Update externals stubs atomically

Since babel can run in many workers, and since we write the external stubs as a side-effect of discovering that they're needed from within babel, it's possible that a stub is written at the exact moment another worker is reading it. So we need to make sure our writes are atomic.

packages/core/src/atomic-write.ts

@@ -0,0 +1,14 @@
+import { randomBytes } from 'crypto';
+import { outputFileSync } from 'fs-extra';
+import { renameSync } from 'fs';
+
+// if the path already exists, this will replace it atomically so that nobody
+// will ever read an inconsistent state.
+//
+// if it doesn't already exist, this will create any intervening directories and
+// then create the file.
+export function atomicWrite(path: string, content: string) {
+  let suffix = randomBytes(8).toString('hex');
+  outputFileSync(path + suffix, content);
+  renameSync(path + suffix, path);
+}

packages/core/src/babel-plugin-adjust-imports.ts

@@ -4,11 +4,11 @@ import type { NodePath } from '@babel/traverse';
 import type * as Babel from '@babel/core';
 import type { types as t } from '@babel/core';
 import { PackageCache, Package, V2Package, explicitRelative } from '@embroider/shared-internals';
-import { outputFileSync } from 'fs-extra';
 import { Memoize } from 'typescript-memoize';
 import { compile } from './js-handlebars';
 import { handleImportDeclaration } from './mini-modules-polyfill';
 import { ImportUtil } from 'babel-import-util';
+import { atomicWrite } from './atomic-write';
 
 interface State {
   adjustFile: AdjustFile;
@@ -333,7 +333,7 @@ function handleExternal(specifier: string, sourceFile: AdjustFile, opts: Options
 
 function makeMissingModule(specifier: string, sourceFile: AdjustFile, opts: Options): string {
   let target = join(opts.externalsDir, specifier + '.js');
-  outputFileSync(
+  atomicWrite(
     target,
     dynamicMissingModule({
       moduleName: specifier,
@@ -344,7 +344,7 @@ function makeMissingModule(specifier: string, sourceFile: AdjustFile, opts: Opti
 
 function makeExternal(specifier: string, sourceFile: AdjustFile, opts: Options): string {
   let target = join(opts.externalsDir, specifier + '.js');
-  outputFileSync(
+  atomicWrite(
     target,
     externalTemplate({
       runtimeName: specifier,