refactor: remove isRSCRequestCheck (#65587)

In preparation for the following request adapter work, this removes a
helper function for checking if a request is an RSC one, instead
preferring to use the metadata directly.

This helps readability because we know where it's coming from, and it
keeps the implementation simple.

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

@@ -844,8 +844,7 @@ export default abstract class Server<
           this.handleRequestImpl(req, res, parsedUrl).finally(() => {
             if (!span) return
 
-            const isRSCRequest = isRSCRequestCheck(req) ?? false
-
+            const isRSCRequest = getRequestMeta(req, 'isRSCRequest') ?? false
             span.setAttributes({
               'http.status_code': res.statusCode,
               'next.rsc': isRSCRequest,
@@ -940,6 +939,7 @@ export default abstract class Server<
         )
       }
 
+      // Update the `x-forwarded-*` headers.
       const { originalRequest = null } = isNodeNextRequest(req) ? req : {}
       const xForwardedProto = originalRequest?.headers['x-forwarded-proto']
       const isHttps = xForwardedProto
@@ -1831,7 +1831,7 @@ export default abstract class Server<
     resolvedPathname: string
   ): void {
     const baseVaryHeader = `${RSC_HEADER}, ${NEXT_ROUTER_STATE_TREE}, ${NEXT_ROUTER_PREFETCH_HEADER}`
-    const isRSCRequest = isRSCRequestCheck(req)
+    const isRSCRequest = getRequestMeta(req, 'isRSCRequest') ?? false
 
     let addedNextUrlToVary = false
 
@@ -1955,9 +1955,11 @@ export default abstract class Server<
      * prefetch request.
      */
     const isPrefetchRSCRequest =
-      (req.headers[NEXT_ROUTER_PREFETCH_HEADER.toLowerCase()] === '1' ||
-        getRequestMeta(req, 'isPrefetchRSCRequest')) ??
-      false
+      getRequestMeta(req, 'isPrefetchRSCRequest') ?? false
+
+    // NOTE: Don't delete headers[RSC] yet, it still needs to be used in renderToHTML later
+
+    const isRSCRequest = getRequestMeta(req, 'isRSCRequest') ?? false
 
     // when we are handling a middleware prefetch and it doesn't
     // resolve to a static data route we bail early to avoid
@@ -2000,9 +2002,6 @@ export default abstract class Server<
       )
     }
 
-    // Don't delete headers[RSC] yet, it still needs to be used in renderToHTML later
-    const isRSCRequest = isRSCRequestCheck(req)
-
     const { routeModule } = components
 
     /**
@@ -3648,7 +3647,3 @@ export default abstract class Server<
     return this.renderError(null, req, res, pathname!, query, setHeaders)
   }
 }
-
-export function isRSCRequestCheck(req: BaseNextRequest): boolean {
-  return getRequestMeta(req, 'isRSCRequest') === true
-}

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

@@ -59,7 +59,7 @@ import type {
   NextEnabledDirectories,
   BaseRequestHandler,
 } from './base-server'
-import BaseServer, { NoFallbackError, isRSCRequestCheck } from './base-server'
+import BaseServer, { NoFallbackError } from './base-server'
 import { getMaybePagePath, getPagePath, requireFontManifest } from './require'
 import { denormalizePagePath } from '../shared/lib/page-path/denormalize-page-path'
 import { normalizePagePath } from '../shared/lib/page-path/normalize-page-path'
@@ -1130,7 +1130,10 @@ export default class NextNodeServer extends BaseServer<
 
           if (!routeMatch || isMiddlewareRequest) return
 
-          const isRSC = isRSCRequestCheck(normalizedReq)
+          // NOTE: this is only attached after handle has started, this runs
+          // after the response has been sent, so it should have it set.
+          const isRSC = getRequestMeta(normalizedReq, 'isRSCRequest')
+
           const reqEnd = Date.now()
           const fetchMetrics = normalizedReq.fetchMetrics || []
           const reqDuration = reqEnd - reqStart