MsgBlock is the unit of storage in the file store — a single append-only
block file containing sequentially written binary message records. Blocks
are sealed (read-only) when they reach a configurable byte-size limit.
Key features:
- Write: appends MessageRecord-encoded messages with auto-incrementing
sequence numbers and configurable first sequence offset
- Read: positional I/O via RandomAccess.Read for concurrent reader safety
- Delete: soft-delete with on-disk persistence (re-encodes flags byte +
checksum so deletions survive recovery)
- Recovery: rebuilds in-memory index by scanning block file using
MessageRecord.MeasureRecord for record boundary detection
- Thread safety: ReaderWriterLockSlim allows concurrent reads during writes
Also adds MessageRecord.MeasureRecord() — computes a record's byte length
by parsing varint field headers without full decode, needed for sequential
record scanning during block recovery.
Reference: golang/nats-server/server/filestore.go:217-267 (msgBlock struct)
12 tests covering write, read, delete, seal, recovery, concurrency,
and custom sequence offsets.
