Improve wasm errors

internal/fnruntime/jsglue.go

@@ -29,7 +29,14 @@ const wasmBuffer = fs.readFileSync(wasmPath)
 var rl = fs.readFileSync(0, 'utf-8')
 WebAssembly.instantiate(wasmBuffer, go.importObject).then((result) => {
     go.run(result.instance);
-    console.log(` + jsEntrypointFunction + `(rl));
+    var result = ` + jsEntrypointFunction + `(rl);
+
+    // On success, output to console, on failure retrieve and output the error message
+    if (result.includes("kind: ResourceList")) {
+        console.log(result)
+      } else {
+        console.log(` + jsEntrypointFunction + "Errors" + `(rl));
+      }
 });`
 
 const golangWasmJSCode = `const fs = require('fs');

internal/fnruntime/wasmtime.go

@@ -146,6 +146,7 @@ func (f *WasmtimeFn) Run(r io.Reader, w io.Writer) error {
 	if err != nil {
 		return fmt.Errorf("unable to invoke %v: %v", jsEntrypointFunction, err)
 	}
+
 	// We expect `result` to be a *wasmexec.jsString (which is not exportable) with
 	// the following definition: type jsString struct { data string }. It will look
 	// like `&{realPayload}`
@@ -154,17 +155,33 @@ func (f *WasmtimeFn) Run(r io.Reader, w io.Writer) error {
 	// Try to parse the output as yaml.
 	resourceListOutput, err := yaml.Parse(resultStr)
 	if err != nil {
-		return fmt.Errorf("error parsing output resource list %q: %w", resultStr, err)
+		additionalErrorMessage, err2 := retrieveError(f, resourceList)
+		if err2 != nil {
+			additionalErrorMessage = fmt.Sprint("failed to retrieve more error information: %w", err2)
+		}
+		return fmt.Errorf("parsing output resource list with content: %q\n%w\n%s", resultStr, err, additionalErrorMessage)
 	}
 	if resourceListOutput.GetKind() != "ResourceList" {
-		return fmt.Errorf("invalid resource list output from wasm library; got %q", resultStr)
+		additionalErrorMessage, errorResultRetrievalErr := retrieveError(f, resourceList)
+		if errorResultRetrievalErr != nil {
+			additionalErrorMessage = fmt.Sprint("failed to retrieve more error information: %w", errorResultRetrievalErr)
+		}
+		return fmt.Errorf("invalid resource list output from wasm library; got %q\n%s", resultStr, additionalErrorMessage)
 	}
 	if _, err = w.Write([]byte(resultStr)); err != nil {
 		return fmt.Errorf("unable to write the output resource list: %w", err)
 	}
 	return f.loader.cleanup()
 }
 
+func retrieveError(f *WasmtimeFn, resourceList []byte) (string, error) {
+	errResult, err := f.gomod.Call(jsEntrypointFunction+"Errors", string(resourceList))
+	if err != nil {
+		return "", fmt.Errorf("unable to retrieve additional error message from function: %w", err)
+	}
+	return fmt.Sprintf("%s", errResult), nil
+}
+
 var _ wasmexec.Instance = &WasmtimeFn{}
 
 func (f *WasmtimeFn) GetSP() (uint32, error) {
@@ -201,4 +218,4 @@ func (f *WasmtimeFn) Write(fd int, b []byte) (n int, err error) {
 // Error implements the wasmexec.errorLogger interface
 func (f *WasmtimeFn) Error(format string, params ...interface{}) {
 	log.Printf("ERROR: "+format+"\n", params...)
-}
+}