Add wasm ctor and dtor calls to non-main exports in core (#481)

crates/core/src/main.rs

@@ -15,6 +15,8 @@ static mut BYTECODE: OnceCell<Vec<u8>> = OnceCell::new();
 
 #[export_name = "wizer.initialize"]
 pub extern "C" fn init() {
+    let _wasm_ctx = WasmCtx::new();
+
     let runtime = runtime::new_runtime().unwrap();
 
     let mut contents = String::new();
@@ -45,7 +47,42 @@ fn main() {
 ///   length.
 #[export_name = "javy.invoke"]
 pub unsafe extern "C" fn invoke(fn_name_ptr: *mut u8, fn_name_size: usize) {
+    let _wasm_ctx = WasmCtx::new();
+
     let js_fn_name = str::from_utf8_unchecked(slice::from_raw_parts(fn_name_ptr, fn_name_size));
     let runtime = unsafe { RUNTIME.take().unwrap() };
     execution::invoke_function(&runtime, FUNCTION_MODULE_NAME, js_fn_name);
 }
+
+// RAII abstraction for calling Wasm ctors and dtors for exported non-main functions.
+struct WasmCtx;
+
+impl WasmCtx {
+    #[must_use = "Failing to assign the return value will result in the wasm dtors being run immediately"]
+    fn new() -> Self {
+        unsafe { __wasm_call_ctors() };
+        Self
+    }
+}
+
+impl Drop for WasmCtx {
+    fn drop(&mut self) {
+        unsafe { __wasm_call_dtors() };
+    }
+}
+
+extern "C" {
+    // `__wasm_call_ctors` is generated by `wasm-ld` and invokes all of the global constructors.
+    // In a Rust bin crate, the `_start` function will invoke this implicitly but no other exported
+    // Wasm functions will invoke this.
+    // If this is not invoked, access to environment variables and directory preopens will not be
+    // available.
+    // This should only be invoked at the start of exported Wasm functions that are not the `main`
+    // function.
+    // References:
+    // - [Rust 1.67.0 stopped initializing the WASI environment for exported functions](https://github.com/rust-lang/rust/issues/107635)
+    // - [Wizer header in Fastly's JS compute runtime](https://github.com/fastly/js-compute-runtime/blob/main/runtime/js-compute-runtime/third_party/wizer.h#L92)
+    fn __wasm_call_ctors();
+
+    fn __wasm_call_dtors();
+}