Jul 022015

Rapicorn 'visitor' branch

Trying to keep it up, here’s an update on recent developments in Rapicorn and Beast.

Git Branches

For now, Rapicorn and Beast are using Git branches the following way:

  • Topic branches are created for each change. Where possible, commits should compile and pass all tests (i.e. pass make check installcheck).
  • Once completed, topic branches are merged into the master branch. For intermediate merges of huge branches, I’ve recently been adding [ongoing] to the merge commit message. As an aside, branch merges should probably be more elaborate in the future to make devlog articles easier to write and potentially more accurate.
  • The master branch must always compile and pass all tests.
  • OpenHub: The OpenHub repo links have been adjusted to point at Rapicorn’s and Beast’s master branch. Because of problems with spammers and a corresponding reimplementations, code statistic updates on the OpenHub are platform currently stalled however.

Hello and goodbye clang++

Rapicorn C++11 code currently compiles with g++-4.7 and upwards. An initial attempt was made at making the C++11 code compile with clang++-3.4 but the incompatibilities are currently too numerous. A few good fixes have come out of this and are merged into master now, but further work on this branch probably has to wait for a newer clang++ version.

New Widgets

Rapicorn is growing more widgets that implement state rendering via SVG element matching. Recent additions are:

  • LayerPainter – A container that allows rendering widgets on top of each other.
  • ElementPainter – A container that displays state dependent SVG image elements.
  • FocusPainter – An ElementPainter that decorates its child according to focus changes.

IDL Improvements

Several changes around Rapicorn’s IDL compiler and support code made it into master recently:

  • The IDL layer got bind() and connect() mthods (on the ObjectBroker interface). This models the IDL setup phase after the zeromq API. Beast makes use of this when setting up IDL interface layers in the UI and in BSE.
  • The Python binding was rewritten using Cython. Instead of invoking a heap of generated Python glue code and talking to the message passing interfaces directly, the Python binding now sits on top of the C++ binding. This makes the end results operate much faster, is less complex on the maintenance side and more functional with regards to the Python API offered. As an added bonus, it also eases testing of the C++ bindings.
  • And just to prove the previous point, the new Cython port uncovered a major issue lurking in the C++ IDL handling of objects in records and sequences. At least since the introduction of remote reference counting, client side object handles and server side object references are implemented and treated in fundamentally different ways. This requires records (struct) and sequences (std::vector) to have separate implementation types on the client and server sides. Thus, the client and server types are now prefixed with ClnT_ and SrvT_ respectively. Newly generated typedef aliases are hiding the prefixes from user code.
  • IDL files don’t need ‘ = 0 ‘ postfixes for methods any more. After all, generating non-virtual methods wasn’t really used anyway.
  • The Enum introspection facilities got rewritten so things like the Enum name are also accessible now. This area probably isn’t fully finished yet, for future Any integration a more versatile API is needed still.
  • Auxillary information for properties is now accessible through an __aida_aux_data__() method on generated interfaces.
  • Generated records now provide a template method __accept__<>(Visitor) to visit all record fields by value reference and name string. Exemplary visitor implementations are provided to serialize/deserialize records to XML and INI file formats.

BEAST Developments

For the most part, changes in Beast are driving or chasing Rapicorn at the moment. This means that often the tip of Rapicorn master is required to build Beast’s master branch. Here is why:

  • Beast now uses RAPIDRES(1) to embedd compressed files. Rapicorn::Blob and Rapicorn::Res make these accessible.
  • Beast now makes use of Rapicorn’s IDL compiler to generate beastrc config structures and to add a new ‘Bse‘ IDL layer into libbse that allows the UI code to interface with Bse objects via C++ interfaces. Of course, lots of additional porting work is needed to complete this.
  • Beast procedures (a kind of ‘remote method’ implemented in C with lots of boilerplate code) are now migrated to C++ methods one by one which majorly simplifies the code base, but also causes lots of laborious adaptions on the call sites, the UI and the undo system. An excursion into the changes this brings for the undo implementation is provided in DevLog: A day of templates.
  • The GParamSpec introspection objects for properties that Beast uses for GUI generation can now be constructed from __aida_aux_data__()  strings, which enabled the beastrc config structure migration.
  • An explanatory file HACKING.md was added which describes the ongoing migration efforts and provides help in accessing the object types involved.

What’s next?

For the moment, porting the object system in Beast from GObject to IDL based C++11 interfaces and related procedure, signal and property migrations is keeping me more than busy. I’ll try to focus on completing the majority of work in this area first. But for outlooks, adding a Python REPL might make a good followup step. 😉

Jun 272015

c++11Yesterday I spent some 14+ hours on getting a templated undo method wrapper going.
Just to throw it all away this morning.

