Sync event databases on persist_ping_lifetime_data

[badboy]

Instead of #3220

I'd like to defer this until after the release.

[chutten]

Needs CHANGELOG.md

API design: Yeah, okay, it kinda tracks. Events are ping-lifetime and this does persist them.

Tests: I guess we need to record events and then forceably crash the app to prove that persist_ping_lifetime_data is necessary to persist events to disk now?

Instrumentation: Do we want to monitor how often sync_all errors out? Seems likely. Though if it's recorded in a ping-lifetime metric we might not ever receive it.

Logging: Is it an error? There's nothing anyone can do about it. Seems like at most a warn. Do we want to spend some words on why this might matter (e.g. "Recently-recorded events for ping {file} have not been persisted to disk and may be lost")?

[badboy]

Logging: Is it an error? There's nothing anyone can do about it. Seems like at most a warn. Do we want to spend some words on why this might matter (e.g. "Recently-recorded events for ping {file} have not been persisted to disk and may be lost")?

The problem with that wording is: it's not accurate. if fsync fails we don't really know whether data is safely persisted or not. Or which data. Anyway, fun!

[badboy]

Tests: I guess we need to record events and then forceably crash the app to prove that persist_ping_lifetime_data is necessary to persist events to disk now?

It would be much more complicated than that. Even on crashes most likely data is written to disk just fine if the app worked normally before the crash.
Testing that code path would require injecting IO faults.

[chutten]

(about testing) Fair 'nuff.

(Still needs CHANGELOG)