[next]: ensure unmatched action rewrites are routed to correct handler (#11686)

User defined rewrites are "normalized" so that our internal rewrites are still properly handled. Before normalizing these rewrites, the Next.js builder will attempt to match server action requests to a`.action` variant. Then the user-defined rewrites flow through the afterFiles normalization ([this part](https://github.com/vercel/vercel/blob/fix/unmatched-action-rewrites/packages/next/src/server-build.ts#L254-L279)) so that when we add `.action` in the builder, we don't drop the suffix. 

But this normalization can lead to a malformed `dest`. e.g., if I had rewrite like this:

```js
{
  source: '/greedy-rewrite/static/:path*',
  destination: '/static/:path*',
}
```

The builder would go through this flow on an action request to `/greedy-rewrite/static`:

1. It'll attempt to match it to a `.action` output, so `/greedy-rewrite/static` -> `/greedy-rewrite/static.action`
2. The afterFiles normalization will take place, so the original `dest` of `/static/$1` will become `/static/$1$rscsuff`
3. $1 will be an empty string, because it doesn't match the existing capture group. So now `/greedy-rewrite/static.action` -> `/greedy-rewrite/static/.action`
4. `static/.action` is not a valid output, so it'll 404 and the action will break. 

Existing handling exists for `.rsc` outputs for a similar reason, but only on the index route. I added a similar fix for this in #11688.

.changeset/mean-peas-visit.md

@@ -0,0 +1,5 @@
+---
+'@vercel/next': patch
+---
+
+ensure unmatched action rewrites are routed to correct handler

packages/next/src/server-build.ts

@@ -2089,6 +2089,25 @@ export async function serverBuild({
       // with that routing section
       ...afterFilesRewrites,
 
+      // Ensure that after we normalize `afterFilesRewrites`, unmatched actions are routed to the correct handler
+      // e.g. /foo/.action -> /foo.action. This should only ever match in cases where we're routing to an action handler
+      // and the rewrite normalization led to something like /foo/$1$rscsuff, and $1 had no match.
+      // This is meant to have parity with the .rsc handling below.
+      ...(hasActionOutputSupport
+        ? [
+            {
+              src: `${path.posix.join('/', entryDirectory, '/\\.action$')}`,
+              dest: `${path.posix.join('/', entryDirectory, '/index.action')}`,
+              check: true,
+            },
+            {
+              src: `${path.posix.join('/', entryDirectory, '(.+)/\\.action$')}`,
+              dest: `${path.posix.join('/', entryDirectory, '$1.action')}`,
+              check: true,
+            },
+          ]
+        : []),
+
       // ensure non-normalized /.rsc from rewrites is handled
       ...(appPathRoutesManifest
         ? [

packages/next/test/fixtures/00-app-dir-actions-experimental-streaming/app/edge/static/page.js

@@ -0,0 +1,25 @@
+"use client";
+
+import { useState } from "react";
+import { increment } from "../../actions";
+
+export default function Home() {
+  const [count, setCount] = useState(0);
+
+  return (
+    <div>
+      {count}
+      <button
+        onClick={async () => {
+          const actionResult = await increment(count);
+          // @ts-ignore
+          setCount(actionResult);
+          console.log(actionResult);
+        }}
+      >
+        Trigger
+      </button>
+      Static
+    </div>
+  );
+}

packages/next/test/fixtures/00-app-dir-actions-experimental-streaming/app/page.js

@@ -0,0 +1,25 @@
+"use client";
+
+import { useState } from "react";
+import { increment } from "./actions";
+
+export default function Home() {
+  const [count, setCount] = useState(0);
+
+  return (
+    <div>
+      {count}
+      <button
+        onClick={async () => {
+          const actionResult = await increment(count);
+          // @ts-ignore
+          setCount(actionResult);
+          console.log(actionResult);
+        }}
+      >
+        Trigger
+      </button>
+      Static
+    </div>
+  );
+}

packages/next/test/fixtures/00-app-dir-actions-experimental-streaming/app/static/page.js

@@ -0,0 +1,25 @@
+"use client";
+
+import { useState } from "react";
+import { increment } from "../actions";
+
+export default function Home() {
+  const [count, setCount] = useState(0);
+
+  return (
+    <div>
+      {count}
+      <button
+        onClick={async () => {
+          const actionResult = await increment(count);
+          // @ts-ignore
+          setCount(actionResult);
+          console.log(actionResult);
+        }}
+      >
+        Trigger
+      </button>
+      Static
+    </div>
+  );
+}

packages/next/test/fixtures/00-app-dir-actions-experimental-streaming/index.test.js

@@ -293,6 +293,43 @@ describe(`${__dirname.split(path.sep).pop()}`, () => {
         expect(res.headers.get('x-edge-runtime')).toBe('1');
       }
     });
+
+    it('should work when a rewrite greedy matches an action rewrite', async () => {
+      const targetPath = `${basePath}/static`;
+      const canonicalPath = `/greedy-rewrite/${basePath}/static`;
+      const actionId = findActionId(targetPath, runtime);
+
+      const res = await fetch(
+        `${ctx.deploymentUrl}${canonicalPath}`,
+        generateFormDataPayload(actionId)
+      );
+
+      expect(res.status).toEqual(200);
+      expect(res.headers.get('x-matched-path')).toBe(targetPath + '.action');
+      expect(res.headers.get('content-type')).toBe('text/x-component');
+      if (runtime === 'node') {
+        expect(res.headers.get('x-vercel-cache')).toBe('MISS');
+      } else {
+        expect(res.headers.get('x-edge-runtime')).toBe('1');
+      }
+    });
+  });
+
+  describe('rewrite to index', () => {
+    it('should work when user has a rewrite to the index route', async () => {
+      const canonicalPath = '/rewritten-to-index';
+      const actionId = findActionId('', 'node');
+
+      const res = await fetch(
+        `${ctx.deploymentUrl}${canonicalPath}`,
+        generateFormDataPayload(actionId)
+      );
+
+      expect(res.status).toEqual(200);
+      expect(res.headers.get('x-matched-path')).toBe('/index.action');
+      expect(res.headers.get('content-type')).toBe('text/x-component');
+      expect(res.headers.get('x-vercel-cache')).toBe('MISS');
+    });
   });
 
   describe('pages', () => {

packages/next/test/fixtures/00-app-dir-actions-experimental-streaming/next.config.js

@@ -9,6 +9,18 @@ module.exports = {
         source: '/rewrite/edge/rsc/static',
         destination: '/edge/rsc/static',
       },
+      {
+        source: '/greedy-rewrite/static/:path*',
+        destination: '/static/:path*',
+      },
+      {
+        source: '/greedy-rewrite/edge/static/:path*',
+        destination: '/edge/static/:path*',
+      },
+      {
+        source: '/rewritten-to-index',
+        destination: '/?fromRewrite=1',
+      },
     ];
   },
 };