fsync while writting last tombstone in purge

Author: tilakraj94

server/filestore.go

@@ -9044,6 +9044,13 @@ func (fs *fileStore) purge(fseq uint64) (uint64, error) {
 		// Leave a tombstone so we can remember our starting sequence in case
 		// full state becomes corrupted.
 		fs.writeTombstone(lseq, lmb.last.ts)
+		if lmb.mfd != nil {
+			// Sync is critical here: after purge, the message block may be 0 bytes (especially for workqueue streams), and
+			// the index may also be 0 bytes. The tombstone will be the only persistent record of the last sequence number.
+			//  We must ensure it's durably written to disk before returning; otherwise, a crash could leave the
+			// stream in an unrecoverable state with no sequence history.
+			lmb.mfd.Sync()
+		}
 	}
 
 	cb := fs.scb