Provisionally support using prebuilt shell binaries via DotSlash (#53436)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/53436

Changelog: [Internal]

Adds a `flavor` option to `unstable_spawnDebuggerShellWithArgs` to select between two modes:

1. `flavor: 'dev'` (current behaviour) - launching a stock Electron binary (from the `electron` package) and pointing it directly at the shell code from the `src/electron` directory.
2. `flavor: 'prebuilt'` (new in this diff) - launching the prebuilt React Native DevTools binary included in the package (built continuously at Meta and committed as a DotSlash file in automated diffs e.g. D79836825). Note that this binary includes Electron *and* a frozen version of the shell code from `src/electron`.

Going forward, `'dev'` will only be used when developing the package (e.g. in D78351934 we will move `electron` to `devDependencies`). The published version of the package is only intended to work with `flavor: 'prebuilt'`.

Reviewed By: huntie

Differential Revision: D78351931

fbshipit-source-id: d0e66b54c142dc2910619ba3d6d149d88324c872

packages/debugger-shell/src/node/__tests__/debugger-shell-test.js

@@ -0,0 +1,48 @@
+/**
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @flow strict-local
+ * @format
+ * @oncall react_native
+ */
+
+const {unstable_spawnDebuggerShellWithArgs} = require('../../');
+
+describe('debugger-shell Node package', () => {
+  test('can spawn in detached+prebuilt mode without crashing', async () => {
+    await expect(
+      unstable_spawnDebuggerShellWithArgs(['--version'], {
+        flavor: 'prebuilt',
+        mode: 'detached',
+      }),
+    ).resolves.toBeUndefined();
+  });
+
+  // When running in the internal react-native-oss-js job, Electron isn't
+  // installed correctly (postinstall scripts don't run) but the internal
+  // `electron` workspace isn't available either. Detecting this dynamically
+  // weakens the test somewhat in environments where it *should* pass, but this
+  // is a dev-only feature anyway so this is fine.
+  if (isElectronInstalled()) {
+    test('can spawn in detached+dev mode without crashing', async () => {
+      await expect(
+        unstable_spawnDebuggerShellWithArgs(['--version'], {
+          flavor: 'dev',
+          mode: 'detached',
+        }),
+      ).resolves.toBeUndefined();
+    });
+  }
+});
+
+function isElectronInstalled() {
+  try {
+    require('electron');
+    return true;
+  } catch {
+    return false;
+  }
+}

packages/debugger-shell/src/node/index.flow.js

@@ -9,43 +9,43 @@
  */
 
 const {spawn} = require('cross-spawn');
+const path = require('path');
+
+// The 'prebuilt' flavor will use the prebuilt shell binary (and the JavaScript embedded in it).
+// The 'dev' flavor will use a stock Electron binary and run the shell code from the `electron/` directory.
+type DebuggerShellFlavor = 'prebuilt' | 'dev';
 
 async function unstable_spawnDebuggerShellWithArgs(
   args: string[],
   {
     mode = 'detached',
+    flavor = 'dev',
   }: $ReadOnly<{
     // In 'syncAndExit' mode, the current process will block until the spawned process exits, and then it will exit
     // with the same exit code as the spawned process.
     // In 'detached' mode, the spawned process will be detached from the current process and the current process will
     // continue to run normally.
     mode?: 'syncThenExit' | 'detached',
+    flavor?: DebuggerShellFlavor,
   }> = {},
 ): Promise<void> {
-  // NOTE: Internally at Meta, this is aliased to a workspace that is
-  // API-compatible with the 'electron' package, but contains prebuilt binaries
-  // that do not need to be downloaded in a postinstall action.
-  const electronPath = require('electron');
+  const [binaryPath, baseArgs] = getShellBinaryAndArgs(flavor);
 
   return new Promise((resolve, reject) => {
-    const child = spawn(
-      electronPath,
-      [require.resolve('../electron'), ...args],
-      {
-        stdio: 'inherit',
-        windowsHide: true,
-        detached: mode === 'detached',
-      },
-    );
+    const child = spawn(binaryPath, [...baseArgs, ...args], {
+      stdio: 'inherit',
+      windowsHide: true,
+      detached: mode === 'detached',
+    });
     if (mode === 'detached') {
       child.on('spawn', () => {
         resolve();
       });
-      child.on('close', (code /*: number */) => {
+      child.on('close', (code: number) => {
         if (code !== 0) {
           reject(
             new Error(
-              `Failed to open debugger shell: ${electronPath} exited with code ${code}`,
+              `Failed to open debugger shell: exited with code ${code}`,
             ),
           );
         }
@@ -54,7 +54,7 @@ async function unstable_spawnDebuggerShellWithArgs(
     } else if (mode === 'syncThenExit') {
       child.on('close', function (code, signal) {
         if (code === null) {
-          console.error(electronPath, 'exited with signal', signal);
+          console.error('Debugger shell exited with signal', signal);
           process.exit(1);
         }
         process.exit(code);
@@ -74,4 +74,28 @@ async function unstable_spawnDebuggerShellWithArgs(
   });
 }
 
+function getShellBinaryAndArgs(
+  flavor: DebuggerShellFlavor,
+): [string, Array<string>] {
+  switch (flavor) {
+    case 'prebuilt':
+      return [
+        // $FlowIssue[cannot-resolve-module] fb-dotslash includes Flow types but Flow does not pick them up
+        require('fb-dotslash'),
+        [path.join(__dirname, '../../bin/react-native-devtools')],
+      ];
+    case 'dev':
+      return [
+        // NOTE: Internally at Meta, this is aliased to a workspace that is
+        // API-compatible with the 'electron' package, but contains prebuilt binaries
+        // that do not need to be downloaded in a postinstall action.
+        require('electron'),
+        [require.resolve('../electron')],
+      ];
+    default:
+      flavor as empty;
+      throw new Error(`Unknown flavor: ${flavor}`);
+  }
+}
+
 export {unstable_spawnDebuggerShellWithArgs};