Concurrently creating firebase users of the same email succeeds

La reproducción se describe en firebase_service.py y test.py; empieza ejecutando el caso de ThreadPoolExecutor contra Firebase Authentication y compáralo con el caso de asyncio. Investiga la ruta del SDK utilizada por auth.create_user y determina si las solicitudes concurrentes se gestionan de forma consistente. Se considera terminado cuando el caso concurrente ya no crea varios usuarios con el mismo correo electrónico y se verifica el comportamiento esperado de EmailAlreadyExistsError.

[READ] Step 1: Are you in the right place?

• For issues related to the code in this repository file a GitHub issue.
• If the issue pertains to Cloud Firestore, report directly in the
  Python Firestore GitHub repo. Firestore
  bugs reported in this repo will be closed with a reference to the Python Firestore
  project.
• For general technical questions, post a question on StackOverflow
  with the firebase tag.
• For general Firebase discussion, use the firebase-talk
  google group.
• For help troubleshooting your application that does not fall under one
  of the above categories, reach out to the personalized
  Firebase support channel.

[REQUIRED] Step 2: Describe your environment

• Operating System version: MacOS sonoma 14.3
• Firebase SDK version: 6.5.0
• Firebase Product: auth
• Python version: 3.11
• Pip version: 23.1.2

Note that I have used other tenants on Google Identity Platform in this gcp project but I scrapped that and went back to the single tenant.
This issue is happening on the default firebase tenant. (Sharing this fact in case it somehow affects the outcome)

[REQUIRED] Step 3: Describe the problem

I discovered that my tests for some CRUD functionality were creating duplicate firebase users with the same email which is problematic because we rely on the firebase_admin._auth_utils.EmailAlreadyExistsError to safeguard functionality. This appeared as my webserver API was called to create the same user concurrently and it actually made multiple firebase users.

For context, my firebase project -> authentication -> settings -> user account linking -> link accounts with same email is active.

Steps to reproduce:

Creating a new user with an existing email address succeeds when its ran concurrently. I have tested 3 scenarios:

1. Create the new user (with existing email) a second after the existing user was created
   1.1. I get the firebase_admin._auth_utils.EmailAlreadyExistsError like I expect

2. Attempt to create 4 users with the same email address using asyncio library to create them quickly
   2.1 Returns one created user and fires 3 firebase_admin._auth_utils.EmailAlreadyExistsError like I expect

3. Attempt to create 4 users with the same email address concurrently using ThreadPoolExecutor
3.1 Returns 4 new Firebase users who all share the same email address. Not good.

Calling the auth.get_user_by_email(email) returns the latest firebase user created, when I delete the latest one then the function returns the newest one after that and so on.

I plan to add in concurrency/idempotent protections to my API logic in the mean time as this would cause a mess downstream ( as uncommon as it would occur)

Relevant Code:

This is a rough outline of my tests but in general just call create_user concurrently to hopefully see the same results. I use a Fastapi server so you can ignore some of this extracted code using async where its not needed. This is just to demo the issue.
firebase_service.py

import firebase_admin
from firebase_admin import auth
if not firebase_admin._apps:
    firebase_admin.initialize_app()

class FirebaseService:

  # async wrapper
  async def create_user(self, email: str, uid: str | None) -> auth.UserRecord | None:
          """Create a user."""
          try:
              user = auth.create_user(email=email, uid=uid)
              return user
          except Exception as e: 
              logger.exception(f"error creating user {email} : {e}")
              return None
  
  # non async implementation	
      def create_user_sync(self, email: str, uid: str | None) -> auth.UserRecord | None:
          """Create a user."""
          try:
              user = auth.create_user(email=email, uid=uid)
              return user
          except Exception as e: 
              logger.exception(f"error creating user {email} : {e}")
              return None

test.py

import asyncio

common_email = "email_goes_here"
no_uid = None

async def test_multiple_user_same_email_create_asyncio() -> None:
    """Test multiple user creation with same email with asyncio gather."""
    tasks = []
    for i in range(4):
        tasks.append(asyncio.create_task(firebase_service.create_user(uid=no_uid, email=common_email)))
    results = await asyncio.gather(*tasks)
    logger.info(results) # correctly creates 1 user and raises an error for the rest

async def test_multiple_user_same_email_create_threadpool() -> None:
    """Test multiple user creation with same email concurrently"""
    uid = None

    with ThreadPoolExecutor(max_workers=4) as executor:
        futures = [executor.submit(firebase_service.create_user_sync,common_email, no_uid ) for _ in range(4)]
        results = [future.result() for future in as_completed(futures)]
        logger.info(results) # actually creates 4 users with the same email

if __name__ == "__main__":
    # asyncio.run(test_multiple_user_same_email_create_asyncio())
     # asyncio.run(test_multiple_user_same_email_create_threadpool())