Port xbstream_fifo_test.sh to pytest (xbstream_fifo_tests.py)

[saikumar-vs]

Summary

Ports xbstream_fifo_test.sh (the bash FIFO-streamed backup test, updated to use SeaweedFS in #11, now merged) to the Python/pytest framework, following the conventions of innodb_myrocks_backup_tests.py and inc_backup_load_tests.py.

• test_helper.py — FIFO_STREAM/FIFO_DIR constants and new BackupTestHelper methods: take_backup_fifo (runs xtrabackup --backup in the background + xbcloud put in the foreground, both reading/writing the same named pipes), take_full_backup_fifo/take_incremental_backup_fifo (full and full+N-incremental orchestration, mirroring the bash script's full_backup_and_restore()/incremental_backup_and_restore()), and cleanup_fifo_state. These stream xtrabackup/xbstream and xbcloud through named pipes as two concurrent processes — distinct from take_backup()'s existing "cloud" backup_type, which is a single shell pipe.
  Two existing methods were extended (not just added-to): xbcloud_get gained a use_fifo keyword (default False, so existing callers are unaffected) that switches it from the single-pipe xbcloud get | xbstream -x to the two-process FIFO form; and take_backup()'s inline stop/move-datadir/copy-back/restart restore logic was factored out into a new shared restore_datadir_from() method (also used by the FIFO path), removing the duplication between the two.
• seaweedfs_helper.py (new) — SeaweedFSHelper class managing the local SeaweedFS S3-gateway docker container, ported from the xbstream_fifo_test.sh start_seaweedfs()/cleanup_exit() shell logic merged in Replace MinIO with SeaweedFS in xbstream_fifo_test.sh, fix keyring_file bugs #11.
• xbstream_fifo_tests.py (new) — the test file itself: a session-scoped seaweedfs fixture (starts the container once, stops it at session end) and 7 tests mirroring the bash script's scenarios (full, incremental, compressed, partition-table, keyring_file, keyring_kmip, xbcrypt-encrypted backup).
• README.md — new section documenting env vars, how to run, and the test reference table, matching the existing per-file structure.
• kmip_helper.py / test_helper.py / hashicorp_kmip_setup.py — enabled the hashicorp KMIP vault type (was commented out of KMIP_CONFIGS/DEFAULT_KMIP_CONFIGS, and even if enabled, setup_hashicorp() was invoking the downloaded setup script via bash instead of python3, which would have failed immediately). Added a HASHICORP_VAULT_LICENSE env-var skip-guard everywhere the shared KMIP_CONFIGS now surfaces a hashicorp variant (this file's test_fifo_kmip_backup, plus the existing test_kmip_component_backup and test_crash_backup_encrypted_kmip in inc_backup_load_tests.py), matching the existing FORTANIX_EMAIL/FORTANIX_PASSWORD guard pattern.
• Improved error surfacing in kmip_helper.py (setup_hashicorp now records self.last_error for empty-download/exec failures) and seaweedfs_helper.py (start() now checks and reports docker start's exit code instead of ignoring it); also fixed an or FIFO_STREAM/or FIFO_DIR pattern in test_helper.py that would have silently overridden an explicit fifo_streams=0/fifo_dir="" with the default (should only fall back on None).
• Branch has been rebased onto main (post-Replace MinIO with SeaweedFS in xbstream_fifo_test.sh, fix keyring_file bugs #11) so this PR's diff no longer shows against a stale base.

Test plan

Verified end-to-end against real PXB 8.4.0-7 / PS 8.4.10-10 builds and a live SeaweedFS container:

• test_fifo_full_backup_and_restore
• test_fifo_incremental_backup
• test_fifo_compressed_backup (confirmed real zstd compression in the backup log)
• test_fifo_partition_tables
• test_fifo_keyring_file_backup
• test_fifo_encrypted_backup
• test_fifo_kmip_backup[pykmip] — run 3 times: passed, then hit a genuine xtrabackup --prepare SIGSEGV crash (__printf_buffer/__vsnprintf_internal), then passed again. Non-deterministic and reproducible independently of this port's code — a product-level PXB 8.4.0-7 stability issue during prepare of an encrypted (keyring_kmip) backup, in the same general class as the separately-noted keyring_file InnoDB-assertion crash. Worth its own PXB bug report; not something this PR can fix.
• test_fifo_kmip_backup[fortanix] / [hashicorp] — confirmed both skip cleanly without FORTANIX_EMAIL/FORTANIX_PASSWORD / HASHICORP_VAULT_LICENSE set (neither available in this environment; the credential/license-driven setup paths themselves are unverified end-to-end)
• Re-verified take_full_backup_fifo/take_incremental_backup_fifo/take_backup_fifo after the method renaming and the xbcloud_get/restore_datadir_from consolidation (full + incremental FIFO backups still pass)

Regression-checked: innodb_myrocks_backup_tests.py and inc_backup_load_tests.py still collect cleanly (72 tests total across all three files).

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