Assets: add function to cancel all remote asset listing downloads

Add a Python function to cancel all remote asset listing downloads.

Calling this function will leave the on-disk listings in an unusable
state. This isn't user friendly, so in the future it would be nice to
store a backup of the listing's files, and restore those on
cancellation. That is for a future PR, though, as the current limitation
already exists when quitting Blender while the listing is being
downloaded.

Pull Request: https://projects.blender.org/blender/blender/pulls/158495

scripts/addons_core/bl_pkg/__init__.py

@@ -485,6 +485,20 @@ def remote_asset_library_sync(
     _downloaders.append(downloader)
 
 
+def remote_asset_library_sync_cancel() -> None:
+    """Cancel all remote asset library sync operations.
+
+    This will cancel all running downloads & shut down the downloaders. Any
+    partially-downloaded listing will have to be re-downloaded to be fully
+    correct again.
+    """
+
+    # This calls the relevant _remote_asset_library_sync_done() functions as well, ensuring that each downloader is
+    # properly cleaned up. That includes removing items from _downloaders, hence the copy of that list.
+    for downloader in _downloaders[:]:
+        downloader.cancel_and_shutdown()
+
+
 def _remote_asset_library_sync_done(downloader: _RemoteAssetListingDownloader) -> None:
     """
     Called when the downloading of the remote asset listing is done.

scripts/modules/_bpy_internal/assets/remote_library_listing/listing_downloader.py

@@ -541,7 +541,8 @@ class RemoteAssetListingDownloader:
     def _shutdown_if_done(self) -> None:
         if self._num_asset_pages_pending == 0 and self._bg_downloader.all_downloads_done:
             # Done downloading everything, let's shut down.
-            self.shutdown(DownloadStatus.FINISHED_SUCCESSFULLY)
+            self._status = DownloadStatus.FINISHED_SUCCESSFULLY
+            self.shutdown()
 
     @staticmethod
     def _cache_bust_stamp(*, _mocked_now: _datetime | None = None) -> int:
@@ -631,7 +632,8 @@ class RemoteAssetListingDownloader:
             "exception while handling downloaded file ({!r}, saved to {!r})".format(
                 http_req_descr, local_file))
         self.report({'ERROR'}, "Asset library index had an issue, download aborted")
-        self.shutdown(DownloadStatus.FAILED)
+        self._status = DownloadStatus.FAILED
+        self.shutdown()
 
     def _queue_download(
         self,
@@ -660,10 +662,18 @@ class RemoteAssetListingDownloader:
         if 'ERROR' in level:
             self._error_message = message
 
-    def shutdown(self, status: DownloadStatus) -> None:
-        """Stop the background downloader, update the status and call the 'done' callback."""
+    def cancel_and_shutdown(self) -> None:
+        """Cancel all downloads and shut down the background downloader."""
 
-        self._status = status
+        if self._status == DownloadStatus.LOADING:
+            self._status = DownloadStatus.FAILED
+
+        # The downloads themselves don't have to be explicitly cancelled,
+        # shutting down the downloader will do that implicitly.
+        self.shutdown()
+
+    def shutdown(self) -> None:
+        """Stop the background downloader and call the 'done' callback."""
 
         # The timer is no longer necessary, the bg_downloader.shutdown() call
         # takes care of the last queued messages.
@@ -693,7 +703,8 @@ class RemoteAssetListingDownloader:
             self._bg_downloader.update()
         except http_dl.BackgroundProcessNotRunningError:
             logger.error("Background downloader subprocess died, aborting.")
-            self.shutdown(DownloadStatus.FAILED)
+            self._status = DownloadStatus.FAILED
+            self.shutdown()
             return 0  # Deactivate the timer.
         except Exception:
             logger.exception(
@@ -744,12 +755,14 @@ class RemoteAssetListingDownloader:
             if self._num_asset_pages_pending:
                 self.report({'WARNING'}, "Cancelled {} pending download".format(self._num_asset_pages_pending))
             logger.warning("Download cancelled: %s", http_req_descr)
-            self.shutdown(DownloadStatus.FAILED)
+            self._status = DownloadStatus.FAILED
+            self.shutdown()
             return
 
         self.report({'ERROR'}, "Error downloading {}: {}".format(http_req_descr.url, error))
         logger.error("Error downloading %s: %s", http_req_descr, error)
-        self.shutdown(DownloadStatus.FAILED)
+        self._status = DownloadStatus.FAILED
+        self.shutdown()
 
     def download_progress(
         self,