Revert changes on the TreeProcessor (#6006 and #5885)

[mjesun]

The TreeProcessor refactor introduced a change on the behavior of beforeEach inside it. While this is an edge case, Facebook (and potentially anyone using inline requires) is relying in it.

Some fixes have been suggested, but for the moment we'll just revert the change. cc / @niieani

[niieani]

@mjesun this will break webpack. It had just merged the huge PR migrating its tests to jest.

I have identified the line which was causing the issue and fixed it here: #6100. Was this fix not enough or is there a different reason for doing the revert?

[SimenB]

+1 for #6100 instead. The order was undocumented and unintuitive before the changes. An option seems like an acceptable workaround to me, until FB is able to fix their tests

[SimenB]

and potentially anyone using inline requires

How does inline requires impact this? Jest itself uses inline requires with no issues before or after this change

[mjesun]

Our internal transformer uses inline requires over test code. Some tests are using a helper utility file, which contains:

beforeEach(() => {
  jest.resetModules();
});

The require for this file is at the top level, but because of inline-requiring, the first time the file is loaded it happens inside an it. Think about the following scenario:

const myHelper = require('./my-helper');
const foo = require('foo');

beforeEach(() => {
  jest.mock('foo', () => {});
});

it('does magic', () => {
  myHelper.createAnotherStuff();
  foo.bar();
});

This test case passes before the change to the tree processor, but breaks now. The reason is that before the change, the order of the beforeEachs was:

• First the jest.resetModules() inside the beforeEach of the helper function.
• Then the jest.mock inside the beforeEach of the test.

With the change to the tree processor, the new order is:

• First the jest.mock inside the beforeEach of the test.
• Then the jest.resetModules() inside the beforeEach of the helper function.

Which essentially means that the later resetModules kills all pre-initialization. This will affect anyone using inline requires with helper files.

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