feat(gatsby): let serve use getServerData headers

packages/gatsby/cache-dir/page-ssr/index.d.ts

@@ -27,7 +27,12 @@ interface IQueryResult {
 // https://codemix.com/opaque-types-in-javascript/
 type Opaque<K, T> = T & { __TYPE__: K }
 // redacted details as this is meant to be opaque internal type that shouldn't be relied on by integrators (can change any time)
-type ISSRData = Opaque<"ISSRData", {}>
+type ISSRData = Opaque<
+  "ISSRData",
+  {
+    serverDataHeaders?: Record<string, string>
+  }
+>
 
 type PageContext = Record<string, any>
 
@@ -52,6 +57,12 @@ export function getData(args: {
     runQuery(query: string, context: Record<string, any>): Promise<IQueryResult>
     findPageByPath(pathName: string): IGatsbyPage | undefined
   }
+  req: Partial<{
+    query: Record<string, unknown>
+    method: string
+    url: string
+    headers: Record<string, string>
+  }>
 }): Promise<ISSRData>
 
 export function renderPageData(args: {

packages/gatsby/src/commands/serve.ts

@@ -232,12 +232,12 @@ module.exports = async (program: IServeProgram): Promise<void> => {
         program.directory,
         `.cache`,
         `query-engine`
-      ))
+      )) as typeof import("../schema/graphql-engine/entry")
       const { getData, renderPageData, renderHTML } = require(path.join(
         program.directory,
         `.cache`,
         `page-ssr`
-      ))
+      )) as typeof import("../utils/page-ssr-module/entry")
       const graphqlEngine = new GraphQLEngine({
         dbPath: path.join(program.directory, `.cache`, `data`, `datastore`),
       })
@@ -260,6 +260,13 @@ module.exports = async (program: IServeProgram): Promise<void> => {
               req,
             })
             const results = await renderPageData({ data })
+            if (page.mode === `SSR` && data.serverDataHeaders) {
+              for (const [name, value] of Object.entries(
+                data.serverDataHeaders
+              )) {
+                res.setHeader(name, value)
+              }
+            }
             return void res.send(results)
           }
 
@@ -279,6 +286,13 @@ module.exports = async (program: IServeProgram): Promise<void> => {
               req,
             })
             const results = await renderHTML({ data })
+            if (page.mode === `SSR` && data.serverDataHeaders) {
+              for (const [name, value] of Object.entries(
+                data.serverDataHeaders
+              )) {
+                res.setHeader(name, value)
+              }
+            }
             return res.send(results)
           }
 

packages/gatsby/src/utils/page-ssr-module/entry.ts

@@ -16,7 +16,7 @@ import {
 } from "../page-data-helpers"
 // @ts-ignore render-page import will become valid later on (it's marked as external)
 import htmlComponentRenderer from "./render-page"
-import { getServerData } from "../get-server-data"
+import { getServerData, IServerData } from "../get-server-data"
 
 export interface ITemplateDetails {
   query: string
@@ -28,6 +28,7 @@ export interface ISSRData {
   page: IGatsbyPage
   templateDetails: ITemplateDetails
   potentialPagePath: string
+  serverDataHeaders?: Record<string, string>
 }
 
 const pageTemplateDetailsMap: Record<
@@ -71,7 +72,7 @@ export async function getData({
   // 3. Execute query
   // query-runner handles case when query is not there - so maybe we should consider using that somehow
   let results: IExecutionResult = {}
-  let serverData: any = undefined
+  let serverData: IServerData | undefined
   if (templateDetails.query) {
     executionPromises.push(
       graphqlEngine
@@ -104,7 +105,13 @@ export async function getData({
   }
   results.pageContext = page.context
 
-  return { results, page, templateDetails, potentialPagePath }
+  return {
+    results,
+    page,
+    templateDetails,
+    potentialPagePath,
+    serverDataHeaders: serverData?.headers,
+  }
 }
 
 export async function renderPageData({