Intercepting ajax request with "async: false" cause cypress to freeze

Current behavior

We have code (JS/jQuery) invoking the POST request:

$.ajax({
    type: 'POST',
    url: '/web/api2/v1/term/{0}'.format(tbId),
    dataType: 'json',
    contentType: 'application/json',
    cache: false,
    async: false,
    data: JSON.stringify({
        conceptId: conceptId,
        lang: input.attr('name'),
        text: input.val()
    }),
    success: function (data) {
        ...
    },
    error: function (req, status) {
        ...
    }
});

When we run tests on production we then intercept all the calls to add additional header and only on the request triggered like this cypress completely freeze. Screenshots and Debug logz are attached. Logs.txt

Desired behavior

Cypress should intercept the request.

Test code to reproduce

application code (JS/jQuery) invoking the POST request

$.ajax({
    type: 'POST',
    url: '/web/api2/v1/term/{0}'.format(tbId),
    dataType: 'json',
    contentType: 'application/json',
    cache: false,
    async: false,
    data: JSON.stringify({
        conceptId: conceptId,
        lang: input.attr('name'),
        text: input.val()
    }),
    success: function (data) {
        ...
    },
    error: function (req, status) {
        ...
    }
});

test.spec.js

cy.get('#tb-insert-button').click();
  cy.get('#tb-insert-tbody').find(`input[type="text"][name=${targetLang}]`).type(term);
  cy.get('#tb-insert-submit').click();`

index.js

`beforeEach('Adds headers to web012 env', () => {
  if (Cypress.env('ENVIRONMENT') === 'web012') {
    cy.log(Cypress.env('ENVIRONMENT'));
    cy.intercept(/^https:\/\/some\.domain\.com\/web\/(?!assets).*/, (req) => {
      req.headers['X-Memsource-Web-Canary-Request'] = 'any';
    });
  }
});

Cypress Version

9.6.2

Other

No response

[tbiethman]

@Donaab thank you for the report, I can reproduce this. The interception of synchronous requests seems to lock up cypress indefinitely. If I don’t intercept the synchronous request, the app will hang while the request executes but continues functioning after the request completes. I can also safely intercept the request if I provide a stubbed response.

I also put together a small reproduction as part of my investigation: https://github.com/tbiethman/cypress-test-tiny/tree/sync-request-intercept

As far as I can tell, when the app makes the synchronous request, it’s not able to handle events emitted from the middleware that allow the interception to be possible, which in result never allows the request to complete. More investigation will be required to determine how we should be handling these types of requests; we definitely shouldn’t allow Cypress to get into a scenario that we know will cause a hang.

[nogwater]

Is there any way to skip interception of async:false requests? I’m trying to follow this sample code for throttling all requests, but I keep getting stuck with pending requests and Cypress frozen. https://docs.cypress.io/api/commands/intercept#Throttle-or-delay-response-all-incoming-responses