turbo-tasks-backend: batch find_and_schedule_dirty using for_each_task_meta - #91497

What?

Batch-process find_and_schedule_dirty in aggregation_update.rs by collecting all queued jobs (up to FIND_AND_SCHEDULE_BATCH_SIZE = 10 000) into a SmallVec and pre-fetching their task metadata with a batched ctx.for_each_task_meta(...) call.

Why?

find_and_schedule can accumulate thousands of tasks during invalidation cascades. The previous implementation issued one ctx.task(...) call per task inside process(), serializing backing-storage fetches one at a time.

ctx.for_each_task_meta triggers a batched fetch of all task metadata from the backing store: keys are sorted by hash for cache-friendly sequential access to the storage layer. The callback is invoked per-task once data is ready, with the task guard handed directly to the callback — no second lock acquisition needed.

The batch limit is set to 10 000, because find-and-schedule jobs are cheap (metadata read + optional schedule) compared to aggregation-update jobs, so yielding less often is safe and beneficial.

How?

fn process — find_and_schedule branch:

• Replace the one-at-a-time loop with a single drain(..FIND_AND_SCHEDULE_BATCH_SIZE) that collects up to 10 000 FindAndScheduleJob structs into a SmallVec, then calls find_and_schedule_dirty once with the whole batch.

fn find_and_schedule_dirty:

• Change parameter from task_id: TaskId to jobs: SmallVec<[FindAndScheduleJob; 4]>.
• Call ctx.for_each_task_meta(...) to batch-prefetch all task metadata (sorted by hash for cache-friendly access) and process each task in the callback.

prepare_tasks_with_callback fix:

• Release TaskLockCounter before calling prepared_task_callback instead of after. This ensures the counter is 0 when the callback runs, so callbacks that drop their task guard and then call ctx.task() (like find_and_schedule_dirty_internal → ctx.schedule()) no longer trigger the "Concurrent task lock acquisition detected" panic in debug builds.
• for_each_task now uses acquire() instead of reacquire() since counter is guaranteed 0 at callback entry. reacquire() is removed.

Cleanup:

• Use FxHashMap (already imported) instead of std::collections::HashMap.
• Combine consecutive #[cfg(trace_find_and_schedule)] let bindings for clarity.
• Add FIND_AND_SCHEDULE_BATCH_SIZE = 10_000 as a self-contained constant (not derived from MAX_COUNT_BEFORE_YIELD).