commonjs: requireReturnsDefault: "preferred" yields unstable results

[thorn0]

• Rollup Plugin Name: commonjs
• Rollup Plugin Version: 18.0.0
• Rollup Version: 2.45.2
• Operating System (or Browser): Linux Ubuntu 20, Windows 10 x64
• Node Version: 16.0.0
• Link to reproduction: https://github.com/thorn0/rollup-commonjs-issue

Expected Behavior

Calling Rollup multiple times with the same settings should lead to the same output.

Actual Behavior

Only the first time the output is correct and as expected. Subsequent outputs are incorrect and different from the first one.

Additional Information

for (let i = 0; i <= 3; i++) {
  const result = await rollup({
    input: "input.js",
    plugins: [commonjs({ requireReturnsDefault: "preferred" })],
  });

  await result.write({ file: `output/result${i}.mjs` });
}

First output:

var numbers = [1, 2, 3, 4, 5, 6];

console.log(numbers);

var input = {

};

export default input; 

Subsequent output:

var constants = [1, 2, 3, 4, 5, 6];

var constants$1 = /*#__PURE__*/Object.freeze({
	__proto__: null,
	'default': constants
});

console.log(constants$1);

var input = {

};

export default input; 

This works as expected:

const plugins = [commonjs({ requireReturnsDefault: "preferred" })];

for (let i = 0; i <= 3; i++) {
  const result = await rollup({
    input: "input.js",
    plugins,
  });

  await result.write({ file: `output/result${i}.mjs` });
}

[thorn0]

The reason is likely this variable shared between instances:

plugins/packages/commonjs/src/is-cjs.js

Line 1 in a82166b

const isCjsPromises = new Map();

[guybedford]

Thanks for spotting this bug. Would you be interested in working on a PR here?

[thorn0]

It blocks a release of a package I maintain, so potentially - yes. But it can take some time. If somebody can fix this more quickly, do it, don't wait for me. ;)

upd: Found #658 and checked if it fixes this issue. It doesn't.