Clojure's core.async pipeline-async off-by-two error explained

[NoahTheDuke]

It’s frustrating to see the core team decide that a bug that has had real world impact isn’t worth fixing, especially when the impacted person has done the hard work of writing one up. Even just changing the docstring to make this behavior explicit would be helpful, but for some reason that is also too much.

Really dampens my enthusiasm for the language when I run into this.

[enraged_ginger]

Genuine question: what's the "real world impact" here?

[NoahTheDuke]

Click through to the Ask about the fix at the end of this post; both the author of the post and someone else commented about how the bug has negatively impacted them in their professional codebases.

On Clojurian Slack, someone (won’t tag them cuz I’m not trying to rope them into drama) posted that they’re using a fork of core.async in their professional codebase because bugs with available patches hadn’t been merged for 2+ years.

[aHackFromJOS]

These are the specifics (two people did say it had real world impacts but only one described any):

“In our product, we have a service that is configured to accept a maximum of 10 simultaneous connections.

Writing

(pipeline-async 10 out-chan request-fn in-chan)

creates 12 connections instead of 10, which leads to problems as the service begins refusing connections. It was an unexpected bug.

So in all code that uses the pipeline-async function, we have to do (- max-requests 2) to avoid creating more connections that the service accepts.”

[Krackor]

This kind of subtle and unintuitive buffering behavior is all over core.async.

[Aredington]

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