feat(build): builds use a chunk group hash

webpackCompilationHash has been replaced with a generated named chunk group hash. This reduces that
blast radius of changed built files between builds

packages/gatsby/src/commands/build.js

@@ -102,6 +102,41 @@ module.exports = async function build(program: BuildArgs) {
     })
     activity.start()
 
+    const { namedChunkGroups, chunks } = stats.toJson({
+      all: false,
+      chunkGroups: true,
+      chunks: true,
+    })
+
+    // Create a chunkGroupHash per namedChunkGroups entry
+
+    const getHashForChunk = chunkId => {
+      const chunk = chunks.find(({ id }) => id === chunkId)
+      return chunk && chunk.hash
+    }
+    const getHashForNamedChunk = ({ chunks }) =>
+      chunks.reduce(
+        (namedChunkHash, chunkId) =>
+          `${namedChunkHash}${getHashForChunk(chunkId)}`,
+        ``
+      )
+
+    // Get app chunk hash as that will be needed for all other named chunk groups
+
+    // Construct hashes for remaining named chunk groups
+    const namedChunkHashes = Object.keys(namedChunkGroups).reduce(
+      (groupHashes, key) => {
+        groupHashes[key] = `${getHashForNamedChunk(namedChunkGroups[key])}`
+        return groupHashes
+      },
+      {}
+    )
+
+    store.dispatch({
+      type: `SET_WEBPACK_CHUNK_GROUP_HASHES`,
+      payload: namedChunkHashes,
+    })
+
     // We need to update all page-data.json files with the new
     // compilation hash. As a performance optimization however, we
     // don't update the files for `pageQueryIds` (dirty queries),
@@ -113,7 +148,7 @@ module.exports = async function build(program: BuildArgs) {
     await pageDataUtil.updateCompilationHashes(
       { publicDir, workerPool },
       cleanPagePaths,
-      webpackCompilationHash
+      namedChunkHashes
     )
 
     activity.end()

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

@@ -28,7 +28,7 @@ module.exports = async (queryJob: QueryJob) => {
     schema,
     schemaCustomization,
     program,
-    webpackCompilationHash,
+    webpackChunkGroupHashes,
   } = store.getState()
 
   const graphql = (query, context) =>
@@ -105,7 +105,7 @@ ${formatErrorDetails(errorDetails)}`)
         { publicDir },
         page,
         result,
-        webpackCompilationHash
+        webpackChunkGroupHashes[page.componentChunkName]
       )
     } else {
       // The babel plugin is hard-coded to load static queries from

packages/gatsby/src/redux/__tests__/__snapshots__/index.js.snap

@@ -20,6 +20,7 @@ Object {
   "status": Object {
     "plugins": Object {},
   },
+  "webpackChunkGroupHashes": "",
   "webpackCompilationHash": "",
 }
 `;

packages/gatsby/src/redux/index.js

@@ -60,6 +60,7 @@ const saveState = () => {
     `components`,
     `staticQueryComponents`,
     `webpackCompilationHash`,
+    `webpackChunkGroupHashes`,
   ])
 
   return writeToCache(pickedState)

packages/gatsby/src/redux/reducers/index.js

@@ -38,6 +38,7 @@ module.exports = {
   jobs: require(`./jobs`),
   webpack: require(`./webpack`),
   webpackCompilationHash: require(`./webpack-compilation-hash`),
+  webpackChunkGroupHashes: require(`./webpack-chunk-group-hashes`),
   redirects: require(`./redirects`),
   babelrc: require(`./babelrc`),
   schemaCustomization: require(`./schema-customization`),

packages/gatsby/src/redux/reducers/webpack-chunk-group-hashes.js

@@ -0,0 +1,8 @@
+module.exports = (state = ``, action) => {
+  switch (action.type) {
+    case `SET_WEBPACK_CHUNK_GROUP_HASHES`:
+      return action.payload
+    default:
+      return state
+  }
+}

packages/gatsby/src/utils/gatsby-webpack-stats-extractor.js

@@ -27,9 +27,10 @@ class GatsbyWebpackStatsExtractor {
         }
       }
       const webpackStats = {
-        ...stats.toJson({ all: false, chunkGroups: true }),
+        ...stats.toJson({ all: false, chunkGroups: true, chunks: true }),
         assetsByChunkName: assets,
       }
+
       fs.writeFile(
         path.join(`public`, `chunk-map.json`),
         JSON.stringify(assetsMap),

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

@@ -28,15 +28,12 @@ const write = async ({ publicDir }, page, result, webpackCompilationHash) => {
 const updateCompilationHashes = (
   { publicDir, workerPool },
   pagePaths,
-  webpackCompilationHash
+  namedChunkHashes
 ) => {
   const segments = chunk(pagePaths, 50)
+  console.log(`Passed namedChunkHashes:: `, JSON.stringify(namedChunkHashes))
   return Promise.map(segments, segment =>
-    workerPool.updateCompilationHashes(
-      { publicDir },
-      segment,
-      webpackCompilationHash
-    )
+    workerPool.updateCompilationHashes({ publicDir }, segment, namedChunkHashes)
   )
 }
 

packages/gatsby/src/utils/worker/page-data.js

@@ -7,22 +7,18 @@ const getFilePath = ({ publicDir }, pagePath) => {
   return path.join(publicDir, `page-data`, fixedPagePath, `page-data.json`)
 }
 
-const updateJsonFileField = async (filename, fieldname, value) => {
+const updateJsonFileField = async (filename, fieldname, namedChunkHashes) => {
   const object = JSON.parse(await fs.readFile(filename, `utf-8`))
-  object[fieldname] = value
+  object[fieldname] = namedChunkHashes[object.componentChunkName]
   await fs.outputFile(filename, JSON.stringify(object), `utf-8`)
 }
 
-const updateCompilationHashes = (
-  { publicDir },
-  pagePaths,
-  webpackCompilationHash
-) =>
+const updateCompilationHashes = ({ publicDir }, pagePaths, namedChunkHashes) =>
   Promise.map(pagePaths, pagePath =>
     updateJsonFileField(
       getFilePath({ publicDir }, pagePath),
       `webpackCompilationHash`,
-      webpackCompilationHash
+      namedChunkHashes
     )
   )