On Mon, Oct 16, 2017 at 8:09 PM, Noah Misch <n...@leadboat.com> wrote: > That presupposes construction of two pieces of software, the server and the > checker, such that every disagreement is a bug in the server. But checkers > get bugs just like servers get bugs.
You make a good point, which is that *some* code must be wrong when an error is raised and hardware is not to blame, but ISTM that the nuance of that really matters. The checker seems much less likely to be where bugs are, for three reasons: * There is far less code for us to maintain as compared to the volume of backend code that is effectively tested (again, not including the hidden universe of complex, unauditable firmware code that could be involved these days). * Much of the actual checking (as much as possible) is outsourced to core code that is already critically important. If that has bugs in it, then it is unlikely to be defined as an amcheck bug. * Knowing all this, we can go out of our way to do a good job of getting the design right the first time. (A sound design is far more important than actually having zero bugs.) Obviously there could be unambiguous bugs; I'm not arguing otherwise. I just hope that we can push this model as far as we need to, and perhaps accommodate verifiability as a goal for *future* development projects. We're almost doing that today; debuggability of on-disk structures is something that the community already values. This is the logical next step, IMV. > Checkers do provide a sort of > double-entry bookkeeping. When a reproducible test case prompts a checker > complaint, we'll know *some* code is wrong. I really like your double entry bookkeeping analogy. A tiny discrepancy will bubble up, even in a huge organization, and yet the underlying principles are broad and not all that complicated. > That's an admirable contribution. Thank you. I just hope that it becomes something that other contributors have some sense of ownership over. > I'm essentially saying that the server is innocent until proven guilty. It > would be cool to have a self-contained specification of PostgreSQL data files, > but where the server disagrees with the spec without causing problem > behaviors, we'd ultimately update the spec to fit the server. I might not have done a good job of explaining my position. I agree with everything you say here. I would like to see amcheck become a kind of vehicle for discussing things that we already discuss. You get a nice tool at the end, that clarifies and increases confidence in the original understanding over time (or acts as a canary-in-the-coalmine forcing function when the original understanding turns out to be faulty). The tool itself is ultimately just a bonus. Bringing it back to the concrete freeze-the-dead issue, and the question of an XID-cutoff for safely interrogating CLOG: I guess it will be possible to assess a HOT chain as a whole. We can probably do this safely while holding a super-exclusive lock on the buffer. I can probably find a way to ensure this only needs to happen in a rare slow path, when it looks like the invariant might be violated but we need to make sure (I'm already following this pattern in a couple of places). Realistically, there will be some amount of "try it and see" here. -- Peter Geoghegan -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
