test/example: add testing constructors and destructors

rust/examples/reactor-hello.rs

@@ -49,5 +49,16 @@ pub async fn main() -> anyhow::Result<()> {
     f.call(&mut store, &[], &mut []).unwrap();
     trace_ctx.shutdown().await;
 
+    // call __wasm_call_dtors
+    // Normally reactors don't emit __wasm_call_dtors, but
+    // reactor-hello.c.instr.wasm includes it as an example of specifying
+    // a cleanup function in a reactor.
+    {
+        let f = instance
+            .get_func(&mut store, "__wasm_call_dtors")
+            .expect("function exists");
+        f.call(&mut store, &[], &mut []).unwrap();
+    }
+
     Ok(())
 }

test/constructor-destructor.c

@@ -0,0 +1,11 @@
+#include <stdio.h>
+
+__attribute__((constructor)) static void constructor(void) {
+  printf("constructor\n");
+}
+
+int main(void) {}
+
+__attribute__((destructor)) static void destructor(void) {
+  printf("destructor\n");
+}

test/reactor-hello.c

@@ -1,4 +1,12 @@
 #include <stdio.h>
 #define EXPORT_AS(name) __attribute__((export_name(name)))
 
+__attribute__((constructor)) static void constructor(void) {
+  printf("constructor\n");
+}
+
 EXPORT_AS("hello") void hello(void) { printf("hello world\n"); }
+
+EXPORT_AS("__wasm_call_dtors") void __wasm_call_dtors(void) {
+  printf("real destructor\n");
+}