Send bulk emails in background task with Flask

I'm using Flask Mail to send emails from my Flask App. No problem using it from the main thread. However I have a route that can send a quite large amount of emails (enough I think to exceed the HTTP Timeout) and I'd like to quickly return a response and running the mail send in the background.

I tried the following solution :

from concurrent.futures import ThreadPoolExecutor

executor = ThreadPoolExecutor(max_workers=1)
mailer = Mail(app) #Flask Mail Mailer

@app.route("/admin/sendmails")
def sendmails():

  #
  # Code to get recipients from DB here
  # 


  executor.submit(send_links,recipients,mailer)
  return "Mail sending began"

Here is the code for the send_mails function :

  def send_links(recipients,mailing=None):
    subject = "My subject"
    if mailing == None : return logger.warning("No mailing available : Login links could not be sent")
    with mailing.connect() as conn:
        for [recipient,token,fullname] in recipients:
            try:
                raw_body = render_template("mails/login.txt",token=token, fullname=fullname)
                html_body = render_template("mails/login.html",token=token, fullname=fullname)
                msg = Message(subject,
                            recipients=[recipient])
                msg.body = raw_body
                msg.html = html_body
                msg.attach('logo.png','image/png',open(os.path.join(os.path.dirname(os.path.abspath(__file__)),'static/image/logo.png'), 'rb').read(), 'inline', [['Content-ID', '<LOGO>']])
                conn.send(msg)
            except Exception as e:
                logger.warning("Impossible to send login token \"{}\" to {}".format(token, fullname))
                logger.warning(traceback.format_exc())
    return True

However the send_links function uses render_template from within the Flask context (and the mailer probably uses it as well I guess) so I get the following error...

Traceback (most recent call last):
  File "/var/www/rbt-rdd/server/utils.py", line 114, in send_links
    raw_body = render_template("mails/login.txt",token=token, fullname=fullname)
  File "/var/www/rbt-rdd/venv/lib/python3.9/site-packages/flask/templating.py", line 146, in render_template
    ctx.app.update_template_context(context)
AttributeError: 'NoneType' object has no attribute 'app'

Do you have any idea how to fix this ?

Manually push a context:

def send_links(recipients, mailing=None):
    # ...                    # -
    with app.app_context():  # +
        ...                  # +

Reference: https://flask.palletsprojects.com/en/2.0.x/appcontext/#manually-push-a-context

You can implement it as a decorator, especially if you have many such functions:

from functools import wraps

def with_app_context(func):
    @wraps(func)
    def _func(*args, **kwargs):
        with app.app_context():
            return func(*args, **kwargs)
    return _func

Usage:

@with_app_context  # +
def send_links(recipients, mailing=None):
    ...