From: "Eregon (Benoit Daloze) via ruby-core" Date: 2025-12-02T16:18:59+00:00 Subject: [ruby-core:123979] [Ruby Feature#21721] Allow `Queue` and `SizedQueue` to be used as LIFO queues Issue #21721 has been updated by Eregon (Benoit Daloze). byroot (Jean Boussier) wrote: > Define different classes for LIFO queues > > * But I find that less flexible, and it means exposing new constant names in the global namespace. Queue is actually defined under `Thread` so that's not that big of a concern. If we add this I'd want a separate class for sure to not make many compromises on the implementation of Queue. I'm not sure how big of a bottleneck this is though, can you share the profile? It'd be interesting to know how much can be gained. Maybe Mutex & ConditionVariable could also be optimized better. In general I feel this is pretty much the intended use case for ConditionVariable, so if it's not fast enough then maybe ConditionVariable needs to be optimized better. ---------------------------------------- Feature #21721: Allow `Queue` and `SizedQueue` to be used as LIFO queues https://bugs.ruby-lang.org/issues/21721#change-115409 * Author: byroot (Jean Boussier) * Status: Open ---------------------------------------- ### Context Since `Queue` and `SizedQueue` gained a proper timeout mechanism, I've been wanting to use them to implement simpler and more efficient connection pools. However for connection pools you ideally want a LIFO queue because it's preferable to checkout the most recently used connection. ### Problem Both `Queue` and `SizedQueue` only support FIFO because you can only enqueue elements at the beginning of the backing array, and only dequeue at the end. `Queue#push` (aliased as `Queue#<<` and `Queue#enq`) calls `Array#unshift` on the backing array and `Queue#pop` (aliased as `Queue#deq` and `Queue#shift`) calls `Array#pop`. Hence it is impossible to use these two classes for anything other than FIFO queues. I tried to use `git blame` to see if there was a justification for this, but I ended up in a git blame loop around May 2000 https://github.com/ruby/ruby/commit/9da4f78db46764be6dae5e7e83ff48cbecb3fb23 ### Feature I'd like to introduce either of two new methods (or both) to allow for LIFO: - A method to dequeue from the beginning of the backing array (`array_shift`). - A method to enqueue from the end of the backing array (`array_push`). A difficulty I have however is that since the common `shift/pop/push` terms are already used with uncommon meaning, it's hard to come up with good names. Ideas welcome. ### Possible alternatives - Define different classes for LIFO queues - But I find that less flexible, and it means exposing new constant names in the global namespace. - Add an initializer argument to define the queue ordering, e.g. `Queue.new([], lifo: true)` - Similarly, I find this less flexible than to decide the order during enqueue/dequeue - Add an argument to `Queue#pop` and `Queue#push`, e.g. `queue.push(element, front: true)`. -- https://bugs.ruby-lang.org/ ______________________________________________ ruby-core mailing list -- ruby-core@ml.ruby-lang.org To unsubscribe send an email to ruby-core-leave@ml.ruby-lang.org ruby-core info -- https://ml.ruby-lang.org/mailman3/lists/ruby-core.ml.ruby-lang.org/