On Sat, Feb 19, 2022 at 5:54 PM Andres Freund <and...@anarazel.de> wrote: > How does that cause the endless loop?
Attached is the page image itself, dumped via gdb (and gzip'd). This was on recent HEAD (commit 8f388f6f, actually), plus 0001-Add-adversarial-ConditionalLockBuff[...]. No other changes. No defragmenting in pg_surgery, nothing like that. > It doesn't do so on HEAD + 0001-Add-adversarial-ConditionalLockBuff[...] for > me. So something needs have changed with your patch? It doesn't always happen -- only about half the time on my machine. Maybe it's timing sensitive? We hit the "goto retry" on offnum 2, which is the first tuple with storage (you can see "the ghost" of the tuple from the LP_DEAD item at offnum 1, since the page isn't defragmented in pg_surgery). I think that this happens because the heap-only tuple at offnum 2 is fully DEAD to lazy_scan_prune, but hasn't been recognized as such by heap_page_prune. There is no way that they'll ever "agree" on the tuple being DEAD right now, because pruning still doesn't assume that an orphaned heap-only tuple is fully DEAD. We can either do that, or we can throw an error concerning corruption when heap_page_prune notices orphaned tuples. Neither seems particularly appealing. But it definitely makes no sense to allow lazy_scan_prune to spin in a futile attempt to reach agreement with heap_page_prune about a DEAD tuple really being DEAD. > Given that heap_surgery's raison d'etre is correcting corruption etc, I think > it makes sense for it to do as minimal work as possible. Iterating through a > HOT chain would be a problem if you e.g. tried to repair a page with HOT > corruption. I guess that's also true. There is at least a legitimate argument to be made for not leaving behind any orphaned heap-only tuples. The interface is a TID, and so the user may already believe that they're killing the heap-only, not just the root item (since ctid suggests that the TID of a heap-only tuple is the TID of the root item, which is kind of misleading). Anyway, we can decide on what to do in heap_surgery later, once the main issue is under control. My point was mostly just that orphaned heap-only tuples are definitely not okay, in general. They are the least worst option when corruption has already happened, maybe -- but maybe not. -- Peter Geoghegan
corrupt-hot-chain.page.gz
Description: GNU Zip compressed data
