Make copying to/from storage systems memory efficient (#419)

The copy_locally and copy_local_to_storage (very inconsistent terms BTW)
were simply slurping in everything in RAM and writing it out at once.
(the copy_locally was actually memory efficient if the remote system was local)

Use shutil.copyfileobj which does chunked reads/writes on file objects.
The default buffer size is 16kb, and as each chunk means a separate HTTP
request for e.g. cloudfiles, we use a chunksize of 4MB here (which has
just been arbitrarily set by me without tests).

This should help with the failure to upload large files issue #419.

mediagoblin/storage/__init__.py

@@ -160,12 +160,13 @@ class StorageInterface(object):
         appropriate.
         """
         if self.local_storage:
-            shutil.copy(
-                self.get_local_path(filepath), dest_path)
+            # Note: this will copy in small chunks
+            shutil.copy(self.get_local_path(filepath), dest_path)
         else:
             with self.get_file(filepath, 'rb') as source_file:
                 with file(dest_path, 'wb') as dest_file:
-                    dest_file.write(source_file.read())
+                    # Copy from remote storage in 4M chunks
+                    shutil.copyfileobj(source_file, dest_file, length=4*1048576)
 
     def copy_local_to_storage(self, filename, filepath):
         """
@@ -177,7 +178,8 @@ class StorageInterface(object):
         """
         with self.get_file(filepath, 'wb') as dest_file:
             with file(filename, 'rb') as source_file:
-                dest_file.write(source_file.read())
+                # Copy to storage system in 4M chunks
+                shutil.copyfileobj(source_file, dest_file, length=4*1048576)
 
 
 ###########

mediagoblin/storage/filestorage.py

@@ -87,6 +87,5 @@ class BasicFileStorage(StorageInterface):
             directory = self._resolve_filepath(filepath[:-1])
             if not os.path.exists(directory):
                 os.makedirs(directory)
-
-        shutil.copy(
-            filename, self.get_local_path(filepath))
+        # This uses chunked copying of 16kb buffers (Py2.7):
+        shutil.copy(filename, self.get_local_path(filepath))