Firebase functions promises not firing correctly - node.js

Question

I'm having a hard time understanding promises in Firebase functions. I have a function that listens for new files in a storage bucket and then emails the user as well as sending them a Discord message. I'm getting inconsistent results and I'm pretty sure its to do with promises and callbacks being setup incorrectly.
exports.sendSetup = functions.storage.bucket('*the-bucket-id*').object().onFinalize((object) => {
// Get a URL for the new config file
console.log('New conf file: ' + object.name);
const { Storage } = require('#google-cloud/storage');
const storage = new Storage({
projectId: '*the-project-id*',
keyFilename: 'googleServiceAccountKey.json'
});
var bucket = storage.bucket('*the-bucket-name*');
const file = bucket.file(object.name);
console.log('Generating download url for file ' + object.name);
return file.getSignedUrl({
promptSaveAs: '*the-file-name*',
action: 'read',
expires: '03-09-2491'
}).then(signedUrls => {
var split = object.name.split("/");
var env = split[0];
var customer_id = split[1];
getCustomerDetails(customer_id, signedUrls[0], env);
});
});
function getCustomerDetails(customer_id, fileUrl, env) {
console.log('Retrieving customer details for customer id ' + customer_id + ' from Stripe');
var stripe = stripeLive;
if (env == 'test') {
stripe = stripeTest;
}
stripe.customers.retrieve(
customer_id,
function (err, customer) {
if (err == null) {
sendMail(fileUrl, customer.email, customer_id, customer.metadata.name);
console.log('discordId= ' + customer.metadata.discordId);
if (customer.metadata.discordId != 'undefined') {
sendDiscord(fileUrl, customer.metadata.discordId, customer.metadata.discordName);
}
console.log('Finished');
} else {
console.log(err);
}
}
);
}
function sendDiscord(fileUrl, discordId, discordName) {
console.log('Attempting to send a discord message to Discord id ' + discordId);
const Discord = require('discord.js');
const client = new Discord.Client();
client.login('*discord-api-key*');
client.once('ready', () => {
console.log('Discord client ready');
client.fetchUser(discordId)
.then((User) => {
console.log('Got Discord user object. Attempting to send message');
return User.send({
embed: {
color: 3447003,
fields: [
{
name: 'Hey ' + discordName + '!',
value: 'Below are the instructions to get you up and running'
},
{
name: '**Step 1**',
value: 'some instructions'
}
]
}
});
})
.catch((err) => {
console.log(err);
})
});
}
function sendMail(fileUrl, customer_email, customer_id, customer_name) {
console.log('customer_name in sendMail function = ' + customer_name);
var firstName = customer_name.substring(0, customer_name.indexOf(' '));
console.log(firstName);
const sgMail = require('#sendgrid/mail');
sgMail.setApiKey(*sendGridApiKey*);
sgMail.setSubstitutionWrappers('{{', '}}'); // Configure the substitution tag wrappers globally
const msg = {
to: customer_email,
subject: 'Welcome!',
from: {
email: 'noreply#example.com.au',
name: 'me'
},
text: 'Let\'s get you setup...',
html: '<p></p>',
templateId: '*template-id*',
substitutions: {
first_name: firstName,
file_url: fileUrl
},
};
console.log('Sending email to ' + customer_email + ' customer id:' + customer_id);
sgMail.send(msg);
}
I've read a heap of articles about promises and callbacks but can't seem to wrap my head around it. The "sendSetup" function actually returns OK but appears to stop right at the start of the getCustomerDetails function. Appreciate any assistance! I'm a bit lost!