[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

  set_payload:
    steps:
      - uses: eas/posthog_flag_rollout
        with:
          flag: eas-workflow-test-flag
          active: true
          payload:
            welcome_message: hello from a workflow

  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

(Validate posthog side, feature flag is updated with enw values)

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 59.63%. Comparing base (dcd93a6) to head (f37a747).

Additional details and impacted files

@@                           Coverage Diff                            @@
##           gwdp/eng-21551-posthog-capture-event    #3944      +/-   ##
========================================================================
+ Coverage                                 59.55%   59.63%   +0.09%     
========================================================================
  Files                                       940      941       +1     
  Lines                                     41414    41498      +84     
  Branches                                   8725     8746      +21     
========================================================================
+ Hits                                      24661    24745      +84     
  Misses                                    16659    16659              
  Partials                                     94       94              

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[gwdp]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• [build-tools] Add eas/posthog_wait_for_query workflow function #3949
• [build-tools] Add eas/posthog_annotation workflow function #3948
• [build-tools] Add eas/posthog_upload_sourcemaps workflow function #3947
• [build-tools] Add eas/posthog_wait_for_metric workflow function #3945
• [build-tools] Add eas/posthog_flag_rollout workflow function #3944 👈 (View in Graphite)
• [build-tools] Add eas/posthog_capture_event workflow function #3934
• main

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[github-actions]

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[sjchmiela]

ok now i think we might want a PostHogClient

[gwdp]

100%, that got way cleaner now, thank you!

[github-actions]

✅ Thank you for adding the changelog entry!