When creating a subscription requires await

[s5bug]

How do I handle the case in which (I believe) creating a subscription requires async/await?

Specifically, I'm in a Chrome extension, and getting the ID of the tab I want to throw a port onto requires awaiting:

new Observable((subscriber) => {
  let extantTabs = await chrome.tabs.query({
  // etc

Furthermore, to provide an Unsubscriber (to close the port, clean up resources, etc), I have to get all this work done first to know what Chrome port to send a message to, which doesn't happen until the port connects:

let connectedPort = await new Promise(resolve => {
  let uniquePortName = ...
  // blah blah blah
  let listener = port => {
    if(port.name !== uniquePortName) return
    resolve(port)
  }
  // addListener(listener) to chrome port connections, blah blah blah
  createNewPort(uniquePortName)
})

return function unsubscribe() {
  // tell the webpage to unhook observers/event listeners/etc
  connectedPort.send(unsubscribeMessage)

[s5bug]

I've figured out a better way to solve my problem, which lets me give out a subscription which is ready for the port to be created later. Unsubscribe attaches a .then to the port creation that cleans up everything. Will close for now, but I'm still interested in the theory behind how subscribe and unsubscribe are expected to behave / why they're meant to be synchronous.