Save client version to the settings file on shutdown #21171

Following an IRC conversation today, this patch proposes to persist CLIENT_VERSION as "lastrunversion" to the settings.json file on shutdown.

This allows, for example, to detect if the user upgraded with code like

if (CLIENT_VERSION > chain.getRwSetting("lastrunversion").get_int())

so as to take various actions.

Reviewers, this patch can be tested manually by opening <datadir>/signet/settings.json in your editor, starting and stopping signet, and watching the file be updated. One can also manually edit the setting and verify that the version is updated on shutdown.

{
    "lastrunversion": 219900,
    "wallet": [
        ""
    ]
}

Concept ACK (assuming no downsides)

I think this could be useful for providing better logging around upgrades. Currently logging does not recognize when an upgrade is happening. Something like "First time running new version" "Successful upgrade to version x" etc

Changed fieldname from "version" to "lastrunversion" per IRC suggestion by @luke-jr (thanks!)

Concept ACK

I didn't read the IRC link, but this does not seem like a good idea. If you can't detect a new feature directly and need a flag to indicate when it's been enabled, you should probably set that flag at the time the feature is actually enabled (not shutdown) and choose a flag name and value clearly tied to the feature. This way behavior can be straightforward and reliable, not confusing and based on a flaky heuristic.

(If in addition, the feature is backwards incompatible and you need to prevent bitcoin versions that don't support the feature from loading after it's enabled, implementing the -require=<feature> flag suggested #15935 (comment) could be a good approach.)

In any case, would encourage opening an issue that clearly describes the problem so a good solution can be found.

Thanks @ryanofsky. I learned more about the settings.json file doing this. Opened a patch for the specific case in #21181 instead.

21171