client: fix panic caused by too large channel size in pkg/varlog.subscribedLogEntriesQueue

[ijsong]

Subscribe API creates a queue to keep log entries received from each log stream. The queue, subscribedLogEntriesQueue, uses a buffered channel as an MPSC queue. Since its buffer size is as large as the subscribe span length, it can panic if the span is too broad. For example, users can subscribe to log entries without stopping by passing MaxGLSN as an end position of range to the Subscribe API. However, it makes the channel size too large, so panic occurs.

varlog/pkg/varlog/subscribe.go

Lines 452 to 461 in 9e6713b

	func newSubscribedLogEntiresQueue(begin, end types.GLSN, closer SubscribeCloser, logger *zap.Logger) *subscribedLogEntriesQueue {
	q := &subscribedLogEntriesQueue{
	c: make(chan client.SubscribeResult, end-begin),
	wanted: begin,
	end: end,
	closer: closer,
	logger: logger.Named("subscribed_log_entries_queue"),
	}
	return q
	}

It is essential to ensure that the queue length is not too large, regardless of the subscription range. The queue size should be small enough to avoid wasting memory but not so small that it requires frequent calls to the Subscribe API. We may need to refactor the overall architecture to address this issue.