[build-tools] Add eas/posthog_flag_rollout workflow function

[gwdp]

Why

Manage a PostHog feature flag straight from an EAS workflow. Flip it on or ramp its rollout as part of a deploy, instead of doing it by hand in PostHog.

How

Adds eas/posthog_flag_rollout. It uses the personal API key from integrations:posthog:connect (POSTHOG_CLI_API_KEY/POSTHOG_CLI_PROJECT_ID), or the api_key/project_id inputs. There's no host input on purpose, so the personal key only ever goes to POSTHOG_CLI_HOST.

The rollout_percentage input updates the flag's catch-all release condition and leaves targeted groups alone. Failures fail the step. Set ignore_error: true to log a warning and continue instead. Bad inputs, unknown flags and 403 scope errors always fail. Lookup and update aren't atomic, so a parallel dashboard edit can be lost.

Test Plan

CI passes. Enabled a real flag from a local run with this workflow. It needs a personal key with feature-flag scopes. The last job fails the lookup on purpose and just warns.

Test workflow

name: PostHog flag rollout test

jobs:
  enable_flag:
    steps:
      - uses: eas/posthog_flag_rollout
        with:
          flag: eas-workflow-test-flag
          active: true

  gradual_rollout:
    steps:
      - uses: eas/posthog_flag_rollout
        with:
          flag: eas-workflow-test-flag
          rollout_percentage: 50

  ignored_failure:
    steps:
      - uses: eas/posthog_flag_rollout
        with:
          flag: eas-workflow-test-flag
          active: true
          api_key: phx_invalid_local_test
          ignore_error: true

Examples

These are simple workflows just to exercise the step. More fun and useful ones, mixed with EAS Update and deploys, are coming in the docs.

Ship a feature behind a flag

Publish the update, then enable the flag at 10% once the code is out.

name: Ship a feature behind a PostHog flag

jobs:
  update:
    type: update
    params:
      branch: main
      message: Ship new-checkout behind a flag

  enable_flag:
    needs: [update]
    steps:
      - uses: eas/posthog_flag_rollout
        with:
          flag: new-checkout
          active: true
          rollout_percentage: 10

Kill switch

Disable the flag, record an audit event, and leave a note on the run.

name: Kill switch, disable new-checkout now

jobs:
  disable_flag:
    steps:
      - uses: eas/posthog_flag_rollout
        with:
          flag: new-checkout
          active: false

  audit_trail:
    needs: [disable_flag]
    steps:
      - uses: eas/posthog_capture_event
        with:
          event: kill_switch_pulled
          properties:
            flag: new-checkout
            source: eas-workflow

[linear-code]

ENG-21551

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 59.33%. Comparing base (0d8006c) to head (3745235).

Additional details and impacted files

@@                           Coverage Diff                            @@
##           gwdp/eng-21551-posthog-capture-event    #3944      +/-   ##
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+ Coverage                                 59.33%   59.33%   +0.01%     
========================================================================
  Files                                       937      938       +1     
  Lines                                     41144    41223      +79     
  Branches                                   8671     8685      +14     
========================================================================
+ Hits                                      24407    24454      +47     
- Misses                                    16643    16674      +31     
- Partials                                     94       95       +1     

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[github-actions]

✅ Thank you for adding the changelog entry!