fix: Only set initial campaign params on person creation

[robbie-c]

Problem

See #7710

Campaign params can be set on a later visit, so a user that was originally organic traffic (no campaign params) could be attributed to a different channel if they click a link with a UTM tag set

Changes

On Person update, don't update initial campaign properties. These can only be set when the person is created, and future events that include campaign properties don't update the initial campaign properties.

TRICKY: To make upgrading anonymous users work, the pipeline now assumes that if you send any $initial props, you will send all relevant $initial campaign params, as this is what posthog-js does. To sanity-check this, I added a counter in this PR to see if this would negatively affect anyone, see this grafana chart. It only says web, which is expected, and the fact that no other SDKs are present means that this would not negatively impact anyone.

This is an improvement on the workaround in #7710 (comment)

• it does not require an SDK update
• will always be up to date with the latest campaign params
• does not increase the bundle size
• it will work for users created before this fix
• unlike the other potential fix in fix: Set campaign params to null on person creation #25132, it doesn't need us to store a bunch of unnecessary null properties in PG

The BIG downside (maybe deal-breaker) is that it touches some person processing code that we might not want to touch this quarter.

Does this work well for both Cloud and self-hosted?

Yes

How did you test this code?

I added some tests for the new cases covered, including the edge cases around upgrading anonymous users to identified.

I created some new snapshot tests for real events from posthog.com

[posthog-bot]

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[posthog-bot]

This PR was closed due to lack of activity. Feel free to reopen if it's still relevant.