New PreviewManager to handle generating previews asynchronously (#86)

* Add PreviewManager

* Refactoring and better error handling

* Integrate PreviewManager into api_server.py

* Integrate PreviewManager into distributed_job_manager.py

* Add method to preview_manager.py to delete previews and integrate it into api_server

* Misc logging improvements

* Misc code cleanup

* Replace existing preview on job completion - Minor code fixes

src/distributed_job_manager.py

@@ -10,6 +10,7 @@ import requests
 from plyer import notification
 from pubsub import pub
 
+from src.api.preview_manager import PreviewManager
 from src.api.server_proxy import RenderServerProxy
 from src.engines.engine_manager import EngineManager
 from src.render_queue import RenderQueue
@@ -52,12 +53,18 @@ class DistributedJobManager:
             return
 
         logger.debug(f"Job {job_id} status change: {old_status} -> {new_status}")
+
+        # Update previews - Force replace if job has completed successfully
+        PreviewManager.update_previews_for_job(render_job,
+                                               replace_existing=(render_job.status == RenderStatus.COMPLETED))
+
+        # Notify parent if necessary
         if render_job.parent:  # If local job is a subjob from a remote server
             parent_id, hostname = render_job.parent.split('@')[0], render_job.parent.split('@')[-1]
             RenderServerProxy(hostname).notify_parent_of_status_change(parent_id=parent_id, subjob=render_job)
 
-        # handle cancelling all the children
-        elif render_job.children and new_status in [RenderStatus.CANCELLED, RenderStatus.ERROR]:
+        # Cancelling children if necessary
+        if render_job.children and new_status in [RenderStatus.CANCELLED, RenderStatus.ERROR]:
             for child in render_job.children:
                 child_id, hostname = child.split('@')
                 RenderServerProxy(hostname).cancel_job(child_id, confirm=True)
@@ -143,6 +150,7 @@ class DistributedJobManager:
             logger.debug("Not splitting into subjobs")
 
         RenderQueue.add_to_render_queue(worker, force_start=job_data.get('force_start', False))
+        PreviewManager.update_previews_for_job(worker)
 
         return worker