Re: MDB v2: Replace meta pages with "meta position" word

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.