Hello everyone,

hereby, I'd like to raise your awareness about pending API changes that affect block-level components. I post this here because block-level components outside our source tree will need an adaptation to the new API.

The change consists of two parts, one for the server side and one of the client side. Both parts are already on our staging branch and will reach the master branch soon.

The new server-side API is located at 'os/include/block/request_stream.h'. It is already part of Genode 19.02 but remained largely unused at the point of the release. In contrast to the traditional API at 'os/include/block/component.h', the new API anticipates the design of block services as state machines and is designed for the efficient batched processing of block operations. You can find an illustrative example of a block server using this API at 'os/src/test/block_request_stream'.

The new client-side API has become part of the 'Block::Connection' located in 'os/include/block_session/connection.h'. Like the server-side API, it is designed for an asynchronous and batched mode of operation. It achieves this with the added notion of 'Job' objects associated to a connection. For more details, you can find the corresponding issue at [1]. The first component I updated to the new API is the block tester located at 'os/src/app/block_tester/'. So there is already a nice playground in the form of the block_tester.run and block_request_stream.run scripts, both located at 'os/run/'.

[1] https://github.com/genodelabs/genode/issues/3283

The client-side as well as the server-side APIs completely hide the underlying packet stream mechanism so that we can - in the future once all components are migrated to the new API - simplify this transport mechanism. By hiding the packet stream, components become much simpler because the application code is no longer bothered with the handling of packet-stream corner cases.

If you maintain custom block-level components, I hereby encourage you giving the new API a try and report your findings. At the current time, the original APIs are still maintained. So there can be smooth transition phase. Still, a few cosmetic adaptations even in the case of intermediately sticking to the old API are needed.

* 'Block::Connection' has become a template. To make your code compile again, the default template argument may become handy. Just rename the type to 'Block::Connection<>'.

* The 'Block::Session::info' RPC function has a new signature. Instead of using C-ish out parameters, the new version returns a compound object of type 'Info'.

* The original 'Block::Session::Operations' type is gone. In practice, it was merely used to distinguished read-only from read-writeable block devices. Now, it has been superseded with a 'writeable' attribute of the 'Info' type. Much simpler.

* SYNC and TRIM have become first-class asynchronous block operations. The former 'sync' RPC function does not exist anymore.

I hope you like the direction for the API changes. Your feedback is very welcome. If this direction turns out well, we may take the design as blue print for other asynchronous inter-component communication APIs (NIC and file system come in mind) in the future.

Cheers Norman