fix: moved down segment path handling after cache headers are set (#7…

…4893)

Previously, the segment path handling was returning before the
`x-nextjs-cache` headers were sent, this moves it below that to ensure
that the correct cache headers are added.

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

@@ -3062,56 +3062,6 @@ export default abstract class Server<
       }
     )
 
-    if (isRoutePPREnabled && typeof segmentPrefetchHeader === 'string') {
-      // This is a prefetch request issued by the client Segment Cache. These
-      // should never reach the application layer (lambda). We should either
-      // respond from the cache (HIT) or respond with 204 No Content (MISS).
-
-      // Set a header to indicate that PPR is enabled for this route. This
-      // lets the client distinguish between a regular cache miss and a cache
-      // miss due to PPR being disabled. In other contexts this header is used
-      // to indicate that the response contains dynamic data, but here we're
-      // only using it to indicate that the feature is enabled — the segment
-      // response itself contains whether the data is dynamic.
-      res.setHeader(NEXT_DID_POSTPONE_HEADER, '2')
-
-      if (
-        cacheEntry !== null &&
-        // This is always true at runtime but is needed to refine the type
-        // of cacheEntry.value to CachedAppPageValue, because the outer
-        // ResponseCacheEntry is not a discriminated union.
-        cacheEntry.value?.kind === CachedRouteKind.APP_PAGE &&
-        cacheEntry.value.segmentData
-      ) {
-        const matchedSegment = cacheEntry.value.segmentData.get(
-          segmentPrefetchHeader
-        )
-        if (matchedSegment !== undefined) {
-          // Cache hit
-          return {
-            type: 'rsc',
-            body: RenderResult.fromStatic(matchedSegment),
-            // TODO: Eventually this should use revalidate time of the
-            // individual segment, not the whole page.
-            revalidate: cacheEntry.revalidate,
-          }
-        }
-      }
-
-      // Cache miss. Either a cache entry for this route has not been generated
-      // (which technically should not be possible when PPR is enabled, because
-      // at a minimum there should always be a fallback entry) or there's no
-      // match for the requested segment. Respond with a 204 No Content. We
-      // don't bother to respond with 404, because these requests are only
-      // issued as part of a prefetch.
-      res.statusCode = 204
-      return {
-        type: 'rsc',
-        body: RenderResult.fromStatic(''),
-        revalidate: cacheEntry?.revalidate,
-      }
-    }
-
     if (isPreviewMode) {
       res.setHeader(
         'Cache-Control',
@@ -3204,6 +3154,53 @@ export default abstract class Server<
 
     const { value: cachedData } = cacheEntry
 
+    if (isRoutePPREnabled && typeof segmentPrefetchHeader === 'string') {
+      // This is a prefetch request issued by the client Segment Cache. These
+      // should never reach the application layer (lambda). We should either
+      // respond from the cache (HIT) or respond with 204 No Content (MISS).
+
+      // Set a header to indicate that PPR is enabled for this route. This
+      // lets the client distinguish between a regular cache miss and a cache
+      // miss due to PPR being disabled. In other contexts this header is used
+      // to indicate that the response contains dynamic data, but here we're
+      // only using it to indicate that the feature is enabled — the segment
+      // response itself contains whether the data is dynamic.
+      res.setHeader(NEXT_DID_POSTPONE_HEADER, '2')
+
+      if (
+        // This is always true at runtime but is needed to refine the type
+        // of cacheEntry.value to CachedAppPageValue, because the outer
+        // ResponseCacheEntry is not a discriminated union.
+        cachedData?.kind === CachedRouteKind.APP_PAGE &&
+        cachedData.segmentData
+      ) {
+        const matchedSegment = cachedData.segmentData.get(segmentPrefetchHeader)
+        if (matchedSegment !== undefined) {
+          // Cache hit
+          return {
+            type: 'rsc',
+            body: RenderResult.fromStatic(matchedSegment),
+            // TODO: Eventually this should use revalidate time of the
+            // individual segment, not the whole page.
+            revalidate: cacheEntry.revalidate,
+          }
+        }
+      }
+
+      // Cache miss. Either a cache entry for this route has not been generated
+      // (which technically should not be possible when PPR is enabled, because
+      // at a minimum there should always be a fallback entry) or there's no
+      // match for the requested segment. Respond with a 204 No Content. We
+      // don't bother to respond with 404, because these requests are only
+      // issued as part of a prefetch.
+      res.statusCode = 204
+      return {
+        type: 'rsc',
+        body: RenderResult.fromStatic(''),
+        revalidate: cacheEntry?.revalidate,
+      }
+    }
+
     // If the cache value is an image, we should error early.
     if (cachedData?.kind === CachedRouteKind.IMAGE) {
       throw new Error('invariant SSG should not return an image cache value')