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Re: pg_waldump: support decoding of WAL inside tarfile

Tom Lane <tgl@sss.pgh.pa.us>

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From: Tom Lane <tgl@sss.pgh.pa.us>
To: Thomas Munro <thomas.munro@gmail.com>
Cc: Andres Freund <andres@anarazel.de>, Michael Paquier <michael@paquier.xyz>, Andrew Dunstan <andrew@dunslane.net>, Amul Sul <sulamul@gmail.com>, Zsolt Parragi <zsolt.parragi@percona.com>, Robert Haas <robertmhaas@gmail.com>, Chao Li <li.evan.chao@gmail.com>, Anthonin Bonnefoy <anthonin.bonnefoy@datadoghq.com>, Fujii Masao <masao.fujii@gmail.com>, Jakub Wartak <jakub.wartak@enterprisedb.com>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Date: 2026-03-28T22:18:03Z
Lists: pgsql-hackers 
Thomas Munro <thomas.munro@gmail.com> writes:
> It seems like the place we'd be looking for the file is in
> astreamer_tar_header(), so I added in some caveman debugging:

> XXXX [PaxHeader/000000010000000000000002] XXXX
> XXXX [GNUSparseFile.0/000000010000000000000002] XXXX

After a bit of research, I see the problem: your tar has decided to
store 000000010000000000000002 as a sparse file, and evidently it
inserts "GNUSparseFile.0/" into the file name when it does that.
Our tar-parsing logic will not recognize that name as being one
of the WAL files it wants; and even if it did, it doesn't know
how to de-sparse the file contents, so it would get the wrong
output.

I'm pretty disinclined to try to support sparse files here,
especially with ~10 days till feature freeze.

However ... I do not find any indication in the GNU tar docs
that it produces sparse files by default.  It looks like you
need to say -S/--sparse to make that happen.  Maybe you have
a version that's been hacked to make that the default?

			regards, tom lane

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. Use size_t instead of Size in pg_waldump

  2. More tar portability adjustments.

  3. Further harden tests that might use not-so-compatible tar versions.

  4. Harden astreamer tar parsing logic against archives it can't handle.

  5. Fix pg_waldump/t/001_basic.pl with BSD tar on ZFS.

  6. Remove a low-value, high-risk optimization in pg_waldump.

  7. Fix misuse of simplehash.h hash operations in pg_waldump.

  8. Fix file descriptor leakages in pg_waldump.

  9. Fix poorly-sized buffers in astreamer compression modules.

  10. Remove read_archive_file()'s "count" parameter.

  11. Report detailed errors from XLogFindNextRecord() failures.

  12. Fix assorted bugs in archive_waldump.c.

  13. Remove nonfunctional tar file trailer size check.

  14. Fix finalization of decompressor astreamers.

  15. Move tar detection and compression logic to common.

  16. pg_verifybackup: Enable WAL parsing for tar-format backups

  17. pg_waldump: Add support for reading WAL from tar archives

  18. pg_waldump: Preparatory refactoring for tar archive WAL decoding.

  19. pg_waldump: Remove file-level global WalSegSz.

  20. pg_verifybackup: Verify tar-format backups.