Fixing pg_basebackup with tablespaces found in $PGDATA

Hi,

As much as I've seen people frown upon $subject, it still happens in the
wild, and Magnus seems to agree that the current failure mode of our
pg_basebackup tool when confronted to the situation is a bug.

So here's a fix, attached.

To reproduce, mkdir -p $PGDATA/tbs/foo then CREATE TABLESPACE there, and
then pg_basebackup your server. If doing so from the same server, as I
did, then pick the tar format, as here:

pg_basebackup -Ft -z -c fast -v -X fetch -D /tmp/backup

Then use tar to see that the base backup contains the whole content of
your foo tablespace, and if you did create another tablespace within
$PGDATA/pg_tblspc (which is the other common way to trigger that issue)
then add it to what you want to see:

tar tzvf /tmp/backup/base.tar.gz pg_tblspc tbs/foo pg_tblspc/bar

Note that empty directories are expected, so tar should output their
entries. Those directories are where you need to be restoring the
tablespace tarballs.

When using pg_basebackup in plain mode, the error is that you get a copy
of all your tablespaces first, then the main PGDATA is copied over and
as the destination directories already do exists (and not empty) the
whole backup fails there.

The bug should be fixed against all revisions of pg_basebackup, though I
didn't try to apply this very patch on all target branches.

Regards,
--
Dimitri Fontaine
http://2ndQuadrant.fr PostgreSQL : Expertise, Formation et Support