[External Storage] Define API and implement SimpleCopy backlend

[troopa81]

This is the first PR of the proposed QEP qgis/QGIS-Enhancement-Proposals#196

It implements:

• External storage API
• Registry that contains all external storage backends
• SimpleCopy external storage that stores the selected external resource on a specific location on disk

Widget modifications and WebDAV backend will come in separated PRs

I had to change the SIP_CONCAT_PARTS to 16 (I have errors when building pycore with 13, pyanalysis with 14 and 15).

cc @Jean-Roc
Funded by Lille Metropole https://www.lillemetropole.fr/

[pblottiere]

Hello @troopa81 ✋,

I took a quick look on code style. Let me know if I missed some things :).

[troopa81]

Hi @pblottiere, and thanks for your review, I'm on my way to take your comments into consideration.

Regarding using unique_ptr in QgsExternalStorageRegistry, it brings a little bit of complexity:

• I need to take a unique_ptr in registerExternalStorage and that doesn't fit well with SIP (though I could expose 2 functions, one for SIP, an another with unique_ptr)
• the projectStorages() method would need to iterate over the hash table to build an other list which returns pointers and not unique_ptr

I'm wondering if these issues are worth using a unique_ptr. As it is, I can't see a way that could lead to memory leak.

[3nids]

As pointed out, I am wondering if we should not create an object which could be started rather than starting automatically and have to check for different statuses? What do you think?

[troopa81]

Unrelated HANA test failure

@3nids I take all your comment into consideration, is it OK for merge?

[mkemeter]

Sorry for that. The unrelated test failure should be gone now.

[3nids]

Looks good to merge, thanks for the follow-ups!

[troopa81]

thanks @3nids