Dynamically Add Handlers

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I am trying to dynamically create handlers for a set of specific urls that come from wordpress. The requirement comes from the need to have a set of top level slugs for a CPT.

To do this I call an endpoint and get a list of the slugs, then the idea is to add a handler (or multiple handlers) to catch these and serve the correct page.

I hit an issue when I try to run any async code before I add the handler in the index file, for example:

const before = async ({ libraries, actions }) => {
  libraries.source.handlers.push(menuHandler);
  libraries.source.handlers.push(officeHandler);

  const slugs = await getOfficeNames(libraries, actions);

  libraries.source.handlers.push(officePageHandler(libraries, actions,"/slug"));

  await actions.source.fetch("primaryMenu");
  await actions.source.fetch("offices");
};

This function is called in both Both beforeSSR and beforeCSR:

  beforeSSR: async ({ libraries, actions }) => {
        await before({ libraries, actions });
      },
      beforeCSR: async ({ libraries, actions }) => {
        await before({ libraries, actions });
      },

The getOfficeNames function currently does nothing for testing (it will get the list of slugs):

async function getOfficeNames(libraries, actions) {
  // const requestUrl = "/wp/v2/office?per_page=100&parent=0&page=";
  const x = new Promise((resolveInner) => {
    setTimeout(resolveInner, 1000);
  });
  const y = await x;
  return "TEST";

When I run it in this configuration it doesnt work navigating to /slug. However in this configuration:

const before = async ({ libraries, actions }) => {
  libraries.source.handlers.push(menuHandler);
  libraries.source.handlers.push(officeHandler);

  libraries.source.handlers.push(officePageHandler(libraries, actions,"/slug"));

  const slugs = await getOfficeNames(libraries, actions);


  await actions.source.fetch("primaryMenu");
  await actions.source.fetch("offices");
};

it works.

Does any one know why running something async before adding to the handlers cause it to break? I have checked the handler list and it seems to have added the handler correctly, it just doesnt get called.

{
    name: 'Office Pages TEST',
    priority: 10,
    pattern: 'RegExp:(/slug/)',
    func: [AsyncFunction: func]
  }