Move Vite manualChunks SSR workaround into Plugin

We use the plugin to work around a bug in Vite 2 which has been fixed in
Vite 3 that breaks the SSR build if the manualChunks Rollup config is
used.

See vitejs/vite#8836

RescriptRelayVitePlugin.mjs

@@ -85,6 +85,23 @@ export let rescriptRelayVitePlugin = ({
 
   return {
     name: "rescript-relay",
+    /**
+     * Workaround until we can upgrade to Vite 3.
+     *
+     * Remove manualChunks if this is SSR, since it doesn't work in SSR mode.
+     * See https://github.com/vitejs/vite/issues/8836
+     */
+    config(userConfig) {
+      //
+      if (
+        Boolean(userConfig.build.ssr) &&
+        userConfig.build?.rollupOptions?.output?.manualChunks != null
+      ) {
+        delete userConfig.build.rollupOptions.output.manualChunks;
+      }
+
+      return userConfig;
+    },
     /**
      * @param {ResolvedConfig} resolvedConfig
      */

package.json

@@ -11,7 +11,7 @@
     "build:rescript": "rescript build -with-deps",
     "build:vite": "run-s build:vite:*",
     "build:vite:client": "vite build --outDir dist/client --ssrManifest --manifest",
-    "build:vite:server": "cross-env IS_VITE_SSR=1 vite build --outDir dist/server --ssr src/EntryServer.mjs",
+    "build:vite:server": "vite build --outDir dist/server --ssr src/EntryServer.mjs",
     "build:vite:server-fix": "perl -i -pe 's/import \\* as ReactRelay/import ReactRelay/' dist/server/EntryServer.js",
     "preview": "cross-env ENABLE_FILESERVER=true yarn start",
     "start": "cross-env NODE_ENV=production node Server.mjs",

vite.config.js

@@ -24,20 +24,11 @@ export default defineConfig({
       plugins: [visualizer()],
       output: {
         format: "esm",
-        // Only enable output chunking for our client bundle.
-        // At the time of writing Vite does not allow us to know when --ssr
-        // is passed so we use a custom env variable.
-        ...(
-          process.env.IS_VITE_SSR === "1"
-            ? {}
-            : {
-              manualChunks: {
-                react: ["react", "react-dom"],
-                relay: ["react-relay", "relay-runtime"],
-                vendor: ["react-helmet"],
-              }
-            }
-        ),
+        manualChunks: {
+          react: ["react", "react-dom"],
+          relay: ["react-relay", "relay-runtime"],
+          vendor: ["react-helmet"],
+        },
       },
     },
   },