rust: avoid rerunning constructors after WASI startup

crates/guest-rust/src/rt/mod.rs

@@ -198,9 +198,21 @@ pub unsafe fn cabi_realloc(
 /// wit-bindgen-rust generates a call to this function at the start of all
 /// component export functions. Importantly, it is not called as part of
 /// `cabi_realloc`, which is a *core* export func, but should not execute ctors.
+/// Constructors already invoked by `_initialize` or `_start` do not need to be
+/// run again when an export is called.
 #[cfg(target_arch = "wasm32")]
 pub fn run_ctors_once() {
     static mut RUN: bool = false;
+
+    extern "C" fn mark_run() {
+        unsafe { RUN = true }
+    }
+
+    // Record startup regardless of who calls `__wasm_call_ctors`.
+    #[used]
+    #[unsafe(link_section = ".init_array")]
+    static MARK_RUN: extern "C" fn() = mark_run;
+
     unsafe {
         if !RUN {
             // This function is synthesized by `wasm-ld` to run all static

tests/runtime/rust/ctors-run-once/runner.rs

@@ -0,0 +1,14 @@
+include!(env!("BINDINGS"));
+
+struct Component;
+
+export!(Component);
+
+impl Guest for Component {
+    fn run() {
+        // Startup may have run the constructors at instantiation or at the
+        // first export call. Neither export call should run them again.
+        assert_eq!(the::test::i::ctor_runs(), 1);
+        assert_eq!(the::test::i::ctor_runs(), 1);
+    }
+}

tests/runtime/rust/ctors-run-once/test.rs

@@ -0,0 +1,23 @@
+include!(env!("BINDINGS"));
+
+use std::sync::atomic::{AtomicU32, Ordering::Relaxed};
+
+static CTOR_RUNS: AtomicU32 = AtomicU32::new(0);
+
+extern "C" fn ctor() {
+    CTOR_RUNS.fetch_add(1, Relaxed);
+}
+
+#[used]
+#[unsafe(link_section = ".init_array")]
+static CTOR: extern "C" fn() = ctor;
+
+struct Test;
+
+export!(Test);
+
+impl exports::the::test::i::Guest for Test {
+    fn ctor_runs() -> u32 {
+        CTOR_RUNS.load(Relaxed)
+    }
+}

tests/runtime/rust/ctors-run-once/test.wit

@@ -0,0 +1,14 @@
+package the:test;
+
+interface i {
+  ctor-runs: func() -> u32;
+}
+
+world runner {
+  import i;
+  export run: func();
+}
+
+world test {
+  export i;
+}