Fixes#16486
## Problem
Callbacks configured via litellm.success_callback (e.g., PostHog, LangSmith)
were not being invoked for litellm.acompletion() in short-lived scripts.
The callbacks worked correctly for synchronous completions but async
completions would queue callbacks that were lost when the script exited
before GLOBAL_LOGGING_WORKER could process them.
Root cause: asyncio.run() closes the event loop immediately after the
async function completes, preventing the background worker from processing
queued callbacks.
## Solution
Implemented a two-level atexit handler approach:
1. GLOBAL_LOGGING_WORKER atexit handler (logging_worker.py):
- Processes remaining callbacks from queue before exit
- Creates new event loop to run pending coroutines synchronously
- Applies time and iteration limits to prevent blocking shutdown
2. Integration-specific atexit handlers (posthog.py as example):
- Flushes internal queue to external service
- Uses synchronous HTTP client for reliable delivery
- Each integration needs its own handler due to varying sync APIs
## Changes
- litellm/litellm_core_utils/logging_worker.py:
- Added _flush_on_exit() method
- Registered atexit handler in __init__
- Processes up to MAX_ITERATIONS_TO_CLEAR_QUEUE events
- Time-limited to MAX_TIME_TO_CLEAR_QUEUE seconds
- litellm/integrations/posthog.py:
- Added _flush_on_exit() method
- Registered atexit handler in __init__
- Groups events by credentials for batch sending
- Uses sync_client for reliable HTTP delivery
- tests/logging_callback_tests/test_posthog.py:
- Added test_async_callback_atexit_handler_exists()
- Added test_posthog_atexit_flushes_internal_queue()
- Added test_sync_callback_not_affected_by_atexit()
## Testing
- All existing tests pass
- Manual end-to-end testing confirms fix:
- Async events now arrive in PostHog
- Sync events continue working (no regression)
- Unit tests verify atexit handlers registered and functional
## Impact
- Fixes async callback delivery for ALL integrations using GLOBAL_LOGGING_WORKER
- No breaking changes - only adds missing functionality
- Sync path unchanged - no performance impact
Shivam Rawat