Cache the lookup pepper in the HashingMetadataStore.

[reivilibre]

The primary motivation for this is to make the casefolding migration script more performant
and to reduce the size of a strace log due to the repeated SQLite transactions involved with
fetching the lookup pepper for every single entry.

[squahtx]

Presumably running two sydent instances off the same database is not supported?

[reivilibre]

Presumably running two sydent instances off the same database is not supported?

As far as I know, no. It's worth noting that the lookup pepper is only generated/stored if it's absent on startup — concurrent Sydents using the same DB could already race and wind up rehashing over each other before, so I think it's fine to accept that we don't support that.

[DMRobertson]

Thanks. I admit that it's probably over paranoid to update the cache if the pepper gets re-stored... but I'll sleep easier at night.

[squahtx]

concurrent Sydents using the same DB could already race and wind up rehashing over each other before, so I think it's fine to accept that we don't support that.

True. The race is now slightly worse than before, since one of those instances will cache an outdated pepper and use that for new entries. Note that the rehash takes place in the same transaction as the pepper update, so that bit is "safe".

[reivilibre]

True. The race is now slightly worse than before, since one of those instances will cache an outdated pepper and use that for new entries. Note that the rehash takes place in the same transaction as the pepper update, so that bit is "safe".

Oh that's true actually. 'Each function will queue some rehashing db transactions' is a bit misleading; those functions actually DO the transactions.

However I don't really see this being any worse than running two Synapses on the same database and expecting things to work just fine; you just wouldn't do it in the first place.

Do you think the PR's fine as is?

[squahtx]

Do you think the PR's fine as is?

I'm not objecting to the PR! Just wanted to make sure we're aware of the limitations.