Here’s what I was trying to achieve, the C version of BEAST implements undo as follows:

// bse_track_remove_tick():
BseTrack *track;
uint tick;
BsePart *part;
bse_item_push_undo_proc (track, "insert-part", tick, part);

That is, it queues an undo step, that if executed, will call the “insert-part” procedure
on a BseTrack object that inserts a BsePart object at a ‘tick’.
This all happens through a varargs interface with lots of magic behind the scenes. In
particular the reference to ‘part’ is tricky. Future modifications to the BseTrack (or
project) may cause the removal and destruction of the BsePart object involved here.
While the execution of future undo steps will re-create a BsePart to be inserted here
before the step at hand is executed, the ‘part’ object pointer will have to be changed
to the re-created one instead of the destroyed one.
To achieve this, bse_item_push_undo_proc() internally converts the ‘part’ pointer into
a serializable descriptor string that allows to re-identify the BsePart object and the
undo machinery will resolve that before “insert-part” is called.

Now on to C++. I wanted the new pendant in the C++ version of Beast to look like:

// TrackImpl::remove_tick():
TrackImpl *this;
const uint tick;
PartImpl &part;
push_undo ("Remove Tick", *this, &TrackImpl::insert_part, tick, part);


Under the hood that means push_undo() (which is a template method on ItemImpl, a base type of TrackImpl) needs to process its variable argument list to:

  • A) Put each argument into a wrapper structure and store away the argument list (i.e. std::tuple<Wrapper<Args>…>).
  • B) Special case the wrapper structure for objects to store a descriptor internally (i.e. template specialisation on Wrapper<Arg> for Arg=ItemImpl& or derived).
  • C) Copy the wrapped argument list into a closure to be called when the undo step is executed.
  • D) When the closure is called, “unwrap” each of the wrapped arguments to yield its original type (i.e. construct a std::tuple<Args…> from std::tuple<Wrapper<Args>…>).
  • E) When unwrapping an object, resolve the descriptor stored internally (i.e. put more magic into Wrapper<Arg> to yield a valid Arg& object).
  • F) Construct a variable argument call to &TrackImpl::insert_part(…) (i.e. apply a C++ argument pack).

In short, I got A, B, C, D, F working after significant efforts.
A is somewhat straight forward with C++11 variable template arguments. C can be accomplished with a C++11 lambda capture list and F involves copying over std::integer_sequence from the C++14 proposals and hacking its std::apply() template to support instance + method calls. Last, D can be implemented in a related fashion to F.
What’s left is B and E, i.e. writing a wrapper that will store and yield ordinary arguments such as int or std::string and convert ItemImpl& derived types back and forth between a string representation.
Probably laborious but doable — or so I thought.

It turns out that because of all the argument and tuple packing hassle (template recursion, integer sequencing and more) involved in implementing A, D, F, it would be hard to pass needed serialization context into Wrapper<>. And what’s much worse is that g++-4.9 started to choke on template errors during the Wrapper<> development, aborting with “confused by earlier errors” after pages and pages of template error messages. clang++-3.4 isn’t yet capable of processing the C++11 used by Rapicorn, so it wasn’t of help here either (I plan on another attempt at porting my C++11 code to be clang++ compatible once I get my hands on a newer clang++ version).
I.e. in the end, I gave up after an overlong day in the middle of E, everything else having been accomplished. g++-4.9 choking was a main let down, but probably even more important is that I had the necessary state and mood to process multiple pages of template error messages yesterday, but the same cannot be expected of every push_undo() user in the future if any push_undo() argument ever mismatches.

This morning, I threw away yesterdays templating excess and within an hour got an alternative interface to work:

// undoing part removal needs an undo_descriptor because future
// deletions may invalidate and recreate the part object
TrackImpl *this;
const uint tick;
PartImpl &part;
UndoDescriptor<PartImpl> part_descriptor = undo_descriptor (part);
auto lambda = [tick, part_descriptor] (TrackImpl &self) {
  PartImpl &part = self.undo_resolve (part_descriptor);
  self.insert_part (utick, part);
push_undo ("Remove Tick", *this, lambda);

That is, this interface is fully type-safe, but the ‘part’ wrapping has to be done manually, which involves writing a small lambda around TrackImpl::insert_part(). If any argument of the lambda or push_undo() calls is erroneous, the compiler will point at a single failing variable assignment in the implementation of push_undo<>() and list the mismatching arguments.
That is much more digestible than multiple template recursion error pages, so it’s a plus on the side of future maintenance.

The short version of push_undo<>() that takes a method pointer instead of a lambda is still available for implementing undo steps that don’t involve object references, incidentally covering the majority of uses.