Async environment for mods to do concurrent tasks (#11131)

doc/lua_api.txt

@@ -5767,6 +5767,68 @@ Timing
 * `job:cancel()`
     * Cancels the job function from being called
 
+Async environment
+-----------------
+
+The engine allows you to submit jobs to be ran in an isolated environment
+concurrently with normal server operation.
+A job consists of a function to be ran in the async environment, any amount of
+arguments (will be serialized) and a callback that will be called with the return
+value of the job function once it is finished.
+
+The async environment does *not* have access to the map, entities, players or any
+globals defined in the 'usual' environment. Consequently, functions like
+`minetest.get_node()` or `minetest.get_player_by_name()` simply do not exist in it.
+
+Arguments and return values passed through this can contain certain userdata
+objects that will be seamlessly copied (not shared) to the async environment.
+This allows you easy interoperability for delegating work to jobs.
+
+* `minetest.handle_async(func, callback, ...)`:
+    * Queue the function `func` to be ran in an async environment.
+      Note that there are multiple persistent workers and any of them may
+      end up running a given job. The engine will scale the amount of
+      worker threads automatically.
+    * When `func` returns the callback is called (in the normal environment)
+      with all of the return values as arguments.
+    * Optional: Variable number of arguments that are passed to `func`
+* `minetest.register_async_dofile(path)`:
+    * Register a path to a Lua file to be imported when an async environment
+      is initialized. You can use this to preload code which you can then call
+      later using `minetest.handle_async()`.
+
+### List of APIs available in an async environment
+
+Classes:
+* `ItemStack`
+* `PerlinNoise`
+* `PerlinNoiseMap`
+* `PseudoRandom`
+* `PcgRandom`
+* `SecureRandom`
+* `VoxelArea`
+* `VoxelManip`
+    * only if transferred into environment; can't read/write to map
+* `Settings`
+
+Class instances that can be transferred between environments:
+* `ItemStack`
+* `PerlinNoise`
+* `PerlinNoiseMap`
+* `VoxelManip`
+
+Functions:
+* Standalone helpers such as logging, filesystem, encoding,
+  hashing or compression APIs
+* `minetest.request_insecure_environment` (same restrictions apply)
+
+Variables:
+* `minetest.settings`
+* `minetest.registered_items`, `registered_nodes`, `registered_tools`,
+  `registered_craftitems` and `registered_aliases`
+    * with all functions and userdata values replaced by `true`, calling any
+      callbacks here is obviously not possible
+
 Server
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