pg_tde: harden archive/restore helpers to clean up tmpfs on failure

[leventov]

The pg_tde helper binaries decrypt/reencrypt WAL via a temporary file under TMPFS_DIRECTORY (/dev/shm). On very low‑memory systems this tmpfs can fill, and previous behavior left temporary files/directories behind on failures or abrupt termination, compounding ENOSPC and retries.

Changes:

• Always remove the temporary file and directory on normal exit and on
  SIGINT/SIGTERM (atexit() + signal handlers).
• Check the tmpfs dir capacity before creating the temp file. The temp file size is taken to be:
  • In archive (pg_tde_restore_encrypt.c): size = encrypted source file size + 4MB slack.
  • In restore (pg_tde_archive_decrypt.c): read xlog_seg_size from pg_control (derived from DEST-PATH
    data dir); fallback to 16MB if unavailable + 4MB slack.
  • If the check fails due to insufficient remaining /dev/shm capacity, exit nonzero with a clear error so Postgres will retry later rather than failing mid‑write.

FWIW, the initial condition that has led this failures was a programming mistake on my part rather than pg_tde's defect, but even then, I think it's better to harden the achirve/restore binaries so that they don't exacerbate the problem by cluttering tmpfs.

I've not tested this patch in a realistic environment yet (I indent to do that, but maybe only the next week or so), yet I would like to publish the patch already for review and feedback.

I'm not an expert in C, Postgres, or pgBackRest; the patch is authored by AI coding agent; but I've reviewed it and it makes sense to me.

[it-percona-cla]

All committers have signed the CLA.

[jeltz]

Thanks for the PR. This is for sure a real bug and we have reported it in our issue tracking system as PG-1932. As a C developer I am not really a fan of what the AI has coded so we will likely write our own patch for it which I hope you do not mind. Again thanks a lot for the bug report and the PR!

[leventov]

@jeltz thanks for looking into this. No, I don't mind at all. Would be curious to learn the proper fix!