prometeu-runtime/discussion/workflow/plans/PLN-0089-introduce-render-handoff-abstraction.md

id	ticket	title	status	created	ref_decisions	tags
PLN-0089	vm-render-parallel-execution-boundary	Introduce Render Handoff Abstraction	done	2026-06-05	DEC-0031	runtime renderer handoff latest-wins

Briefing

DEC-0031 requires a single-slot latest-wins handoff where the producer never blocks on render ACK and replacement before consumption is observable.

Objective

Introduce a render handoff abstraction that can be used synchronously today and by a worker later.

Dependencies

• Source decision: DEC-0031.
• PLN-0093 should define telemetry counters before final instrumentation is wired.

Scope

• Add a small handoff type or trait around one pending owned RenderSubmission.
• Support publish, take latest, replace pending, and drain/discard operations.
• Report replacement/drop events to telemetry hooks.
• Integrate the abstraction into RenderManager without starting a worker.

Non-Goals

• Do not spawn a thread.
• Do not implement frame pacing.
• Do not change Shell UI default rendering policy.

Execution Method

1. Locate RenderManager::latest_complete_submission ownership and publication flow.
2. Introduce a handoff module with single pending slot semantics.
3. Move RenderSubmission into the slot on publication.
4. Make take_latest transfer ownership to the consumer.
5. Count replacement when a pending submission is overwritten before consumption.
6. Keep current synchronous publication working by immediately taking and rendering through the same abstraction.

Acceptance Criteria

• RenderManager no longer relies on an unconstrained raw Option<RenderSubmission> for the future worker boundary.
• Publishing a newer pending submission replaces the old one without blocking.
• The consumer receives owned submissions.
• Replacement before consume is observable.

Tests

• Unit tests for empty take, publish/take, publish/replace/take, and drain/discard.
• Integration smoke test that existing synchronous render still presents frames.

Affected Artifacts

• crates/console/prometeu-system/src/services/vm_runtime/render_manager.rs
• New or existing render handoff module under crates/console/prometeu-system.
• Render manager tests.