Add middleware to persist form actions in session and follow POST-REDIRECT-GET

form submission pattern

src/middleware.ts

@@ -0,0 +1,46 @@
+import { defineMiddleware } from "astro:middleware";
+import { getActionContext } from "astro:actions";
+import { randomUUID } from "node:crypto";
+
+export const onRequest = defineMiddleware(async (context, next) => {
+  if (context.isPrerendered) return next();
+
+  const { action, setActionResult, serializeActionResult } =
+    getActionContext(context);
+
+  const currentAction = await context.session?.get("currentAction");
+
+  if (currentAction) {
+    const { actionName, actionResult } = JSON.parse(currentAction);
+    setActionResult(actionName, actionResult);
+
+    context.session?.delete("currentAction");
+    return next();
+  }
+
+  if (action?.calledFrom === "form") {
+    const actionResult = await action.handler();
+
+    context.session?.set(
+      "currentAction",
+      JSON.stringify({
+        actionName: action.name,
+        actionResult: serializeActionResult(actionResult),
+      }),
+    );
+
+    if (actionResult.error) {
+      const referer = context.request.headers.get("Referer");
+      if (!referer) {
+        throw new Error(
+          "Internal: Referer unexpectedly missing from Action POST request.",
+        );
+      }
+      return context.redirect(referer);
+    }
+
+    return context.redirect(context.originPathname);
+  }
+
+  return next();
+});