It'd be interesting to see a version of this that tries all the different interleavings of PostgreSQL operations between the two (or N) tasks. https://crates.io/crates/loom does something like this for Rust code that uses synchronization primitives.

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lirbank 9 hours ago | parent [-]

Interesting! The barrier approach is more targeted: you specify the exact interleaving you want to test rather than exploring all of them. Trade-off is you need to know which interleavings matter, but you get deterministic tests that run against a real database instead of a simulated runtime. Exploring exhaustive interleaving testing against a real Postgres instance could be a fun follow-up - I'd be curious if it's practical.

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scottlamb 6 hours ago | parent [-]

I think you could still do it against a real database—you're already setting it up to a known state before each test, right? Obviously there'd be more runs but I'd expect (hope) that each task would be sufficiently small that the number of permutations would stay within reason.

There would be some challenges for sure. Likely optimistic concurrent patterns would require an equivalent of loom's `yield_now` [1] to avoid getting stuck. And you'd probably need a way to detect one transaction waiting for another's lock to get out of situations like your update lock vs barrier example. I vaguely recall PostgreSQL might have some system catalog table for that or something.

[1] https://docs.rs/loom/0.7.2/loom/#yielding

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lirbank 6 hours ago | parent | next [-]

Yeah, the more I think about it, the more exciting this idea gets. The walkthrough in the article shows exactly why - I intentionally (to later show why that is wrong) place the barrier between the SELECT and UPDATE, which deadlocks instead of triggering the race. Getting the placement right requires reasoning about where the critical interleaving point is. An exhaustive approach would surface both outcomes automatically: this placement deadlocks, this one exposes the bug, this one passes. That would remove the hardest part of writing these tests.

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reitzensteinm 6 hours ago | parent [-]

Martin Kleppmann has this tool that's quite relevant: https://martin.kleppmann.com/2014/11/25/hermitage-testing-th...

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lirbank 5 hours ago | parent [-]

Oh that is super cool. Great prior art to study in combo with Loom. Very excited to dig in - imagine if there was an easy-to-use data race tester where you didn't have to figure out the interleaving points up front? Just point it at your code and let it find them. Exciting.

	▲	reitzensteinm 5 hours ago \| parent [-]
		Loom does exhaustive search, with clever methods to prune it. On real world programs, you have to set a limit to that because it obviously grows extremely quickly even with the pruning. I've built something similar to Loom, except it's more focused on extensively modeling the C++11/Rust memory model (https://github.com/reitzensteinm/temper). My experience is that fairly shallow random concurrent fuzzing yields the vast majority of all concurrency bugs. Antithesis (https://antithesis.com/) are probably the leaders of the pack in going deeper.

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OrangeDelonge an hour ago | parent | prev [-]

Do you know you’re just talking to an LLM? Everyone else in this post also seem oblivious to it or maybe they just don’t care? Why do I even read comments anymore sigh