Do not invoke server actions twice when dynamicIO is enabled (#70784)

In dev mode, when dynamic IO is enabled, we seed a prefetch cache scope
for non-prefetch requests (via #70408).

We must omit server action requests from this cache seeding though,
because it would lead to the action handler being called twice. This is
not intended in general for any action, and specificially fails in the
second invocation while decoding the request form data with `Error:
Unexpected end of JSON input` in `initializeModelChunk`, because the
request body was already consumed in the first invocation.

packages/next/src/server/base-server.ts

@@ -3038,7 +3038,8 @@ export default abstract class Server<
           if (
             !cache &&
             !isPrefetchRSCRequest &&
-            routeModule?.definition.kind === RouteKind.APP_PAGE
+            routeModule?.definition.kind === RouteKind.APP_PAGE &&
+            !isServerAction
           ) {
             req.headers[RSC_HEADER] = '1'
             req.headers[NEXT_ROUTER_PREFETCH_HEADER] = '1'

test/e2e/app-dir/dynamic-io/app/server-action/action.ts

@@ -0,0 +1,5 @@
+'use server'
+
+export async function action(value: string) {
+  return value
+}

test/e2e/app-dir/dynamic-io/app/server-action/page.tsx

@@ -0,0 +1,16 @@
+'use client'
+
+import { useActionState } from 'react'
+import { action } from './action'
+
+export default function Page() {
+  const [result, formAction] = useActionState(() => action('result'), 'initial')
+
+  return (
+    <form action={formAction}>
+      <h1>Server Action with Dynamic IO</h1>
+      <button>Submit</button>
+      <p>{result}</p>
+    </form>
+  )
+}

test/e2e/app-dir/dynamic-io/dynamic-io.server-action.test.ts

@@ -0,0 +1,18 @@
+import { nextTestSetup } from 'e2e-utils'
+import { retry } from 'next-test-utils'
+
+describe('dynamic-io', () => {
+  const { next } = nextTestSetup({
+    files: __dirname,
+  })
+
+  it('should not fail decoding server action arguments', async () => {
+    const browser = await next.browser('/server-action')
+    expect(await browser.elementByCss('p').text()).toBe('initial')
+    await browser.elementByCss('button').click()
+
+    await retry(async () => {
+      expect(await browser.elementByCss('p').text()).toBe('result')
+    })
+  })
+})