fix(gatsby): fix hashes used in webpack for output

packages/gatsby/src/bootstrap/index.js

@@ -352,13 +352,14 @@ module.exports = async (args: BootstrapArgs) => {
   )
 
   const browserPluginsRequires = browserPlugins
-    .map(
-      plugin =>
-        `{
-      plugin: require('${plugin.resolve}'),
+    .map(plugin => {
+      // we need a relative import path to keep contenthash the same if directory changes
+      const relativePluginPath = path.relative(siteDir, plugin.resolve)
+      return `{
+      plugin: require('${slash(relativePluginPath)}'),
       options: ${JSON.stringify(plugin.options)},
     }`
-    )
+    })
     .join(`,`)
 
   const browserAPIRunner = `module.exports = [${browserPluginsRequires}]\n`

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

@@ -1,6 +1,8 @@
 const _ = require(`lodash`)
+const path = require(`path`)
 const fs = require(`fs-extra`)
 const crypto = require(`crypto`)
+const slash = require(`slash`)
 const { store, emitter } = require(`../redux/`)
 const reporter = require(`gatsby-cli/lib/reporter`)
 const { match } = require(`@reach/router/lib/utils`)
@@ -133,12 +135,17 @@ const preferDefault = m => m && m.default || m
 const preferDefault = m => m && m.default || m
 \n`
   asyncRequires += `exports.components = {\n${components
-    .map(
-      c =>
-        `  "${c.componentChunkName}": () => import("${joinPath(
-          c.component
-        )}" /* webpackChunkName: "${c.componentChunkName}" */)`
-    )
+    .map(c => {
+      // we need a relative import path to keep contenthash the same if directory changes
+      const relativeComponentPath = path.relative(
+        path.join(program.directory, `.cache`),
+        c.component
+      )
+
+      return `  "${c.componentChunkName}": () => import("${slash(
+        relativeComponentPath
+      )}" /* webpackChunkName: "${c.componentChunkName}" */)`
+    })
     .join(`,\n`)}
 }\n\n`
 

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

@@ -459,7 +459,10 @@ module.exports = async ({
         loaders.css({ ...options, importLoaders: 1 }),
         loaders.postcss({ browsers }),
       ]
-      if (!isSSR) use.unshift(loaders.miniCssExtract({ hmr: !options.modules }))
+      if (!isSSR)
+        use.unshift(
+          loaders.miniCssExtract({ hmr: !PRODUCTION && !options.modules })
+        )
 
       return {
         use,

packages/gatsby/src/utils/webpack.config.js

@@ -469,6 +469,10 @@ module.exports = async (
       runtimeChunk: {
         name: `webpack-runtime`,
       },
+      // use hashes instead of ids for module identifiers
+      // TODO update to deterministic in webpack 5 (hashed is deprecated)
+      // @see https://webpack.js.org/guides/caching/#module-identifiers
+      moduleIds: `hashed`,
       splitChunks: {
         name: false,
         chunks: `all`,
@@ -497,6 +501,8 @@ module.exports = async (
             test: /\.(css|scss|sass|less|styl)$/,
             chunks: `all`,
             enforce: true,
+            // this rule trumps all other rules because of the priority.
+            priority: 10,
           },
         },
       },