fix: distinguish transient plugin-not-installed from truly invalid pipeline ID (#936)

[livepeer-tessa]

Summary

Fixes #936

At cloud session init the frontend sends two concurrent requests — one to install a plugin and one to load its pipeline. If the load request arrives before the plugin finishes registering, _load_pipeline_implementation falls through to a generic ValueError("Invalid pipeline ID: ...")) and _load_pipeline_by_id_sync logs it at ERROR level with the unhelpful consider removing the models directory suggestion. The session recovers silently on the next retry (~1-2 min), but the misleading error creates confusion.

Changes

1. New exception class — PipelineNotYetRegisteredException

Added PipelineNotYetRegisteredException(ValueError) near the top of pipeline_manager.py. This makes the transient case distinguishable from a genuinely unknown pipeline ID.

2. _load_pipeline_implementation — raise typed exception

The final else: raise ValueError(f"Invalid pipeline ID: {pipeline_id}") is now raise PipelineNotYetRegisteredException(...). No other code paths change.

3. _load_pipeline_by_id_sync — catch before generic handler

Added a dedicated except PipelineNotYetRegisteredException clause before the existing except Exception block:

• Logs at WARN (not ERROR) with a friendly, actionable message.
• Sets pipeline status to NOT_LOADED (not ERROR) so the frontend never shows an error state and the load can be retried transparently once the plugin finishes installing.
• Does not publish an error Kafka event (it is not a real error).
• Signals the load event so any blocked waiters are unblocked.

4. Tests

New file tests/test_pipeline_race_condition.py (9 tests):

• Verifies PipelineNotYetRegisteredException is raised for unknown non-builtin IDs.
• Verifies _load_pipeline_by_id_sync returns False without setting status to ERROR.
• Verifies no ERROR-level log is emitted.
• Verifies a WARNING-level log is emitted mentioning plugin/installing.
• Verifies the load event is set (waiters unblocked).
• Verifies built-in pipelines are unaffected.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/936-plugin-race-pipeline-id

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App ID	daydream/scope-pr-938--preview
WebSocket	wss://fal.run/daydream/scope-pr-938--preview/ws
Commit	0d581c3

Livepeer Runner

App ID	daydream/scope-livepeer-pr-938--preview
WebSocket	wss://fal.run/daydream/scope-livepeer-pr-938--preview/ws
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Testing Livepeer Mode

SCOPE_CLOUD_MODE=livepeer SCOPE_CLOUD_APP_ID="daydream/scope-livepeer-pr-938--preview/ws" uv run daydream-scope