extern.load - load shared libraries at runtime (#123)

Allow users to write and load plugins that expose a symbol `seal_open_extern`. The Luau state is passed as a `*mut lua_State` to the shared library which can do whatever it wants.

Author: deviaze

.seal/typedefs/interop/extern.luau

@@ -0,0 +1,57 @@
+
+--[=[
+    Load external libraries at runtime.
+
+    External libraries (also called plugins), can be used to expand *seal*'s functionality,
+    add bindings to C and Rust libraries, and allow performance-critical code execution without mluau or Luau VM overhead.
+]=]
+export type extern = {
+    --[=[
+        Loads the native dynamic library at `path`, calling the symbol `seal_open_extern` in that library with a pointer to the Luau state.
+
+        It is expected that calling `seal_open_extern` pushes 1 value directly onto the Luau stack. This function returns that value unchanged.
+
+        This function does not handle platform-specific behavior; it is expected the caller loads the correct dynamic library for
+        supported platforms/architectures.
+
+        # ⚠️ Safety
+        
+        This function is extremely unsafe.
+            
+        External libraries can easily bypass Rust's and mluau's safety guarantees, including but not limited to:
+        - causing memory safety vulnerabilities, 
+        - incorrectly using the Luau stack (causing hard crashes or UB),
+        - modifying the Luau state in cursed ways,
+        - execute arbitrary code.
+
+        The caller is responsible for ensuring the library:
+            
+        - is compatible with the caller's operating system, platform, and architecture,
+        - is a *seal* extern library, not any other shared/dynamic library,
+        - contains a *not mangled* function symbol `seal_open_extern` that:
+            - is in the C ABI (use `"C-unwind"` in Rust),
+            - takes in a Luau `lua_State` pointer as its first argument,
+            - returns `1` as an `i32 (c_int)`.
+
+        Library/plugin maintainers are responsible for ensuring the library:
+
+        - uses the Luau stack correctly,
+        - does NOT share the passed `lua_State` between multiple OS threads,
+        - does NOT free memory owned by Luau,
+        - correctly links to Luau (not Lua) for using the Luau stack,
+        - should not throw an uncaught foreign exception or Rust panic.
+
+        # Errors
+        
+        Throws an error if:
+
+        - The dynamic library cannot be found (usually with a message like "dlopen failed"),
+        - The dynamic library is missing the symbol `seal_open_extern`,
+        - The dynamic library could not be loaded for a different reason,
+        - `seal_open_extern` does not return 1 (c_int),
+        - `seal_open_extern` returns 1 but does not push exactly 1 value to the Luau stack.
+    ]=]
+    load: (path: string) -> unknown,
+}
+
+return {} :: extern

CONTRIBUTING.md

@@ -42,11 +42,12 @@ A library is a good candidate for `@std` if:
 5. please don't run a formatter over the whole codebase.
    1. I don't mind if `wrap_err!`s go off the RHS of the page if that means vertical space is better used for code.
    2. On the other hand, let's try to keep non-wrap_err! code and comments to 85-110 colwidth?
-6. Documentation goes in `./.seal/typedefs/std/*`.
+6. Inline documentation goes in `./.seal/typedefs/std/*`.
    1. For documentation, try to stick with the newer docs headers like seen in the `@std/process` API docs.
    2. No Moonwave `@attributes`, they make code less readable and we don't use Moonwave.
-7. Register the library in `./src/require/mod.rs` and `./.seal/typedefs/init.luau` in 3 places:
+7. After you finish writing inline documentation, regenerate markdown documentation by running `seal ./docs/docscripts/reference.luau`. Generated docs aren't going to be perfect, we'll fix them later.
+8. Register the library in `./src/require/mod.rs` and `./.seal/typedefs/init.luau` in 3 places:
    1. The Big Beautiful Table (BBT) in `./src/require/mod.rs`, which handles when the library is required directly (`@std/mylib`)
    2. the Small Beautiful Table in `./src/require/mod.rs`, which handles when the entire `@std` is required at once.
    3. in `./.seal/typedefs/init.luau` to provide `require` support when the entire `@std` is required at once.
-8. Add tests in `./tests/luau/std/libname/*` or `./tests/luau/std/libname.luau`.
+9. Add tests in `./tests/luau/std/libname/*` or `./tests/luau/std/libname.luau`.