YouTube scheduler silently ignores sync failures causing double scheduling

Issue

The YouTube Data API-based scheduler encountered synchronization failures, leading to incorrect scheduling behavior. When API sync operations failed (e.g., due to quota exceeded or network errors), the scheduler continued with an empty _scheduled_times list, causing:

• Videos scheduled 3 hours apart instead of 6 hours, leading to double scheduling.
• Overlapping time slots due to new videos being scheduled without regard for existing YouTube schedule.
• Incorrect series interleaving, where parts from different series appeared out of order.

Culprit

The root issue was in the exception handling logic of the scheduler.py module. When synchronization failed, the scheduler silently continued execution with an empty schedule list, assuming no videos were scheduled:

• The _scheduled_times list was only populated during successful synchronization.
• On failure, the list remained empty, and the scheduler treated it as a valid empty state.
• The get_next_available_slot() function returned an incorrect time slot (now + min_days_advance), causing scheduling conflicts.

Solution

Fail-Safe Synchronization Mechanism

Implemented a robust synchronization mechanism with the following improvements:

1. State Flag for Sync Validation: Added a _sync_required flag to enforce successful synchronization before scheduling.
2. Boolean Return from sync_with_youtube(): Modified the function to return a boolean indicating success or failure.
3. Failure Path Blocking: If synchronization fails and no cached schedule exists, block uploads to prevent incorrect scheduling.
4. Cached Schedule Utilization: Use cached schedule data from the last successful sync if available.
5. Integration with YouTube Uploader: Added logic to propagate synchronization errors to the upload system for retry handling.

Key Behavior Changes

Scenario	Old Behavior	New Behavior
First sync fails	Schedule from empty list, causing incorrect scheduling	Block uploads until synchronization succeeds
Sync fails after success	Schedule from empty list, causing conflicts	Use cached schedule from the last successful sync
Quota exceeded	Schedule from empty list, leading to conflicts	Use last known good schedule or block uploads until quota is reset
Restart + quota exceeded	Catastrophic empty schedule, causing scheduling conflicts	Block uploads until synchronization resolves

Result

The solution ensures:

• Scheduling integrity: Prevents incorrect scheduling by blocking uploads when no valid schedule exists.
• Graceful degradation: Uses cached schedule data when available.
• Error propagation: Clearly communicates synchronization failures to the upload system for appropriate handling.
• Robustness: Maintains schedule integrity even during API failures or quota issues.