[csp] Bug with race condition of select() and read() + suggested fix

[azerum]

I assume that select() should unblock with undefined only when the selected channel is closed

However, it may unblock undefined even when the channel is not closed. This happens when read() runs after write(), before continuation of select():

import { channel, select } from '@thi.ng/csp'

void main()

async function main() {
    const ch = channel<number>(1)
    const s = select(ch)

    queueMicrotask(async () => {
        const x = await ch.read()
        console.log('Did read', x)
    })

    ch.write(42)

    const [v, _] = await s
    console.log('Did select', v)
}

Expected output:

Did read 42
<select remains blocked>

Actual output:

Did read 42
Did select undefined

This requires write() to happen within the same call that schedules callback doing read(), so might never happen in real world code. Still, it might a tricky bug

One way to handle this is to do poll() after race(), and re-run the race if the selected channel is empty and not closed

Shameless plug: I've implemented similar library before finding the great @thi.ng, and decided to publish it anyway. Here's select implementation that handles the case

[postspectacular]

Thank you for reporting this @azerum! I did have a long deep look at this all. I tried your proposed approach and several others to address this, but no success so far and I will have to defer this for a while and revisit in the near future. So far every attempt has resulted in fixing this particular issue, but at the same time breaking dozens of other use cases, functions & operators... This issue only seems to be reproducilble via queueMicrotask() and process.nextTick(), but I'm not even sure why one would combine these methods with normal async/await code...

The one (somewhat unrelated) thing I did update though: Re-add the shuffling of inputs given to select() — I cant believe I forgot to do this in the major rewrite of this package 1.5 years ago (they were shuffled previously)...

[azerum]

Interesting, thanks for explaining! I wonder what bugs arise when using race-then-poll-in-a-loop method? I'd love to have a think about it, especially since I use channels in some projects

Could those issues be related to #547?

await resolvedPromise; await ch.read() also works like queueMicrotask(() => ch.read()), e.g.

import { channel, select } from '@thi.ng/csp'

void main()

async function main() {
    const ch = channel<number>(1)
    const s = select(ch)

    void (async () => {
        // Or other promise that happen to be resolved
        await Promise.resolve() 

        const x = await ch.read()
        console.log('Did read', x)
    })()

    await ch.write(42)

    const [x, _] = await s
    console.log('Did select', x)
}

But again, not what the most (all?) code does