent builds end up
blocking on each other, which is annoying).

Thanks,

Stephen