Django Async Deadlock Issue Analysis

Based on the information provided, you're experiencing a deadlock issue after upgrading Django and asgiref, particularly when using async_to_sync in long-running interfaces. This is related to thread management changes in asgiref 3.3.0 and above.

Root Cause Analysis

The core issue appears to be related to the thread_sensitive parameter in sync_to_async() and the corresponding ThreadSensitiveContext in Django's ASGI handler. As mentioned in the Django documentation:

asgiref version 3.3.0 changed the default value of the thread_sensitive parameter to True. This is a safer default, and in many cases interacting with Django the correct value, but be sure to evaluate uses of sync_to_async() if updating asgiref from a prior version.

When a request times out in nginx but the Django process is still running, the thread-sensitive context is causing a deadlock because:

1. The main thread is waiting for the timed-out request's thread to complete
2. The timed-out request's thread is waiting for the main thread to process its async_to_sync call
3. This creates a circular dependency that blocks all subsequent requests

Potential Solutions

1. Use a Proper ASGI Server

Your deployment is using runserver, which is not recommended for production. Instead, use a proper ASGI server:

BASHCopied
daphne myproject.asgi:application

2. Adjust Thread Sensitivity

You can modify your code to explicitly set thread_sensitive=False when using sync_to_async in long-running operations:

PYTHONCopied
def interface_a(request):
    # Long-running code
    cost_long_time_execute()
    # Use thread_sensitive=False to avoid deadlock
    async_to_sync(execute_async, thread_sensitive=False)()
    return HttpResponse(content='{"status": "ok"}")

3. Restructure Your Code

Consider restructuring your code to avoid mixing sync and async operations in long-running interfaces:

PYTHONCopied
async def interface_a(request):
    # Long-running code in a sync function
    await sync_to_async(cost_long_time_execute)()
    # Now directly call the async function without async_to_sync
    await execute_async()
    return HttpResponse(content='{"status": "ok"}")

4. Implement Timeouts

Add timeouts to your async operations to prevent indefinite blocking:

PYTHONCopied
import asyncio
from asgiref.sync import async_to_sync

def interface_a(request):
    cost_long_time_execute()
    try:
        # Add timeout to async_to_sync call
        async_to_sync(lambda: asyncio.wait_for(execute_async(), timeout=30))()
    except asyncio.TimeoutError:
        # Handle timeout gracefully
        pass
    return HttpResponse(content='{"status": "ok"}")

Regarding Your Fix

Removing async with ThreadSensitiveContext() from Django's ASGI handler might solve the immediate issue, but it's not recommended because:

1. It could lead to thread safety issues with database connections
2. It might cause unexpected behavior in middleware and other Django components
3. It's modifying Django's core functionality, which could break with future updates

Performance Considerations

As noted in the Django documentation:

If you put synchronous middleware between an ASGI server and an asynchronous view, it will have to switch into sync mode for the middleware and then back to async mode for the view. Django will also hold the sync thread open for middleware exception propagation. This may not be noticeable at first, but adding this penalty of one thread per request can remove any async performance advantage.

For optimal performance, try to make your entire request stack either fully synchronous or fully asynchronous to minimize context switching.