[ppr] Narrow condition for fallback shell generation at runtime (#79565)

This addresses a bug where a fallback shell was generated at runtime
when it was not expected to do so. This was an issue with how the
`x-now-route-matches` header was parsed and was incorrectly assuming
that a falsy header was the same as comparing it to a string. This
correctly only generates the fallback shell if the header is present and
was an empty string as was sent by Vercel.

Co-authored-by: Hendrik Liebau <mail@hendrik-liebau.de>

Author: wyattjoh

.changeset/giant-bushes-sink.md

@@ -0,0 +1,5 @@
+---
+'next': patch
+---
+
+Resolved bug where hitting the parameterized path directly would cause a fallback shell generation instead of just rendering the route with the parameterized placeholders.

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

@@ -1346,19 +1346,24 @@ export default abstract class Server<
               }
             }
 
-            // handle the actual dynamic route name being requested
+            // If the pathname being requested is the same as the source
+            // pathname, and we don't have valid params, we want to use the
+            // default route matches.
             if (
               utils.defaultRouteMatches &&
               normalizedUrlPath === srcPathname &&
-              !paramsResult.hasValidParams &&
-              !utils.normalizeDynamicRouteParams({ ...params }, true)
-                .hasValidParams
+              !paramsResult.hasValidParams
             ) {
               params = utils.defaultRouteMatches
 
-              // Mark that the default route matches were set on the request
-              // during routing.
-              addRequestMeta(req, 'didSetDefaultRouteMatches', true)
+              // If the route matches header is an empty string, we want to
+              // render a fallback shell. This is because we know this came from
+              // a prerender (it has the header) but it's values were filtered
+              // out (because the allowQuery was empty). If it was undefined
+              // then we know that the request is hitting the lambda directly.
+              if (routeMatchesHeader === '') {
+                addRequestMeta(req, 'renderFallbackShell', true)
+              }
             }
 
             if (params) {
@@ -3223,8 +3228,7 @@ export default abstract class Server<
       const fallbackRouteParams =
         isDynamic &&
         isRoutePPREnabled &&
-        (getRequestMeta(req, 'didSetDefaultRouteMatches') ||
-          isDebugFallbackShell)
+        (getRequestMeta(req, 'renderFallbackShell') || isDebugFallbackShell)
           ? getFallbackRouteParams(pathname)
           : null
 

packages/next/src/server/request-meta.ts

@@ -153,9 +153,9 @@ export interface RequestMeta {
   middlewareInvoke?: boolean
 
   /**
-   * Whether the default route matches were set on the request during routing.
+   * Whether the request should render the fallback shell or not.
    */
-  didSetDefaultRouteMatches?: boolean
+  renderFallbackShell?: boolean
 
   /**
    * Whether the request is for the custom error page.

test/e2e/app-dir/empty-fallback-shells/empty-fallback-shells.test.ts

@@ -118,6 +118,23 @@ describe('empty-fallback-shells', () => {
             expect(res.headers.get('x-nextjs-postponed')).not.toBe('1')
           }
         })
+
+        it('does not render a fallback shell when using a params placeholder', async () => {
+          // This should trigger a blocking prerender of the route shell.
+          const res = await next.fetch(
+            '/with-cached-io/without-suspense/params-in-page/[slug]'
+          )
+
+          expect(res.status).toBe(200)
+
+          const html = await res.text()
+
+          // This should render the encoded param in the route shell, and not
+          // interpret the param as a fallback param, and subsequently try to
+          // render the fallback shell instead, which would fail because of the
+          // missing parent suspense boundary.
+          expect(html).toContain('page-%5Bslug%5D')
+        })
       })
 
       describe('and the params accessed in a cached non-page function', () => {

test/production/standalone-mode/required-server-files/required-server-files-ppr.test.ts

@@ -355,6 +355,7 @@ describe('required server files app router', () => {
         'x-matched-path': '/postpone/isr/[slug]',
         // We don't include the `x-now-route-matches` header because we want to
         // test that the fallback route params are correctly set.
+        'x-now-route-matches': '',
       },
     })