[FIXED] Lost sequences after hard kill in fs.removeMsgBlock of lmb

[MauriceVanVeen]

Supersedes #6778

Fixes an edge case where the stream state could reset during recovery in WorkQueue (WQ) streams.

In WQ streams, messages are removed immediately upon ACK. Once a message block becomes empty, NATS deletes the corresponding block file. After deleting the last message block (lmb), NATS also creates a new block file that carries the latest sequence and timestamp from lmb.

If NATS crashes between deleting the lmb and creating the new block file, on NATS reboot the recovery logic cannot restore the stream state due to the absence of any block files. This leads to a reset to sequence 0, potentially causing inconsistencies such as the consumer sequence being higher than the stream sequence.

This change updates the logic to first create the new block file before deleting the lmb. This ensures that there is always at least one valid block on disk, allowing for consistent recovery even if NATS is terminated unexpectedly during the transition.

Resolves #6600

Signed-off-by: Sourabh Agrawal souravagrawal1111@gmail.com
Signed-off-by: Maurice van Veen github@mauricevanveen.com