Celery Mongo Results Backend get call hangs with non default db

[sukrit007]

Given:
Celery version: 3.1.16

And:
Celery task that returns asyncresult. E.g:

@app.task
def _add(a, b):
    return a + b

@app.task
def sumall(a, b, c, d):
    return (_add.s(a,b) | _add.s(c) | _add.s(d))()

And: Celery configured to use mongodb as backend database but with different database other than celery (Issue does not happen if celery is selected as database)

CELERY_RESULT_BACKEND =  'mongodb://localhost:27017'
CELERY_MONGODB_BACKEND_SETTINGS = {
    'database': 'celery2'
}

When: I run the add task in one process and get the status by id in another process (Note: in same process issue does not happen. )

result = sumall.delay(1,2,3,4)
print(result.id)

Take the id from above process and print the status in another process using same celery settings as worker and process1 (that initiated sumall task)

result = app.AsyncResult('636c865f-df44-4e82-9f3f-d88cf13cbc13').get()  # Works (uses correct db)
result.get()  # Hangs . Uses celery db instead of 'celery2' database.

Then:
second process hangs forever when trying to get second result in the chain.

Workaround-1:

result = app.AsyncResult('636c865f-df44-4e82-9f3f-d88cf13cbc13').get()  # Works (uses correct db)
app.AsyncResult(result.id).get() # This time it uses correct db.

Workaround-2:
Do not change default database.

[sukrit007]

After debugging through celery source, here is what I found :

I did figure out how celery is initializing the backend using default_app. https://github.com/celery/celery/blob/master/celery/_state.py#L83 However, _tls.current_app is always None in my case. Even though set_as_current is True for my celery app object. As a result it fall backs to default app (there by changing the database)

One possible workaround is to use :

app.set_default()

But not sure, why would _tls.current_app be None.

[ask]

current_app is a thread local, so it must be set for each new thread!

Unpickling a result requires a working current_app, because the app is removed from objects when serializing.

[sukrit007]

Setting current_app did help to solve the database issue. However there seems to be a bug with host initialization during de-serialization process in MongoBackend. Even after using current_app, celery was trying to use 'localhost' as host. Here is a patch to fix this issue:

#2347