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Re: pg_waldump: support decoding of WAL inside tarfile

Thomas Munro <thomas.munro@gmail.com>

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From: Thomas Munro <thomas.munro@gmail.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Tomas Vondra <tomas@vondra.me>, 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: 2026-04-03T00:47:28Z
Lists: pgsql-hackers 
On Fri, Apr 3, 2026 at 1:11 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
> In any case, this is all completely moot if we don't write code to
> de-sparse a sparse entry: we will not be able to validate WAL data
> if the WAL file is missing some pages.  So I see little point in
> having code that tolerates pax headers if it doesn't also do that.

Yeah.  FWIW I spent a few hours hacking on that the other day and
could decode many files, but I now realise that the task was made more
difficult by a problem you fixed: without header validation, small
mistakes resulted in corruption or went bananas.  With that now
addressed, I hope I can get it into shape and propose it for the next
cycle...

For what it's worth, I was just speculating about how one might
reasonably handle unrecognised *non-standard* header names, not the
POSIX-standardised ones which, you're right, we'd probably need to
grok properly.  If we assumed reasonable engineering decisions
following (what I understood to be) the spirit of pax, maybe we could
assume that new non-standard headers either don't affect file contents
and thus could be ignored (think: GNU.windows.permissions=...), or do
affect file contents but have measures in place to prevent unknown
encodings from being exposed to unsuspecting software (think:
deathstation.byte=9bit).  That's a position we could choose to take,
anyway, in the absence of a crystal ball...  Fortunately there aren't
really many implementations of POSIX left, so it's not like we're
dealing with the Fermi Paradox here...

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.