fix(engine): cascade on_error through Build-mode GC sweep#1999
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Resolves the spec's §4.2 observability gap: previously, orphan-delete failures during a normal update's commit phase (when a parent's `process()` no longer declared a child it had declared on a prior run) hit the framework's default `error!` log instead of routing through the parent's user-installed exception handler chain. Only `App.drop()` (a Delete-mode root with a raising on_error) propagated descendant failures; `coco.mount`-mounted parents had no observability. Symmetric with `ComponentDeleteContext::on_error`: - `ComponentBuildContext` gains an `on_error: Option<OnError>` field. - `ComponentProcessingAction::new_build` and `new_processor_context_for_build` accept it. - `Component::mount` stores its `on_error` argument on the child's Build context (in addition to passing it to `run_in_background`) — one handler, two surfaces. - `LiveComponentController::update_full` and `run_op`'s `Op::Update` branch mirror the same pattern for the live-component dispatch path. - `App::update` (root), `Component::use_mount` (foreground), and `mount_inner_live`'s synthetic parent context pass `None` — the prior log+swallow default is preserved where intentional. `launch_child_component_gc` switches from `delete_action_on_error()` to the new unified `processing_action_on_error()`, so it cascades the parent's on_error regardless of mode. `delete_action_on_error()` stays as the Delete-only accessor used by `Component::delete`'s spawned task (a Build context's on_error is meant for cascades, not for invoking on the build's own failure — that flows through `run_in_background`'s `on_error` argument directly). Adds `test_orphan_delete_failure_routes_through_parent_handler` in test_exception_handlers.py: parent mounts child A on update #1, doesn't mount A on update #2, sink fails during the orphan delete. Verifies the handler fires exactly once (mount_kind="mount"). Toggling `launch_child_component_gc` back to `delete_action_on_error()` confirms the test fails without this change. Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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Summary
coco.mount-mounted parent's commit-phase GC sweep now route through the parent's exception handler chain (previously:error!log + swallow).ComponentDeleteContext::on_errorcascade used byApp.drop():ComponentBuildContextgains the same field;launch_child_component_gcreads via a new unifiedprocessing_action_on_error()accessor.on_errorpassed toComponent::mountcatches both the child's own task failure (existing) and orphan-delete failures from the child's commit (new).LiveComponentController::update_full/Op::Updatemirror the same pattern.use_mountand rootApp::updatepassNone(preserves the prior log+swallow default by design).Test plan
test_orphan_delete_failure_routes_through_parent_handlerin test_exception_handlers.py confirms the cascade.launch_child_component_gcback todelete_action_on_error()confirms the test fails without the fix (seen == []+ framework's defaulterror!print).pytest python/— 674 passed, 138 skipped, 0 failed locally.cargo test,uv run mypyclean.🤖 Generated with Claude Code