feat: support ESM (#270)

sources/corepackUtils.ts

@@ -2,6 +2,7 @@ import {createHash}                                            from 'crypto';
 import {once}                                                  from 'events';
 import fs                                                      from 'fs';
 import type {Dir}                                              from 'fs';
+import Module                                                  from 'module';
 import path                                                    from 'path';
 import semver                                                  from 'semver';
 
@@ -214,5 +215,10 @@ export async function runVersion(locator: Locator, installSpec: { location: stri
   ];
   process.execArgv = [];
 
-  return nodeUtils.loadMainModule(binPath);
+  // Unset the mainModule and let Node.js set it when needed.
+  process.mainModule = undefined;
+
+  // Use nextTick to unwind the stack, and consequently remove Corepack from
+  // the stack trace of the package manager.
+  process.nextTick(Module.runMain, binPath);
 }

sources/nodeUtils.ts

@@ -1,30 +1,3 @@
-import Module from 'module';
-import path   from 'path';
-
-/**
- * Loads a module as a main module, enabling the `require.main === module` pattern.
- */
-export function loadMainModule(id: string): void {
-  const modulePath = Module._resolveFilename(id, null, true);
-
-  const module = new Module(modulePath, undefined);
-
-  module.filename = modulePath;
-  module.paths = Module._nodeModulePaths(path.dirname(modulePath));
-
-  Module._cache[modulePath] = module;
-
-  process.mainModule = module;
-  module.id = `.`;
-
-  try {
-    return module.load(modulePath);
-  } catch (error) {
-    delete Module._cache[modulePath];
-    throw error;
-  }
-}
-
 export interface NodeError extends Error {
   code: string;
 }

tests/main.test.ts

@@ -560,7 +560,10 @@ it(`should not override the package manager exit code`, async () => {
   });
 });
 
-it(`should not override the package manager exit code when it throws`, async () => {
+it(`should not preserve the process.exitCode when a package manager throws`, async () => {
+  // Node.js doesn't preserve process.exitCode when an exception is thrown
+  // so we need to make sure we don't break this behaviour.
+
   await xfs.mktempPromise(async cwd => {
     await xfs.writeJsonPromise(ppath.join(cwd, `package.json` as Filename), {
       packageManager: `yarn@2.2.2`,
@@ -575,9 +578,9 @@ it(`should not override the package manager exit code when it throws`, async ()
     `);
 
     await expect(runCli(cwd, [`yarn`, `--version`])).resolves.toMatchObject({
-      exitCode: 42,
-      stdout: expect.stringContaining(`foo`),
-      stderr: ``,
+      exitCode: 1,
+      stdout: ``,
+      stderr: expect.stringContaining(`foo`),
     });
   });
 });
@@ -606,3 +609,28 @@ it(`should not set the exit code after successfully launching the package manage
     });
   });
 });
+
+it(`should support package managers in ESM format`, async () => {
+  await xfs.mktempPromise(async cwd => {
+    await xfs.writeJsonPromise(ppath.join(cwd, `package.json` as Filename), {
+      packageManager: `yarn@2.2.2`,
+    });
+
+    const yarnDir = ppath.join(npath.toPortablePath(process.env.COREPACK_HOME!), `yarn/2.2.2` as PortablePath);
+
+    await xfs.mkdirPromise(yarnDir, {recursive: true});
+    await xfs.writeFilePromise(ppath.join(yarnDir, `yarn.js` as PortablePath), `
+      import 'fs';
+      console.log(42);
+    `);
+    await xfs.writeJsonPromise(ppath.join(yarnDir, `package.json` as PortablePath), {
+      type: `module`,
+    });
+
+    await expect(runCli(cwd, [`yarn`, `--version`])).resolves.toMatchObject({
+      exitCode: 0,
+      stdout: `42\n`,
+      stderr: ``,
+    });
+  });
+});