Internal refactor of getDocumentHeaders for re-use with RSC (#13639)

Author: brophdawg11

.changeset/nice-files-grow.md

@@ -0,0 +1,5 @@
+---
+"react-router": patch
+---
+
+[INTERNAL] Slight refactor of internal `headers()` function processing for use with RSC

packages/react-router/lib/server-runtime/headers.ts

@@ -1,12 +1,26 @@
 import { splitCookiesString } from "set-cookie-parser";
 
-import type { ServerBuild } from "./build";
+import type { DataRouteMatch } from "../context";
 import type { StaticHandlerContext } from "../router/router";
+import type { ServerRouteModule } from "../dom/ssr/routeModules";
+import type { ServerBuild } from "./build";
 import invariant from "./invariant";
 
+// Version used by v7 framework mode
 export function getDocumentHeaders(
   build: ServerBuild,
   context: StaticHandlerContext
+): Headers {
+  return getDocumentHeadersImpl(context, (m) => {
+    let route = build.routes[m.route.id];
+    invariant(route, `Route with id "${m.route.id}" not found in build`);
+    return route.module.headers;
+  });
+}
+
+function getDocumentHeadersImpl(
+  context: StaticHandlerContext,
+  getRouteHeadersFn: (match: DataRouteMatch) => ServerRouteModule["headers"]
 ): Headers {
   let boundaryIdx = context.errors
     ? context.matches.findIndex((m) => context.errors![m.route.id])
@@ -38,9 +52,6 @@ export function getDocumentHeaders(
 
   return matches.reduce((parentHeaders, match, idx) => {
     let { id } = match.route;
-    let route = build.routes[id];
-    invariant(route, `Route with id "${id}" not found in build`);
-    let routeModule = route.module;
     let loaderHeaders = context.loaderHeaders[id] || new Headers();
     let actionHeaders = context.actionHeaders[id] || new Headers();
 
@@ -56,8 +67,10 @@ export function getDocumentHeaders(
       errorHeaders !== loaderHeaders &&
       errorHeaders !== actionHeaders;
 
+    let headersFn = getRouteHeadersFn(match);
+
     // Use the parent headers for any route without a `headers` export
-    if (routeModule.headers == null) {
+    if (headersFn == null) {
       let headers = new Headers(parentHeaders);
       if (includeErrorCookies) {
         prependCookies(errorHeaders!, headers);
@@ -68,16 +81,14 @@ export function getDocumentHeaders(
     }
 
     let headers = new Headers(
-      routeModule.headers
-        ? typeof routeModule.headers === "function"
-          ? routeModule.headers({
-              loaderHeaders,
-              parentHeaders,
-              actionHeaders,
-              errorHeaders: includeErrorHeaders ? errorHeaders : undefined,
-            })
-          : routeModule.headers
-        : undefined
+      typeof headersFn === "function"
+        ? headersFn({
+            loaderHeaders,
+            parentHeaders,
+            actionHeaders,
+            errorHeaders: includeErrorHeaders ? errorHeaders : undefined,
+          })
+        : headersFn
     );
 
     // Automatically preserve Set-Cookie headers from bubbled responses,