Re: refactoring basebackup.c

Thanks, Robert for the patches.

I tried to take a backup using gzip compression and got a core.

$ pg_basebackup -t server:/tmp/data_gzip -Xnone --server-compression=gzip
NOTICE: WAL archiving is not enabled; you must ensure that all required
WAL segments are copied through other means to complete the backup
pg_basebackup: error: could not read COPY data: server closed the
connection unexpectedly
This probably means the server terminated abnormally
before or while processing the request.

The backtrace:

gdb) bt
#0 0x0000000000000000 in ?? ()
#1 0x0000558264bfc40a in bbsink_cleanup (sink=0x55826684b5f8) at
../../../src/include/replication/basebackup_sink.h:268
#2 0x0000558264bfc838 in bbsink_forward_cleanup (sink=0x55826684b710) at
basebackup_sink.c:124
#3 0x0000558264bf4cab in bbsink_cleanup (sink=0x55826684b710) at
../../../src/include/replication/basebackup_sink.h:268
#4 0x0000558264bf7738 in SendBaseBackup (cmd=0x55826683bd10) at
basebackup.c:1020
#5 0x0000558264c10915 in exec_replication_command (
cmd_string=0x5582667bc580 "BASE_BACKUP ( LABEL 'pg_basebackup base
backup', PROGRESS, MANIFEST 'yes', TABLESPACE_MAP, TARGET 'server',
TARGET_DETAIL '/tmp/data_g
zip', COMPRESSION 'gzip')") at walsender.c:1731
#6 0x0000558264c8a69b in PostgresMain (dbname=0x5582667e84d8 "",
username=0x5582667e84b8 "hadoop") at postgres.c:4493
#7 0x0000558264bb10a6 in BackendRun (port=0x5582667de160) at
postmaster.c:4560
#8 0x0000558264bb098b in BackendStartup (port=0x5582667de160) at
postmaster.c:4288
#9 0x0000558264bacb55 in ServerLoop () at postmaster.c:1801
#10 0x0000558264bac2ee in PostmasterMain (argc=3, argv=0x5582667b68c0) at
postmaster.c:1473
#11 0x0000558264aa0950 in main (argc=3, argv=0x5582667b68c0) at main.c:198

bbsink_gzip_ops have the cleanup() callback set to NULL, and when the
bbsink_cleanup() callback is triggered, it tries to invoke a function that
is NULL. I think either bbsink_gzip_ops should set the cleanup callback
to bbsink_forward_cleanup or we should be calling the cleanup() callback
from PG_CATCH instead of PG_FINALLY()? But in the latter case, even if
we call from PG_CATCH, it will have a similar problem for gzip and other
sinks which may not need a custom cleanup() callback in case there is any
error before the backup could finish up normally.

I have attached a patch to fix this.

Thoughts?

Regards,
Jeevan Ladhe

On Tue, Oct 26, 2021 at 1:45 AM Robert Haas <robertmhaas(at)gmail(dot)com> wrote:

> On Fri, Oct 15, 2021 at 8:05 AM Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> > > You mean the way gzip allows us to use our own alloc and free functions
> > > by means of providing the function pointers for them. Unfortunately,
> > > no, LZ4 does not have that kind of provision. Maybe that makes a
> > > good proposal for LZ4 library ;-).
> > > I cannot think of another solution to it right away.
> >
> > OK. Will give it some thought.
>
> Here's a new patch set. I've tried adding a "cleanup" callback to the
> bbsink method and ensuring that it gets called even in case of an
> error. The code for that is untested since I have no use for it with
> the existing basebackup sink types, so let me know how it goes when
> you try to use it for LZ4.
>
> I've also added documentation for the new pg_basebackup options in
> this version, and I fixed up a couple of these patches to be
> pgindent-clean when they previously were not.
>
> --
> Robert Haas
> EDB: http://www.enterprisedb.com
>