feat(gatsby): Add defer to createPage

packages/gatsby/src/bootstrap/requires-writer.ts

@@ -108,8 +108,14 @@ const getMatchPaths = (
   const matchPathPages: Array<IMatchPathEntry> = []
 
   pages.forEach((page: IGatsbyPage, index: number): void => {
-    if (page.matchPath) {
-      matchPathPages.push(createMatchPathEntry(page, index))
+    if (_CFLAGS_.GATSBY_MAJOR === `4`) {
+      if (page.matchPath && page.mode === `SSG`) {
+        matchPathPages.push(createMatchPathEntry(page, index))
+      }
+    } else {
+      if (page.matchPath) {
+        matchPathPages.push(createMatchPathEntry(page, index))
+      }
     }
   })
 

packages/gatsby/src/commands/build-utils.ts

@@ -155,14 +155,27 @@ export function calcDirtyHtmlFiles(state: IGatsbyState): {
   }
 
   state.html.trackedHtmlFiles.forEach(function (htmlFile, path) {
-    if (htmlFile.isDeleted || !state.pages.has(path)) {
+    const page = state.pages.get(path)
+    if (htmlFile.isDeleted || !page) {
       // FIXME: checking pages state here because pages are not persisted
       // and because of that `isDeleted` might not be set ...
       markActionForPage(path, `delete`)
-    } else if (htmlFile.dirty || state.html.unsafeBuiltinWasUsedInSSR) {
-      markActionForPage(path, `regenerate`)
     } else {
-      markActionForPage(path, `reuse`)
+      if (_CFLAGS_.GATSBY_MAJOR === `4`) {
+        if (page.mode === `SSG`) {
+          if (htmlFile.dirty || state.html.unsafeBuiltinWasUsedInSSR) {
+            markActionForPage(path, `regenerate`)
+          } else {
+            markActionForPage(path, `reuse`)
+          }
+        }
+      } else {
+        if (htmlFile.dirty || state.html.unsafeBuiltinWasUsedInSSR) {
+          markActionForPage(path, `regenerate`)
+        } else {
+          markActionForPage(path, `reuse`)
+        }
+      }
     }
   })
 

packages/gatsby/src/query/index.ts

@@ -259,6 +259,12 @@ function createPageQueryJob(
   }
 
   const { path, componentPath, context } = page
+  if (_CFLAGS_.GATSBY_MAJOR === `4`) {
+    const { mode } = page
+    if (mode !== `SSG`) {
+      return undefined
+    }
+  }
   const { query } = component
 
   return {

packages/gatsby/src/query/query-runner.ts

@@ -157,6 +157,9 @@ export async function queryRunner(
     delete result.pageContext.componentPath
     delete result.pageContext.context
     delete result.pageContext.isCreatedByStatefulCreatePages
+    if (_CFLAGS_.GATSBY_MAJOR === `4`) {
+      delete result.pageContext.mode
+    }
   }
 
   const resultJSON = JSON.stringify(result)

packages/gatsby/src/redux/actions/public.js

@@ -97,8 +97,11 @@ type PageInput = {
   component: string,
   context?: Object,
   ownerNodeId?: string,
+  defer?: boolean,
 }
 
+type PageMode = "SSG" | "DSR" | "SSR"
+
 type Page = {
   path: string,
   matchPath: ?string,
@@ -108,6 +111,7 @@ type Page = {
   componentChunkName: string,
   updatedAt: number,
   ownerNodeId?: string,
+  mode: PageMode,
 }
 
 type ActionOptions = {
@@ -407,6 +411,14 @@ ${reservedFields.map(f => `  * "${f}"`).join(`\n`)}
     pluginCreatorId: plugin.id ?? ``,
   }
 
+  if (_CFLAGS_.GATSBY_MAJOR === `4`) {
+    let pageMode: PageMode = `SSG`
+    if (page.defer) {
+      pageMode = `DSR`
+    }
+    internalPage.mode = pageMode
+  }
+
   if (page.ownerNodeId) {
     internalPage.ownerNodeId = page.ownerNodeId
   }

packages/gatsby/src/redux/types.ts

@@ -25,6 +25,8 @@ export enum ProgramStatus {
   BOOTSTRAP_QUERY_RUNNING_FINISHED = `BOOTSTRAP_QUERY_RUNNING_FINISHED`,
 }
 
+export type PageMode = "SSG" | "DSR" | "SSR"
+
 export interface IGatsbyPage {
   internalComponentName: string
   path: string
@@ -39,6 +41,7 @@ export interface IGatsbyPage {
   pluginCreatorId: Identifier
   componentPath: SystemPath
   ownerNodeId: Identifier
+  mode: PageMode
 }
 
 export interface IGatsbyFunction {

packages/gatsby/src/utils/page-data.ts

@@ -211,8 +211,9 @@ export async function flush(): Promise<void> {
     // This is why we need this check
     if (page) {
       if (
-        program?._?.[0] === `develop` &&
-        process.env.GATSBY_EXPERIMENTAL_QUERY_ON_DEMAND
+        (program?._?.[0] === `develop` &&
+          process.env.GATSBY_EXPERIMENTAL_QUERY_ON_DEMAND) ||
+        (_CFLAGS_.GATSBY_MAJOR === `4` ? page.mode !== `SSG` : false)
       ) {
         // check if already did run query for this page
         // with query-on-demand we might have pending page-data write due to