refactor(android): stream multipart bundle bodies straight to disk

[AndreiCalazans]

Summary:

We have a very large developer bundle which on a stock Android emulator often causes OOMs while loading the bundle (#52818). You will see below that there is a spike in memory when we are downloading the bundle that seems to go up 1.4x the bundle size and exacerbates the problem. This PR streams the multipart bundle bodies straight to disk to avoid this peak memory.

Before:

Note in the video the spike to 433.8 MB

before-stream-to-disk-react-native.mov

After:

With this implementation memory is contained and during the download phase does not peak past 83.4 MB only taking 316.6 MB after the download phase is finished.

after-stream-to-disk-react-native.mov

Note that we have been using this prior version that I made PR for in January because we are on older RN version.

Changelog:

[ANDROID] [CHANGED] Refactored stream multipart bundle bodies to write straight to disk

Test Plan:

Create the bundle padding mechanism to create a bundle with artificial 100 MB in size.

diff --git a/packages/rn-tester/metro.config.js b/packages/rn-tester/metro.config.js
@@ -14,6 +14,36 @@ const {getDefaultConfig} = require('@react-native/metro-config');
 const {mergeConfig} = require('metro-config');
 const path = require('path');

+const padMB = parseInt(process.env.RN_PAD_BUNDLE_MB || '0', 10);
+// Metro 0.81+ enforces strict package `exports`; `./private/*` re-exports `./src/*.js`.
+const baseJSBundle = require('metro/private/DeltaBundler/Serializers/baseJSBundle');
+const bundleToString = require('metro/private/lib/bundleToString');
+const baseJSBundleFn = baseJSBundle.default || baseJSBundle;
+const bundleToStringFn = bundleToString.default || bundleToString;
+
+function buildPaddedSerializer(padBytes) {
+  const overhead = '\n/* RN_PAD_BUNDLE  */\n'.length;
+  const filler = 'A'.repeat(Math.max(0, padBytes - overhead));
+  const suffix = '\n/* RN_PAD_BUNDLE ' + filler + ' */\n';
+  return (entryPoint, preModules, graph, options) => {
+    const bundle = baseJSBundleFn(entryPoint, preModules, graph, options);
+    const result = bundleToStringFn(bundle);
+    return {code: result.code + suffix, map: result.map};
+  };
+}
+
 const config = {
   watchFolders: [ /* ... */ ],
   resolver: { /* ... */ },
+  ...(padMB > 0
+    ? {serializer: {customSerializer: buildPaddedSerializer(padMB * 1024 * 1024)}}
+    : null),
 };

1. yarn android
2. Stop the Metro bundle and start it yourself with the padding
   RN_PAD_BUNDLE_MB=100 yarn start --reset-cache
3. Run app

Run tests as well

./gradlew :packages:react-native:ReactAndroid:testDebugUnitTest \     
     -PrunPerfTests=true -Preact.internal.useHermesNightly=true \
     --tests "*PerfTest"