Breaking the WAL
antithesis.com
antithesis.com
I'm only sad that I didn't put SQLite under test earlier in the year, or I would have found this issue right away. If you look at the workload[1], you can see how simple it is. Exactly the same kind of workload we write every day to help our customers find bugs.
If you have any questions about our process or how debugging with Antithesis works, please let me know! Thanks for reading!
[1]: https://github.com/antithesishq/sqlite/blob/3.51.2-instrumen...
The workload I put in place does exactly that[1]. It runs a write workload from multiple processes concurrently on the same SQLite database to cause writes to build up in the WAL, and runs checkpoints concurrently. This exercises the portion of the WAL code that, from a trivial read-through, is most likely to contain bugs (and turns out, did!).
Said differently, this is exactly the approach we take with all stateful transactional systems. I am only sad that I didn't do this experiment months earlier, as it would have saved Tailscale and the SQLite team a lot of time.
[1]: https://github.com/antithesishq/sqlite/blob/3.51.2-instrumen...
Suuure. So why exactly haven't you found this bug already a long time ago? I mean it is trivially obvious that this is where bugs in SQLite would be, right?
SQLite is open source and is one of the most popular databases, surely testing it to find bugs would also be obvious?
I'm not gonna lie, though, you're hitting a sore point. I'm absolutely kicking myself for not putting SQLite under test when I joined. The first few projects I put under test after joining were all projects that had a few properties: 1. The project had a path to a commercial deal 2. The project likely had interesting bugs to find
TBH, I didn't expect that one of the most widely used and well-tested pieces of software in existence would have a bug our fuzzer could find in 15 mins. (To be clear, this is not a hit on SQLite, just that our fuzzer is stupid good at finding race conditions. SQLite remains one of my favorite pieces of software ever.)
With this in mind, are there other similar projects that I'm not thinking of that I should put under test? Before you ask, I'm already scaling up my SQLite harness to cover more of its state space and run against master. Maybe I'll find something novel in SQLite after all! Never too late to check.
If you want a challenging open source deterministic game to test, may I suggest Beyond All Reason, powered by the Spring / Recoil engine? It's an RTS game that's been in various incarnations of development for 20+ years. (Disclosure: I'm an occasional code contributor to the project).
Go find bugs that haven't been yet found, and then brag about them :joy:
i don't feel like i gained a strong appreciation for what the actual Antithesis capabilities are or why you had confidence this was anything the Antithesis tools would have helped find? i want to know more about what kind of analysis or capabilities Antithesis has and how it could have helped find the issue. what's clear is that Antithesis could confirm an already found issue. and even there I don't feel like I gained any connection or knowledge or understanding of what the tooling was doing or how it would help. i'd really like more technical meat to this post to understand the value proposition.
Unless you guys routinely add assertions to libraries I don't see how you would have caught this?
I mean for sure databases should guard against lost writes but how would you have known to add that particular assert?
this one falls very flat on that front imo. the "antithesis finds rare bugs" is the overwhelming majority of the actual content here, where as the usual meat of the blog, the hard work, the deep thinking, really distills down to "typed this into claude mobile, typed that into claude mobile". yes, its a statement to the power of the technologies of today that's all it takes to find this bug, but man it's a sad signal for those of us looking for compelling technical reads instead of b2b sales pitches.
this is a pretty wide trend for blog content nowadays so im not suprised overall, i just commented because previous antithesis blogs were better than this to that criteria. just take a look at the other older write ups on their site compared to this one and you’ll see a very clear difference in depth and effort of content.
But reading over tailscale's blogpost, I'm not sure how I would connect the dots to "this issue I'm seeing in production" to "load this up in antithesis". It seemed half the battle was figuring out if the bug was in Sqlite, Tailscale, or Linux, and in that situation if you gave Antithesis all the context, would it reliably find the bug?
I couldn't find it, so its unclear if the prompt was completely "make a test suite" or was lead towards finding it in the first place, which wouldn't be a fair test.
The closest I mention of the prompt I could find was:
> Then I asked it to write a simple workload which exercised the WAL insert and checkpoint code. Notably, this is a completely generic workload.
With a skeptical lens, unclear.
My goal, which I believe I accomplished, was to determine if Antithesis would have found this bug if we had put the SQLite WAL under test.
1. The specific bug isnt mentioned 2. (If youre game) a model with a knowledge-cutoff date before the report is used
Which is: "Given the knowledge of a kind of bug that can happen in one Raft implementation, can a similar workload find similar bugs in other Raft implementations?" Turns out it can.
But please keep writing, I know its super hard to put yourself out there and make content!
Looking forward to seeing the other bugs they found.
I'd be interested in seeing what the SQLite people have to say about them if/when they are submitted upstream
Which says they made two commits to make instrumentation work. Also, no fixes (which seems reasonable to me; need to figure out what's wrong first)
Presumably any bugs found are being sent upstream to fix.
My understanding is that this workload is very uncommon: the Tailscale blog says they used a custom unusual configuration to have many checkpoints like this. So without knowing the bug, it seems unlikely one would build this workload and then find the bug. If that makes sense.
Edit: just want to say that you being able to repro it is awesome, but that the overall claim seems a little overstated to me.
https://news.ycombinator.com/item?id=49278424 https://news.ycombinator.com/item?id=49278521
Also appreciate the nice words at the end :) I'm feeling a bit ganged up on.
I guess what would have been an even more cool thing is "we ran some more general testing with Antithesis, and it found five other bugs". Have you thought along those lines or explored something like that? There have to be other, similar bugs lurking in SQLite :)
Like some others have mentioned, one of my earliest thoughts was “how much of a hint was the LLM given about the bug?” I think if the prompt used was stated clearly/verbatim near the beginning of the article, that would probably dispel a good amount of the criticism.
It states you replicated the bug once the SQLite team fixed it, and published it.
Not sure what’s difficult about replicating behaviour when it’s spelled out for you.
As-is, this is just 20/20 hindsight with concerns about leading the AI on through the prompt hand-waved away. Come on.
Oh btw, we also found some other ones… Stay tuned!