Constant MAX_PENDING_ACKS_PER_FILE 

pub(crate) const MAX_PENDING_ACKS_PER_FILE: usize = 20_000;

Unacked messages that force the open file to close early.

Messages are acked only after the file holding their samples is closed. The file is closed early if this many messages are waiting, so the consumer can never reach its max_ack_pending limit (CONSUMER_MAX_ACK_PENDING) and stall.