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Create new version of RuntimeScheduler (facebook#40944)
Summary: ## Summary This creates a new version of `RuntimeScheduler` that's intended to be backwards compatible but with a few notable changes: 1. `scheduleTask` is now thread-safe. 2. `scheduleWork` is now just an alias of `scheduleTask` with immediate priority (to preserve the yielding semantics it had over other tasks). 3. Yielding mechanism has changed, to make lower priority tasks to yield to higher priority tasks, instead of just yielding to `scheduleWork` and `executeNowOnTheSameThread`. We don't expect this to have any impact in performance or user perceivable behavior, so we consider it a short-lived refactor. When we validate this assumptions in a complex application we'll delete the old version and only keep the fork. ## Motivation The main motivation for this refactor is to reduce the amount of unnecessary interruptions of running tasks (via `shouldYield`) that are only used to schedule asynchronous tasks from native. The `scheduleWork` method is the only available mechanism exposed to native APIs to schedule work in the JS thread (as the existing version of `scheduleTask` is only meant to be called from JS). This mechanism **always** asks for any running tasks in the scheduler to yield, so these tasks are always considered to have the highest priority. This makes sense for discrete user events, but not for many other use cases coming from native (e.g.: notifying network responses could be UserBlocking, Normal or Low depending on the use case). We need a way to schedule tasks from native with other kinds of priorities, so we don't always have to interrupt what's currently executing if it has a higher priority than what we're scheduling. ## Changes **General APIs:** This centralizes scheduling in only 2 APIs in `RuntimeScheduler` (which already exist in the legacy version): * `scheduleTask`, which is non-blocking for the caller and can be used from any thread. This always uses the task queue in the scheduler and a new yielding mechanism. * `executeNowOnTheSameThread`, which is blocking for the caller and asks any task executing in the scheduler to yield. These tasks don't go through the task queue and instead queue through the existing synchronization mechanism in `RuntimeExecutor`. The yielding mechanism for these tasks is preserved. `scheduleWork` will be deprecated and it's just an alias for `scheduleTask` with an immediate priority (to preserve a similar behavior). **Yielding behavior:** Before, tasks would only yield to tasks scheduled via `scheduleWork` and `executeNowOnTheSameThread` (those tasks didn't go through the task queue). With this implementation, tasks would now yield to any task that has a higher position in the task queue. That means we reuse the existing mechanism to avoid lower priority tasks to never execute because higher priority tasks never stop coming. All tasks would yield to requests for synchronous access (via `executeNowOnTheSameThread`) as did the current implementation. Changelog: [internal] Reviewed By: javache, sammy-SC Differential Revision: D49316881
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