Fix mismatch between session and event user_ids after salts are rotated (#5065)

* Refactor test suite to allow seeking multiple sessions

* Fix mismatch between session and event user_ids after salts are rotated

Consider the following scenario
1. We receive a pageview from a user A
2. Salts are rotated after day end
3. We receive another pageview

Previously, this would result in:
- 2 different sessions with mismatching user_ids
- 2 events where each would have different user_ids

This fixes the problem by removing the previous faulty session updating
and forcing the event.user_id to be updated after we found a session
with the previous salt.

* Remove an assertion

CHANGELOG.md

@@ -29,6 +29,7 @@ All notable changes to this project will be documented in this file.
 - Fix Direct / None details modal not opening
 - Fix year over year comparisons being offset by a day for leap years
 - Breakdown modals now display correct comparison values instead of 0 after pagination
+- Fix database mismatch between event and session user_ids after rotating salts
 
 ## v2.1.5-rc.1 - 2025-01-17
 

lib/plausible/clickhouse_event_v2.ex

@@ -73,6 +73,7 @@ defmodule Plausible.ClickhouseEventV2 do
 
   @session_properties [
     :session_id,
+    :user_id,
     :referrer,
     :referrer_source,
     :click_id_param,

lib/plausible/session/cache_store.ex

@@ -78,8 +78,7 @@ defmodule Plausible.Session.CacheStore do
   defp update_session(session, event) do
     %{
       session
-      | user_id: event.user_id,
-        timestamp: event.timestamp,
+      | timestamp: event.timestamp,
         entry_page:
           if(session.entry_page == "" and event.name == "pageview",
             do: event.pathname,