Howard Chu writes: >Hallvard Breien Furuseth wrote: >>I wrote: >>> One thing about ACID - IIRC there are some potential data races which my >>> single-word meta info in the header will fix. E.g. if the user does ^Z >>> at an unfortunate time, and with WRITEMAP updating the meta page while a >>> txn is reading it. >> >> Reproducible by sending frequent SIGSTOPs and SIGCONTs, with mdb.c >> patched to verify (memcmp or checksum) the meta info after copying. >> Happens even with 1 MDB process. >> >> (Re comments elsewhere: Selecting a meta page is atomic, but >> copying/writing it is not.) > > True, but writing the meta page doesn't need to be atomic - it won't be > considered current until its txnid is updated. Until then, its txnid will be > less than the other page's anyway.
I did test it. Maybe the signal caught a reader in the middle of copying a meta page, then several write transactions changed both metas. I see WRITEMAP can only be relevant if something reoders some accesses, which I see no particular reason to happen in this case. Or at least not compared to reordering meta page vs. me_txns->mti_txnid access. -- Hallvard
