[Story 3.3] Child completion callback and parent wakeup #372

Part of #358 (Epic 3: TaskScheduler + Subtask Fan-out).

Depends on: Story 3.2d (Webview-side task-scoping guard — must land before fan-out is first exercised in user-visible UI).

Relates to #126 (concurrent conversation sidebar — this story completes the fan-out loop).

Context

When a child completes, the parent needs to receive the result and resume. Currently this is embedded entirely in reopenParentFromDelegation (disk-based: reads messages from disk, injects tool_result, reopens parent instance). For fan-out, the parent may still be running and its state is live in memory — disk I/O is unnecessary and would race against the live instance. Two injection paths are needed: one per parent state.

This story assumes Stories 3.2b-3.2d have already established task isolation:

• 3.2b: parent and child execution contexts do not leak into each other during child setup
• 3.2c: non-focused tasks do not post into the single webview
• 3.2d: the webview rejects cross-task messages/state and uses per-task sequence guards

Developer Notes

In src/core/task/TaskScheduler.ts:

• Register an onComplete callback per child via RooCodeEventName.TaskCompleted or TaskDelegationCompleted.
• On child completion: (1) release semaphore permit; (2) check if parent is still active in TaskRegistry (fan-out path) or was suspended (sequential path); (3) call the appropriate injection path.

In src/core/webview/ClineProvider.ts:

• Sequential path (parent was suspended): Existing reopenParentFromDelegation flow — reads messages from disk, injects tool_result, reopens parent Task instance. No change to this path.
• Fan-out path (parent is still running): In-memory injection directly into the parent's live userMessageContent array via pushToolResultToUserContent(). Critical: the child completion callback must NOT call parent.setUserMessageContentReady(true). The parent's own dispatch loop is the sole owner of userMessageContentReady — it sets it to true only after all tool results for the current turn are collected. This follows the Single Writer Principle.
• Parent lookup must be by explicit parentTaskId, not by getCurrentTask(). During fan-out the focused task may be the child or another live task; during sequential delegation the parent may be persisted but not live.

Add /** @invariant Single Writer — only the parent's dispatch loop may set this to true. See Single Writer Principle (Thompson, 2011). */ JSDoc at the userMessageContentReady field in Task.ts.

Files: src/core/task/TaskScheduler.ts, src/core/webview/ClineProvider.ts

Tests (extend src/core/task/__tests__/TaskScheduler.spec.ts and delegation tests):

• Child completion releases semaphore permit (semaphore.available increases by 1).
• Sequential path: assert reopenParentFromDelegation is called with correct completionResultSummary.
• Fan-out path: assert pushToolResultToUserContent is called on live parent instance and userMessageContentReady is NOT set by the callback.
• Fan-out path: assert that if the parent has in-flight tools, userMessageContentReady remains false after child completion injection.
• Fan-out path with child focused: completion still injects into the parent by parentTaskId, not into whichever task is current.
• Semaphore permit restored on child abort and on child unhandled crash (the finally guard from Story 3.1 covers this).
• Completed child Task instances removed from activeTasks map after callback.
• Orphan handling: Parent aborts while child is in-flight → assert abortTask(true) called on the orphaned child, semaphore permit released, child removed from activeTasks.

Acceptance Criteria

• Parent receives completionResultSummary as a tool_result in its next API call regardless of which path is taken.
• No memory leak: completed task instances are removed from the scheduler's map.
• Correct injection path selected based on parent's live vs suspended state.
• Completion injection is scoped by explicit parent/child ids and does not depend on focused task.
• Orphan handling: TaskScheduler listens for RooCodeEventName.TaskAborted on the parent and calls abortTask(true) on orphaned children.
• Invariant: Only the parent's dispatch loop sets userMessageContentReady = true. No external callback may set this flag.

Out of Scope

• Story [Story 3.2b] Fan-out: parent stays active while child runs #369 owns parent/child execution-context isolation and delegation rollback safety.
• Story [Story 3.2c] Extension-side task-scoping guard #370 owns extension-host post suppression and live-task focus switching.
• Story [Story 3.2d] Webview-side task-scoping guard + per-task seq #371 owns webview-side rejection and per-task sequence handling.