fix: re-implement loadPageDataSync for onRenderBody in gatsby-ssr

packages/gatsby/cache-dir/ssr-builtin-trackers/tracking-unsafe-module-wrapper.js

@@ -12,7 +12,12 @@ function createProxyHandler(prefix) {
           }
           Error.captureStackTrace(myErrorHolder, wrapper)
 
-          global.unsafeBuiltinUsage.push(myErrorHolder.stack)
+          // loadPageDataSync already is tracked with dedicated warning messages,
+          // so skipping marking it to avoid multiple messages for same usage
+          if (!myErrorHolder.stack.includes(`loadPageDataSync`)) {
+            global.unsafeBuiltinUsage.push(myErrorHolder.stack)
+          }
+
           return value.apply(target, args)
         }
       } else if (typeof value === `object` && value !== null) {

packages/gatsby/cache-dir/static-entry.js

@@ -2,13 +2,9 @@ const React = require(`react`)
 const path = require(`path`)
 const { renderToString, renderToStaticMarkup } = require(`react-dom/server`)
 const { ServerLocation, Router, isRedirect } = require(`@reach/router`)
-const {
-  merge,
-
-  flattenDeep,
-  replace,
-} = require(`lodash`)
+const { merge, flattenDeep, replace } = require(`lodash`)
 const { StaticQueryContext } = require(`gatsby`)
+const fs = require(`fs`)
 
 const { RouteAnnouncerProps } = require(`./route-announcer-props`)
 const apiRunner = require(`./api-runner-ssr`)
@@ -62,10 +58,6 @@ const getStaticQueryUrl = hash =>
 const getAppDataUrl = () =>
   `${__PATH_PREFIX__}/${join(`page-data`, `app-data.json`)}`
 
-function loadPageDataSync() {
-  throw new Error(`"loadPageDataSync" is no longer available`)
-}
-
 const createElement = React.createElement
 
 export const sanitizeComponents = components => {
@@ -121,6 +113,29 @@ export default ({
     let postBodyComponents = []
     let bodyProps = {}
 
+    function loadPageDataSync(_pagePath) {
+      if (_pagePath === pagePath) {
+        // no need to use fs if we are asking for pageData of current page
+        return pageData
+      }
+
+      const pageDataPath = getPageDataPath(_pagePath)
+      const pageDataFile = join(process.cwd(), `public`, pageDataPath)
+      try {
+        // deprecation notice
+        const myErrorHolder = {
+          name: `Usage of loadPageDataSync for page other than currently generated page disables incremental html generation in future builds`,
+        }
+        Error.captureStackTrace(myErrorHolder, loadPageDataSync)
+        global.unsafeBuiltinUsage.push(myErrorHolder.stack)
+        const pageDataJson = fs.readFileSync(pageDataFile)
+        return JSON.parse(pageDataJson)
+      } catch (error) {
+        // not an error if file is not found. There's just no page data
+        return null
+      }
+    }
+
     const replaceBodyHTMLString = body => {
       bodyHtml = body
     }