fix: make sure we don't panic when an exception is thrown in an async context

Makefile

@@ -71,6 +71,14 @@ test: compile-examples
 			echo "Got: $$error_msg"; \
 			exit 1; \
 		fi
+		@output=$$(extism call examples/async_exception.wasm main --wasi --log-level info 2>&1); \
+		if echo "$$output" | grep -q "wasm error: unreachable"; then \
+			echo "Test failed - found 'wasm error: unreachable' in output"; \
+			echo "Output: $$output"; \
+			exit 1; \
+		else \
+			echo "Test passed - no unreachable error in async_exception"; \
+		fi
 
 compile-examples: cli
 		cd examples/react && npm install && npm run build && cd ../..
@@ -82,6 +90,7 @@ compile-examples: cli
 		./target/release/extism-js examples/console/script.js -i examples/console/script.d.ts -o examples/console.wasm
 		./target/release/extism-js examples/base64/script.js -i examples/base64/script.d.ts -o examples/base64.wasm
 		./target/release/extism-js examples/try-catch/script.js -i examples/try-catch/script.d.ts -o examples/try-catch.wasm
+		cd examples/async_exception && npm install && npm run build && cd ../..
 
 kitchen: 
 	cd examples/kitchen-sink && npm install && npm run build && cd ../..

crates/core/src/lib.rs

@@ -18,9 +18,26 @@ static CALL_ARGS: std::sync::Mutex<Vec<Vec<ArgType>>> = std::sync::Mutex::new(ve
 fn check_exception(this: &Ctx, err: rquickjs::Error) -> anyhow::Error {
     let s = match err {
         rquickjs::Error::Exception => {
-            let err = this.catch().into_exception().unwrap();
-            let msg = err.message().unwrap_or_default();
-            format!("Exception: {}\n{}", msg, err.stack().unwrap_or_default())
+            // Check if we got a valid exception or if it's uninitialized (possibly due to async code)
+            // - https://github.com/DelSkayn/rquickjs/issues/421
+            // - https://github.com/DelSkayn/rquickjs/pull/422
+            // - https://github.com/quickjs-ng/quickjs/issues/39
+            // - https://github.com/quickjs-ng/quickjs/pull/1038
+
+            match this.catch().into_exception() {
+                Some(err) => {
+                    let msg = err.message().unwrap_or_default();
+                    let stack = err.stack().unwrap_or_default();
+                    format!("Exception: {}\n{}", msg, stack)
+                }
+                None => {
+                    format!(
+                        "Uncaught exception in async context. \
+                        Async/Promise operations are not supported in this environment. \
+                        Please use only synchronous code."
+                    )
+                }
+            }
         }
         err => err.to_string(),
     };
@@ -102,7 +119,7 @@ fn invoke<'a, T, F: for<'b> Fn(Ctx<'b>, Value<'b>) -> T>(
         let function_invocation_result = function.call_arg(args);
 
         while ctx.execute_pending_job() {
-            continue
+            continue;
         }
 
         match function_invocation_result {

examples/async_exception/.gitignore

@@ -0,0 +1,2 @@
+node_modules/
+dist/

examples/async_exception/esbuild.js

@@ -0,0 +1,12 @@
+const esbuild = require('esbuild');
+
+esbuild
+  .build({
+    entryPoints: ['src/index.ts'],
+    outdir: 'dist',
+    bundle: true,
+    sourcemap: true,
+    minify: false, // might want to use true for production build
+    format: 'cjs', // needs to be CJS for now
+    target: ['es2020'] // don't go over es2020 because quickjs doesn't support it
+  })