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

[Hallvard Breien Furuseth <h.b.furuseth@usit.uio.no>]

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