MDEV-39468 Mariabackup: redo log copier stalls and fails with misleading error message

[Thirunarayanan]

Problem:

During backup process, log copier thread stalls at lsn and fails with the misleading "Was only able to copy log from X to Y, not Z; try increasing innodb_log_file_size", even when the redo was intact and the log far from full.

Problem is that copier advances recv_sys.lsn only on CRC-validated mtr, but it parses the redo at the server is concurrently writing. InnoDB rewrites the current partial write block repeatedly as mini-transaction fills it, so an mmap read (or) pread of a page being written can observe a torn/intermediate image of the tail block. If that tail block happens to parse as a longer, still valid crc valid mtr then recv_sys.lsn, recv_sys.offset gets advanced wrongly and points to middle of the mini-transaction. In that case, later parse return GOT_EOF always and copier thread never reaches target and fails

Solution:

Poll the server's durably-flushed LSN (status var Innodb_lsn_flushed) and limit the redo log copier based on it.

backup_log_parse(): parses one mtr and if it exceeds the limit rolls back recv_sys.lsn/offset and reports GOT_EOF; the copier re-parses those bytes on a later pass once the fence has advanced past them. Used on both the mmap and buffered paths.

parse_limit_from_flushed(): Rounds the flushed LSN down to a write-block boundary, excluding the tail block InnoDB may still be rewriting

log_copying_thread(): opens its own connection (it runs concurrently with the main thread, which owns mysql_connection) and refreshes the max_limit for parsing of redo log in each pass.

In the final backup phase (BACKUP STAGE BLOCK_COMMIT) it gates on the exact target max(metadata_last_lsn, metadata_to_lsn) with no block-align, so the last partial block is copied in full up to the target.

The stall diagnostic / retry spin is suppressed on a reached_parse_limit wait so a normal "caught up, wait" is not misreported as drift.

get_log_flushed_lsn(): Added to get log_flushed_lsn from SHOW STATUS outout;

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[gemini-code-assist]

Warning

Gemini encountered an error creating the review. You can try again by commenting /gemini review.