pg_basebackup, tablespace mapping and path canonicalization

Hi

I stumbled on what appears to be inconsistent handling of double slashes
in tablespace paths when using pg_basebackup with the -T/--tablespace-mapping
option:

ibarwick:postgresql (master)$ mkdir /tmp//foo-old
ibarwick:postgresql (master)$ $PG_HEAD/psql 'dbname=postgres port=9595'
psql (9.5devel)
Type "help" for help.

postgres=# CREATE TABLESPACE foo LOCATION '/tmp//foo-old';
CREATE TABLESPACE
postgres=# \db
List of tablespaces
Name | Owner | Location
------------+----------+--------------
foo | ibarwick | /tmp/foo-old
pg_default | ibarwick |
pg_global | ibarwick |
(3 rows)

So far so good. However attempting to take a base backup (on the same
machine) and remap the tablespace directory:

ibarwick:postgresql (master)$ $PG_HEAD/pg_basebackup -p9595 --pgdata=/tmp//backup --tablespace-mapping=/tmp//foo-old=/tmp//foo-new

produces the following message:

pg_basebackup: directory "/tmp/foo-old" exists but is not empty

which, while undeniably true, is unexpected and could potentially encourage someone
to hastily delete "/tmp/foo-old" after confusing it with the new directory.

The double-slash in the old tablespace path is the culprit:

ibarwick:postgresql (master)$ $PG_HEAD/pg_basebackup -p9595 --pgdata=/tmp//backup --tablespace-mapping=/tmp/foo-old=/tmp//foo-new
NOTICE: pg_stop_backup complete, all required WAL segments have been archived

The documentation does state:

To be effective, olddir must exactly match the path specification of the
tablespace as it is currently defined.

which I understood to mean that e.g. tildes would not be expanded, but it's
somewhat surprising that the path is not canonicalized in the same way
it is pretty much everywhere else (including in "CREATE TABLESPACE").

The attached patch adds the missing canonicalization; I can't see any
reason not to do this. Thoughts?

Regards

Ian Barwick

--
Ian Barwick http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, RemoteDBA, Training & Services