Collection download: Initiate on first API call

Previously the manifest files were read from disk so it didn't matter to
read them all on module import. But now they are fetched from the
Internet. So add a decorator to all API views for initiating the global
variables and fetch collections only once on first API call.

Author: manuq

kolibri_explore_plugin/collectionviews.py

@@ -602,12 +602,15 @@ def _build_remotechannelimport_task(channel_id):
 
 _content_manifests = []
 _content_manifests_by_grade_name = {}
-_collection_download_manager = CollectionDownloadManager()
+_collection_download_manager = None
 
 
-def _read_content_manifests():
+def _initiate():
     global _content_manifests
     global _content_manifests_by_grade_name
+    global _collection_download_manager
+
+    _collection_download_manager = CollectionDownloadManager()
 
     free_space_gb = get_free_space() / 1024**3
 
@@ -631,7 +634,15 @@ def _create_manifest(grade, name):
             _create_manifest(grade, name)
 
 
-_read_content_manifests()
+def ensure_initiated(api_function):
+    """Decorator to initiate only once in the first API call."""
+
+    def wrapper(*args, **kwargs):
+        if _collection_download_manager is None:
+            _initiate()
+        return api_function(*args, **kwargs)
+
+    return wrapper
 
 
 def _save_state_in_request_session(request):
@@ -670,6 +681,7 @@ def _get_channel_metadata(channel_id):
     return ChannelMetadata.objects.get(id=channel_id)
 
 
+@ensure_initiated
 @api_view(["GET"])
 def get_collection_info(request):
     """Return the collection metadata and availability."""
@@ -679,6 +691,7 @@ def get_collection_info(request):
     return Response({"collectionInfo": collection_info})
 
 
+@ensure_initiated
 @api_view(["GET"])
 def get_all_collections_info(request):
     """Return all the collections metadata and their availability."""
@@ -697,6 +710,7 @@ def get_all_collections_info(request):
     return Response({"allCollectionsInfo": info})
 
 
+@ensure_initiated
 @api_view(["GET"])
 def get_should_resume(request):
     """Return if there is a saved state that should be resumed."""
@@ -715,6 +729,7 @@ def get_should_resume(request):
     )
 
 
+@ensure_initiated
 @api_view(["POST"])
 def start_download(request):
     """Start downloading a collection.
@@ -756,6 +771,7 @@ def start_download(request):
     return Response({"status": status})
 
 
+@ensure_initiated
 @api_view(["POST"])
 def resume_download(request):
     """Resume download from a previous session.
@@ -784,6 +800,7 @@ def resume_download(request):
     return Response({"status": status})
 
 
+@ensure_initiated
 @api_view(["POST"])
 def update_download(request):
     """Continue downloading current collection.
@@ -806,6 +823,7 @@ def update_download(request):
     return Response({"status": status})
 
 
+@ensure_initiated
 @api_view(["DELETE"])
 def cancel_download(request):
     """Cancel current download and clear the saved state.
@@ -825,6 +843,7 @@ def cancel_download(request):
     return Response({"status": status})
 
 
+@ensure_initiated
 @api_view(["GET"])
 def get_download_status(request):
     """Return the download status."""