Generate source map for server actions in ESM loader

We use the webpack-sources dependency as a utility but it doesn't
use anything from webpack and it's a small dep. Could maybe inline into
build.

Author: sebmarkbage

fixtures/flight-esm/package.json

@@ -13,14 +13,15 @@
     "prompts": "^2.4.2",
     "react": "experimental",
     "react-dom": "experimental",
-    "undici": "^5.20.0"
+    "undici": "^5.20.0",
+    "webpack-sources": "^3.2.0"
   },
   "scripts": {
     "predev": "cp -r ../../build/oss-experimental/* ./node_modules/",
     "prestart": "cp -r ../../build/oss-experimental/* ./node_modules/",
     "dev": "concurrently \"npm run dev:region\" \"npm run dev:global\"",
     "dev:global": "NODE_ENV=development BUILD_PATH=dist node server/global",
-    "dev:region": "NODE_ENV=development BUILD_PATH=dist nodemon --watch src --watch dist -- --experimental-loader ./loader/region.js --conditions=react-server server/region",
+    "dev:region": "NODE_ENV=development BUILD_PATH=dist nodemon --watch src --watch dist -- --enable-source-maps --experimental-loader ./loader/region.js --conditions=react-server server/region",
     "start": "concurrently \"npm run start:region\" \"npm run start:global\"",
     "start:global": "NODE_ENV=production node server/global",
     "start:region": "NODE_ENV=production node --experimental-loader ./loader/region.js --conditions=react-server server/region"

fixtures/flight-esm/server/region.js

@@ -141,13 +141,7 @@ if (process.env.NODE_ENV === 'development') {
 
       const sourceMap = nodeModule.findSourceMap(requestedFilePath);
       let map;
-      // There are two ways to return a source map depending on what we observe in error.stack.
-      // A real app will have a similar choice to make for which strategy to pick.
-      if (!sourceMap || Error.prepareStackTrace === undefined) {
-        // When --enable-source-maps is enabled, the error.stack that we use to track
-        // stacks will have had the source map already applied so it's pointing to the
-        // original source. We return a blank source map that just maps everything to
-        // the original source in this case.
+      if (!sourceMap) {
         const sourceContent = await readFile(requestedFilePath, 'utf8');
         const lines = sourceContent.split('\n').length;
         map = {
@@ -161,13 +155,6 @@ if (process.env.NODE_ENV === 'development') {
           sourceRoot: '',
         };
       } else {
-        // If something has overridden prepareStackTrace it is likely not getting the
-        // natively applied source mapping to error.stack and so the line will point to
-        // the compiled output similar to how a browser works.
-        // E.g. ironically this can happen with the source-map-support library that is
-        // auto-invoked by @babel/register if external source maps are generated.
-        // In this case we just use the source map that the native source mapping would
-        // have used.
         map = sourceMap.payload;
       }
       res.write(JSON.stringify(map));

fixtures/flight-esm/yarn.lock

@@ -755,6 +755,11 @@ vary@~1.1.2:
   resolved "https://registry.yarnpkg.com/vary/-/vary-1.1.2.tgz#2299f02c6ded30d4a5961b0b9f74524a18f634fc"
   integrity sha512-BNGbWLfd0eUPabhkXUVm0j8uuvREyTh5ovRa/dyow/BqAbZJyC+5fU+IzQOzmAKzYqYRAISoRhdQr3eIZ/PXqg==
 
+webpack-sources@^3.2.0:
+  version "3.2.3"
+  resolved "https://registry.yarnpkg.com/webpack-sources/-/webpack-sources-3.2.3.tgz#2d4daab8451fd4b240cc27055ff6a0c2ccea0cde"
+  integrity sha512-/DyMEOrDgLKKIG0fmvtz+4dUX/3Ghozwgm6iPp8KRhvn+eQf9+Q7GWxVNMk3+uCPWfdXYC4ExGBckIXdFEfH1w==
+
 wrap-ansi@^7.0.0:
   version "7.0.0"
   resolved "https://registry.yarnpkg.com/wrap-ansi/-/wrap-ansi-7.0.0.tgz#67e145cff510a6a6984bdf1152911d69d2eb9e43"

packages/react-server-dom-esm/package.json

@@ -58,6 +58,7 @@
     "react-dom": "^19.0.0"
   },
   "dependencies": {
-    "acorn-loose": "^8.3.0"
+    "acorn-loose": "^8.3.0",
+    "webpack-sources": "^3.2.0"
   }
 }