[node-bridge] Ensure content-type is always set with multi-payloads (#…

…7681)

### Related Issues

This ensures a content-type is always set inside of each part of the multi-part payload as it's needed for proper parsing. This also ensures non-200 status codes/headers are passed back separate when they differ so that they can be handled independently without assuming they all match. 

x-ref: https://vercel.slack.com/archives/C03AYHB6MA9
x-ref: #7507

### 📋 Checklist

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#### Tests

- [ ] The code changed/added as part of this PR has been covered with tests
- [ ] All tests pass locally with `yarn test-unit`

#### Code Review

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- [ ] Issue from task tracker has a link to this PR

packages/node-bridge/bridge.js

@@ -186,9 +186,6 @@ class Bridge {
       'payloads' in normalizedEvent &&
       Array.isArray(normalizedEvent.payloads)
     ) {
-      // statusCode and headers are required to match when using
-      // multiple payloads in a single invocation so we can use
-      // the first
       let statusCode = 200;
       /**
        * @type {import('http').IncomingHttpHeaders}
@@ -200,6 +197,14 @@ class Bridge {
       let combinedBody = '';
       const multipartBoundary = 'payload-separator';
       const CLRF = '\r\n';
+      /**
+       * @type {Record<string, any>[]}
+       */
+      const separateHeaders = [];
+      /**
+       * @type {Set<string>}
+       */
+      const allHeaderKeys = new Set();
 
       // we execute the payloads one at a time to ensure
       // lambda semantics
@@ -209,20 +214,49 @@ class Bridge {
         // build a combined body using multipart
         // https://www.w3.org/Protocols/rfc1341/7_2_Multipart.html
         combinedBody += `--${multipartBoundary}${CLRF}`;
-        if (response.headers['content-type']) {
-          combinedBody += `content-type: ${response.headers['content-type']}${CLRF}${CLRF}`;
-        }
-        combinedBody += response.body;
+        combinedBody += `content-type: ${
+          response.headers['content-type'] || 'text/plain'
+        }${CLRF}${CLRF}`;
+        combinedBody += response.body || '';
         combinedBody += CLRF;
 
         if (i === normalizedEvent.payloads.length - 1) {
           combinedBody += `--${multipartBoundary}--${CLRF}`;
         }
-
-        statusCode = response.statusCode;
-        headers = response.headers;
+        // pass non-200 status code in header so it can be handled
+        // separately from other payloads e.g. HTML payload redirects
+        // (307) but data payload does not (200)
+        if (response.statusCode !== 200) {
+          headers[`x-vercel-payload-${i + 1}-status`] =
+            response.statusCode + '';
+        }
+        separateHeaders.push(response.headers);
+        Object.keys(response.headers).forEach(key => allHeaderKeys.add(key));
       }
 
+      allHeaderKeys.forEach(curKey => {
+        /**
+         * @type string | string[] | undefined
+         */
+        const curValue = separateHeaders[0] && separateHeaders[0][curKey];
+        const canDedupe = separateHeaders.every(
+          headers => headers[curKey] === curValue
+        );
+
+        if (canDedupe) {
+          headers[curKey] = curValue;
+        } else {
+          // if a header is unique per payload ensure it is prefixed
+          // so it can be parsed and provided separately
+          separateHeaders.forEach((curHeaders, idx) => {
+            if (curHeaders[curKey]) {
+              headers[`x-vercel-payload-${idx + 1}-${curKey}`] =
+                curHeaders[curKey];
+            }
+          });
+        }
+      });
+
       headers[
         'content-type'
       ] = `multipart/mixed; boundary="${multipartBoundary}"`;

packages/node-bridge/test/bridge.test.js

@@ -86,6 +86,12 @@ test('`NowProxyEvent` normalizing', async () => {
 
 test('multi-payload handling', async () => {
   const server = new Server((req, res) => {
+    if (req.url === '/redirect') {
+      res.setHeader('Location', '/somewhere');
+      res.statusCode = 307;
+      res.end('/somewhere');
+      return;
+    }
     res.setHeader(
       'content-type',
       req.url.includes('_next/data') ? 'application/json' : 'text/html'
@@ -117,6 +123,11 @@ test('multi-payload handling', async () => {
             headers: { foo: 'baz' },
             path: '/_next/data/build-id/nowproxy.json',
           },
+          {
+            method: 'GET',
+            headers: { foo: 'baz' },
+            path: '/redirect',
+          },
         ],
       }),
     },
@@ -137,20 +148,36 @@ test('multi-payload handling', async () => {
       !item.startsWith('content-type:') &&
       !item.startsWith('--payload')
     ) {
-      bodies.push(
-        JSON.parse(
-          Buffer.from(item.split('--payload-separator')[0], 'base64').toString()
-        )
-      );
+      const content = Buffer.from(
+        item.split('--payload-separator')[0],
+        'base64'
+      ).toString();
+      bodies.push(content.startsWith('{') ? JSON.parse(content) : content);
     }
   });
 
+  // ensure content-type is always specified as is required for
+  // proper parsing of the multipart body
+  assert(payloadParts.some(part => part.includes('content-type: text/plain')));
+
   assert.equal(bodies[0].method, 'GET');
   assert.equal(bodies[0].path, '/nowproxy');
   assert.equal(bodies[0].headers.foo, 'baz');
   assert.equal(bodies[1].method, 'GET');
   assert.equal(bodies[1].path, '/_next/data/build-id/nowproxy.json');
   assert.equal(bodies[1].headers.foo, 'baz');
+  assert.equal(bodies[2], '/somewhere');
+  assert.equal(result.headers['x-vercel-payload-3-status'], '307');
+  assert.equal(result.headers['x-vercel-payload-2-status'], undefined);
+  assert.equal(result.headers['x-vercel-payload-1-status'], undefined);
+  assert.equal(result.headers['x-vercel-payload-1-content-type'], 'text/html');
+  assert.equal(
+    result.headers['x-vercel-payload-2-content-type'],
+    'application/json'
+  );
+  assert.equal(result.headers['x-vercel-payload-3-content-type'], undefined);
+  assert.equal(result.headers['x-vercel-payload-3-location'], '/somewhere');
+  assert.equal(result.headers['x-vercel-payload-2-location'], undefined);
   assert.equal(context.callbackWaitsForEmptyEventLoop, false);
 
   server.close();