Re: Add function to return backup_label and tablespace_map

On Fri, Jul 8, 2022 at 3:31 PM Bharath Rupireddy
<bharath.rupireddyforpostgres@gmail.com> wrote:
>
> On Thu, Jul 7, 2022 at 10:14 PM Fujii Masao <masao.fujii@oss.nttdata.com> wrote:
> >
> > Hi,
> >
> > Since an exclusive backup method was dropped in v15, in v15 or later, we need to create backup_label and
tablespace_mapfiles from the result of pg_backup_stop() when taking a base backup using low level backup API. One issue
whendoing this is that; there is no simple way to create those files from two columns "labelfile" and "spcmapfile" that
pg_backup_stop()returns if we execute it via psql. Probaby we need to store those columns in a temporary file and run
someOS commands or script to separate that file into backup_label and tablespace_map. This is not simple way, and which
wouldprevent users from migrating their backup scripts using psql from an exclusive backup method to non-exclusive one,
I'mafraid. 
> >
> > To enable us to do that more easily, how about adding the pg_backup_label() function that returns backup_label and
tablespace_map?I'm thinking to make this function available just after pg_backup_start() finishes, also even after
pg_backup_stop()finishes. For example, this function allows us to take a backup using the following psql script file. 
> >
> > ------------------------------
> > SELECT * FROM pg_backup_start('test');
> > \! cp -a $PGDATA /backup
> > SELECT * FROM pg_backup_stop();
> >
> > \pset tuples_only on
> > \pset format unaligned
> >
> > \o /backup/data/backup_label
> > SELECT labelfile FROM pg_backup_label();
> >
> > \o /backup/data/tablespace_map
> > SELECT spcmapfile FROM pg_backup_label();
> > ------------------------------
> >
> > Attached is the WIP patch to add pg_backup_label function. No tests nor docs have been added yet, but if we can
successfullyreach the consensus for adding the function, I will update the patch. 
> >
> > Thought?
>
> +1 for making it easy for the user to create backup_label and
> tablespace_map files. With the patch, the label_file and
> tblspc_map_file contents are preserved until the lifecycle of the
> session or the next run of pg_backup_start, I'm not sure if we need to
> worry more about it.
>
> Why can't we have functions like pg_create_backup_label() and
> pg_create_tablespace_map() which create the 'backup_label' and
> 'tablespace_map' files respectively in the data directory and also
> return the contents as output columns?
>
> Also, we can let users run these create functions only once (perhaps
> after the pg_backup_stop is called which is when the contents will be
> consistent). If we allow these functions to read the label_file or
> tblspc_map_file contents during the backup before stop backup, they
> may not be consistent. We can have a new sessionBackupState something
> like SESSION_BACKUP_READY_TO_COLLECT_INFO or SESSION_BACKUP_DONE and
> after the new function calls sessionBackupState  goes to
> SESSION_BACKUP_NONE) and the contents of label_file and
> tblspc_map_file are freed up.
>
> In the docs, it's good if we can clearly specify the steps to use all
> of these functions.

Forgot to mention a comment on the v1 patch: we'll need to revoke
permissions from the public for pg_backup_label (or whatever the new
function(s) that'll be introduced) as well similar to pg_backup_start
and pg_backup_stop.

Regards,
Bharath Rupireddy.