Paper: [Suggestion] Async Chunk/World Saving w/ Callback

[Joannou1]

Since chunks are sent to the FileIOThread for async saving, could we make the whole process async?
Or rather, a method with a callback that would be ran async to simply add all the current chunks to the async saver.

I am able to successfully save a world by calling a normal save() async, although sometimes I get a generic "stopTiming called async" throwable.
Not sure if it's safe? But it does indeed put chunks into the saver if all you're concerned about is a quick save before unloading and archiving.
(This would save 50-80ms on the main thread per forced save!)

In addition, a callback for when the world is completely saved (region files have been written to fully) would be extremely useful for anyone looking to quickly archive a world via zipping/copying immediately after saving without the risk of missing chunks.

_This is an example of what may happen if you try to zip up a world immediately after a manual save() - waiting around 100-500ms after seems to be enough to completely save all the data, but It's never a good idea to wait an arbitrary amount of time in case something does take an unusual amount of time_

I spent a few hours seeing how the world saving system worked, was thinking perhaps creating a list of pending chunks for that save then removing them as they are written in RegionFileCache.e()
If the async idea is just not really possible, would the callback work at least?

Though I think right now it's a bit beyond me and wanted to see if this is plausible before I spent a few more hours just to end up completely breaking it!

(Basically, async the chunk iterator in ChunkProviderServer.a():294, then call the callback when all chunks have been physically written to disk in RegionFileCache.e())

[Joannou1]

Err, ignore the bunch of renames, sorry lol, I was indecisive as to what to call this.

[aikar]

This is completely unsafe and can corrupt your world.

You can not build the NBT structure safely async, as the world data might change underneath you and break the data and make it unable to be loaded.

The absolute best we can do is to batch up saves and do them in PARALLEL while the main thread is paused.

Now, this is reasonable imo. but we wouldnt need a callback as it would be an impl detail.

[aikar]

and for handling archiving, i would unload the world and wait until its unloaded.

[CeIebrimbor]

I think this is officially added in 1.14.2 version of Paper.

[Chew]

Hello! Been a while since this opened and a lot of changes have occurred. The support for async chunks we have now is stable and as a result this issue is not very relevant anymore, if there's still any nitpicks you want added feel free to open up another issue describing your suggestions based off of what we have now. Thank you for your suggestion!