More resilient droplet upload

[kathap]

Previously, the cc-uploader process did not have a draining mechanism. When the process was stopped, any ongoing droplet uploads were aborted immediately, leading to potential failures and incomplete uploads.

This PR introduces a graceful shutdown mechanism to allow ongoing uploads to complete before termination. The key enhancements include:

• Handling shutdown signals:
  • The process now listens for termination signals (SIGINT, SIGTERM) and responds by allowing uploads to finish before exiting.
• Tracking active uploads:
  • A global WaitGroup (uploadWaitGroup) is introduced to track ongoing uploads.
  • A wrapper (trackedPoller) ensures uploads are properly accounted for by incrementing/decrementing the WaitGroup.
• Graceful waiting before shutdown:
  • When a shutdown signal is received, cc-uploader will:
    1. Log the shutdown request.
    2. Wait for all active uploads to complete.
    3. Sleep for 20 seconds as an additional buffer.
    4. Proceed with shutdown.
• Forceful termination fallback:
  • If necessary, a force shutdown mechanism (forceShutdown()) ensures the process does not hang indefinitely.

Code Changes

1. Signal Handling:
   • Introduced a signalChan to capture termination signals.
   • Logs all received signals for debugging.
2. WaitGroup for Uploads:
   • uploadWaitGroup ensures cc-uploader waits for all uploads to finish before stopping.
3. Poller Enhancements:
   • Wrapped the existing Poller to track polling completion, ensuring uploads are properly accounted for.
4. Graceful Shutdown Logic:
   • Modified the shutdown process to allow ongoing uploads to complete before exiting.