# Objective
- Currently exclusive systems and applying buffers run outside of the multithreaded executor and just calls the funtions on the thread the schedule is running on. Stageless changes this to run these using tasks in a scope. Specifically It uses `spawn_on_scope` to run these. For the render thread this is incorrect as calling `spawn_on_scope` there runs tasks on the main thread. It should instead run these on the render thread and only run nonsend systems on the main thread.
## Solution
- Add another executor to `Scope` for spawning tasks on the scope. `spawn_on_scope` now always runs the task on the thread the scope is running on. `spawn_on_external` spawns onto the external executor than is optionally passed in. If None is passed `spawn_on_external` will spawn onto the scope executor.
- Eventually this new machinery will be able to be removed. This will happen once a fix for removing NonSend resources from the world lands. So this is a temporary fix to support stageless.
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## Changelog
- add a spawn_on_external method to allow spawning on the scope's thread or an external thread
## Migration Guide
> No migration guide. The main thread executor was introduced in pipelined rendering which was merged for 0.10. spawn_on_scope now behaves the same way as on 0.9.
# Objective
- Fix panic_when_hierachy_cycle test hanging
- The problem is that the scope only awaits one task at a time in get_results. In stageless this task is the multithreaded executor. That tasks hangs when a system panics and cannot make anymore progress. This wasn't a problem before because the executor was spawned after all the system tasks had been spawned. But in stageless the executor is spawned before all the system tasks are spawned.
## Solution
- We can catch unwind on each system and close the finish channel if one panics. This then causes the receiver end of the finish channel to panic too.
- this might have a small perf impact, but when running many_foxes it seems to be within the noise. So less than 40us.
## Other possible solutions
- It might be possible to fairly poll all the tasks in get_results in the scope. If we could do that then the scope could panic whenever one of tasks panics. It would require a data structure that we could both poll the futures through a shared ref and also push to it. I tried FuturesUnordered, but it requires an exclusive ref to poll it.
- The catch unwind could be moved onto when we create the tasks for scope instead. We would then need something like a oneshot async channel to inform get_results if a task panics.
# Objective
- Trying to move some of the fixes from https://github.com/bevyengine/bevy/pull/7267 to make that one easier to review
- The MainThreadExecutor is how the render world runs nonsend systems on the main thread for pipelined rendering.
- The multithread executor for stageless wasn't using the MainThreadExecutor.
- MainThreadExecutor was declared in the old executor_parallel module that is getting deleted.
- The way the MainThreadExecutor was getting passed to the scope was actually unsound as the resource could be dropped from the World while the schedule was running
## Solution
- Move MainThreadExecutor to the new multithreaded_executor's file.
- Make the multithreaded executor use the MainThreadExecutor
- Clone the MainThreadExecutor onto the stack and pass that ref in
## Changelog
- Move MainThreadExecutor for stageless migration.
# Objective
- After the multithreaded executor finishes running all the systems, we apply the buffers for any system that hasn't applied it's buffers. This is a courtesy apply for users who forget to order their systems before a apply_system_buffers. When checking stageless, it was found that this apply_system_buffers was running on the executor thread instead of the world's thread. This is a problem because anything with world access should be able to access nonsend resources.
## Solution
- Move the final apply_system_buffers outside of the executor and outside of the scope, so it runs on the same thread that schedule.run is called on.
# Objective
- The stageless executor keeps track of systems that have run, but have not applied their system buffers. The bitset for that was being cloned into apply_system_buffers and cleared in that function, but we need to clear the original version instead of the cloned version
## Solution
- move the clear out of the apply_system_buffers function.
Co-authored-by: Carter Anderson <mcanders1@gmail.com>
# Objective
Complete the first part of the migration detailed in bevyengine/rfcs#45.
## Solution
Add all the new stuff.
### TODO
- [x] Impl tuple methods.
- [x] Impl chaining.
- [x] Port ambiguity detection.
- [x] Write docs.
- [x] ~~Write more tests.~~(will do later)
- [ ] Write changelog and examples here?
- [x] ~~Replace `petgraph`.~~ (will do later)
Co-authored-by: james7132 <contact@jamessliu.com>
Co-authored-by: Michael Hsu <mike.hsu@gmail.com>
Co-authored-by: Mike Hsu <mike.hsu@gmail.com>
