feat(experimentation): default to control when rollout segment is created

[gagantrivedi]

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• I have read the Contributing Guide.
• I have added information to docs/ if required so people know about the feature.
• I have filled in the "Changes" section below.
• I have filled in the "How did you test this code" section below.

Changes

When a feature is first added to an experiment's rollout — the moment the rollout system segment is created — zero every variant's allocation on the environment-default feature state, so identities outside the rollout cohort receive control (the unallocated remainder) while the experiment runs.

• The rollout segment override is unchanged: it keeps the experiment's own split.
• Only runs when the rollout segment is first created; later rollout updates leave the default allocations alone, so manual edits to the default survive.
• Goes through the version-aware update_flag path, working under both v1 and v2 versioning.

How did you test this code?

Unit tests (make test): service tests covering first-rollout zeroing and later updates leaving the default alone; view tests asserting create-with-rollout zeroes the default and preserves the override split under both v1 and v2 versioning. mypy, ruff, and flagsmith-lint-tests clean.

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3 Skipped Deployments

Project	Deployment	Actions	Updated (UTC)
docs	Ignored	Preview	Jul 16, 2026 6:41am
flagsmith-frontend-preview	Ignored	Preview	Jul 16, 2026 6:41am
flagsmith-frontend-staging	Ignored	Preview	Jul 16, 2026 6:41am

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.64%. Comparing base (62a8143) to head (93069f0).
⚠️ Report is 16 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff            @@
##             main    #7905    +/-   ##
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  Coverage   98.63%   98.64%            
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  Files        1497     1501     +4     
  Lines       59261    59466   +205     
========================================
+ Hits        58455    58660   +205     
  Misses        806      806            

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[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: ASSERTIVE

Plan: Pro Plus

Run ID: 732561bf-7acb-41ae-866c-162e3757e0ca

📥 Commits

Reviewing files that changed from the base of the PR and between bf5b044 and 93069f0.

📒 Files selected for processing (1)

• api/experimentation/services.py

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📝 Walkthrough

Walkthrough

Experiment rollout application now zeroes environment-default multivariate allocations when creating the first rollout segment, while preserving the rollout segment’s submitted allocation split. Existing rollout updates leave manually edited default allocations unchanged. Feature-state value serialisation is centralised and typed, including boolean formatting. Unit and view tests cover standard and versioned environment paths, and warehouse event documentation references are updated.

Estimated code review effort: 3 (Moderate) | ~25 minutes

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[gagantrivedi]

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[Zaimwa9]

Looks good overall but we should fix the null => "None" before moving forward

[coderabbitai]

Caution

Some comments are outside the diff and can’t be posted inline due to platform limitations.

⚠️ Outside diff range comments (1)

api/experimentation/services.py (1)

683-687: 🗄️ Data Integrity & Integration | 🟠 Major | ⚡ Quick win

Re-check the experiment status after acquiring the lock.

The COMPLETED guard runs before refresh_from_db(). If another transaction completes the experiment between these operations, this transaction can still write a rollout for a completed experiment. Repeat the guard immediately after refreshing the locked row.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: ASSERTIVE

Plan: Pro Plus

Run ID: 6d63392d-3b95-4df3-bacb-9b95934e624b

📥 Commits

Reviewing files that changed from the base of the PR and between 166b3a1 and 6417f9d.

📒 Files selected for processing (3)

• api/experimentation/services.py
• api/tests/unit/experimentation/test_services.py
• docs/docs/deployment-self-hosting/observability/_events-catalogue.md

[Zaimwa9]

👍