Serialize new session cookie synchronously to avoid overlapping sessions (close #1381)

[matus-tomlein]

Issue #1381

The issue reported that after upgrading to v4, there is a higher number of very short sessions in very short sequences. The hypothesis was that this is due to the async cookie access introduced in v4.

I was able to reproduce the problem with a simple setup that I added to the integration tests – a single page that embeds 15 iframe pages. The iframes are loaded at the same time and each initializes a new tracker and tracks a page view. Since the iframes share the same cookies, we should only see a single session id across all tracked page views.

With this setup, I was getting multiple different session ids across the page views. It was usually around 10 different session IDs.

The change I made was to flush the cookies (write them synchronously) each time the session changes. That means the first session or every subsequent new session. When a new event is tracked without triggering a new session, the cookies are still updated async by default.

This fixed the problem in the iframe test setup. Now I am only getting a single session ID across all page views. Have kept this test in the integration tests so that we don't regress on the behaviour.

[bundlemon]

BundleMon

Files added (6)

Status	Path	Size	Limits
✅	trackers/javascript-tracker/dist/sp.js	+24.46KB	30KB / +10%
✅	libraries/browser-tracker-core/dist/index.mod ule.js	+23.48KB	25KB / +10%
✅	libraries/tracker-core/dist/index.module.js	+19.42KB	20KB / +10%
✅	trackers/browser-tracker/dist/index.umd.min.j s	+17.37KB	20KB / +10%
✅	trackers/javascript-tracker/dist/sp.lite.js	+17.31KB	20KB / +10%
✅	trackers/browser-tracker/dist/index.module.js	+3.49KB	5KB / +10%

Total files change +105.53KB 0%

Final result: ✅

View report in BundleMon website ➡️

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Current branch size history

[jethron]

LGTM!