From 7d13f07c9e2ee42ef719564d3d936fd8f8e4ad8a Mon Sep 17 00:00:00 2001
From: Michal Piechowiak <misiek.piechowiak@gmail.com>
Date: Sat, 18 Jul 2020 12:23:58 +0200
Subject: [PATCH] feat(socket-io): run processHotPageDataUpdate if modules
 changed

---
 packages/gatsby/cache-dir/loader.js   |  9 +++++++++
 packages/gatsby/cache-dir/socketIo.js | 27 +++++++++++++++++++++++----
 2 files changed, 32 insertions(+), 4 deletions(-)

diff --git a/packages/gatsby/cache-dir/loader.js b/packages/gatsby/cache-dir/loader.js
index 983142d4a2bd6..a24e71ea790fd 100644
--- a/packages/gatsby/cache-dir/loader.js
+++ b/packages/gatsby/cache-dir/loader.js
@@ -555,3 +555,12 @@ export const publicLoader = {
 }
 
 export default publicLoader
+
+// separate export so it gets tree-shaken in production build
+export function processHotPageDataUpdate(pageData) {
+  // We don't need to update `pageDB` or `pageDataDb` because modules
+  // are not stored in page resources. This function only triggers fetching
+  // and updating modules store.
+
+  return instance.fetchModuleDependencies(pageData.moduleDependencies)
+}
diff --git a/packages/gatsby/cache-dir/socketIo.js b/packages/gatsby/cache-dir/socketIo.js
index c645499a31e42..784d208c8ab21 100644
--- a/packages/gatsby/cache-dir/socketIo.js
+++ b/packages/gatsby/cache-dir/socketIo.js
@@ -1,5 +1,6 @@
 import { reportError, clearError } from "./error-overlay-handler"
 import normalizePagePath from "./normalize-page-path"
+import { processHotPageDataUpdate } from "./loader"
 
 let socket = null
 
@@ -31,6 +32,7 @@ export default function socketIo() {
         }
 
         socket.on(`message`, msg => {
+          let shouldEmit = true
           if (msg.type === `staticQueryResult`) {
             if (didDataChange(msg, staticQueryData)) {
               staticQueryData = {
@@ -40,9 +42,26 @@ export default function socketIo() {
             }
           } else if (msg.type === `pageQueryResult`) {
             if (didDataChange(msg, pageQueryData)) {
-              pageQueryData = {
-                ...pageQueryData,
-                [normalizePagePath(msg.payload.id)]: msg.payload.result,
+              if (msg.payload.result) {
+                const currentEntry =
+                  pageQueryData[normalizePagePath(msg.payload.id)]
+
+                // run `processPageData` if modules changed
+                if (
+                  !currentEntry ||
+                  JSON.stringify(currentEntry.moduleDependencies) !==
+                    JSON.stringify(msg.payload.result.moduleDependencies)
+                ) {
+                  shouldEmit = false
+                  processHotPageDataUpdate(msg.payload.result).then(() => {
+                    pageQueryData = {
+                      ...pageQueryData,
+                      [normalizePagePath(msg.payload.id)]: msg.payload.result,
+                    }
+
+                    ___emitter.emit(msg.type, msg.payload)
+                  })
+                }
               }
             }
           } else if (msg.type === `overlayError`) {
@@ -52,7 +71,7 @@ export default function socketIo() {
               clearError(msg.payload.id)
             }
           }
-          if (msg.type && msg.payload) {
+          if (msg.type && msg.payload && shouldEmit) {
             ___emitter.emit(msg.type, msg.payload)
           }
         })