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Generated C++ code for CS API

The code in Quotient/csapi, Quotient/identity and Quotient/application-service, although stored in Git, is actually generated from the official Matrix Client-Server API definition files. If you're unhappy with something in there and want to improve that, you have to understand the way these files are produced and setup some additional tooling. The shortest possible procedure resembling the below text can be found in .github/workflows/ci.yml (regeneration of those files is tested in CI) - see the tasks "Get CS API definitions; clone and build GTAD" and "Regenerate API code".

Why generate the code at all?

Because otherwise we have to do monkey business of writing boilerplate code, with the same patterns, types etc., literally, for every single API endpoint, and one of libQuotient authors got fed up with it at some point in time. By then about 15 job classes have been written; the whole API is about 100 endpoints, with new ones added and some existing changed in each Matrix protocol version. Other considerations can be found in this talk about API description languages that also briefly touches on GTAD - the tool written for the purpose.

Prerequisites for CS API code generation

  1. Get the source code of GTAD and its dependencies. Since version 0.7, libQuotient includes GTAD as a submodule so you can get everything you need by updating gtad/gtad submodule in libQuotient sources: git submodule update --init --recursive gtad/gtad.

    You can also just clone GTAD sources to keep them separate from libQuotient: git clone --recursive https://github.com/quotient-im/gtad.git

  2. Configure and build GTAD: same as libQuotient, it uses CMake so this should be very straightforward (if not - you're probably not quite ready for this stuff anyway).

  3. Get Matrix CS API definitions from a matrix-spec repo. Although the official repo is at https://github.com/matrix-org/matrix-spec.git` (formerly https://github.com/matrix-org/matrix-doc.git), you cannot use its main branch with GTAD 0.9 (used for the current - 0.8 - version of libQuotient) because the project has recently moved to OpenAPI 3 and GTAD does not support it yet. For that reason as well as build predictability (the official Matrix repo doesn't check whether a particular change breaks code generation), a soft fork of the official definitions is kept at https://github.com/quotient-im/matrix-spec.git - that guarantees buildability of the generated code. This repo closely follows the official one (but maybe not its freshest commit), applying a few adjustments on top. And of course you can use your own repository if you need to change the API definition.

  4. If you plan to submit a PR with the generated code to libQuotient or just would like it to be properly formatted, you should either ensure you have clang-format (version 13 at least) in your PATH or pass -DCLANG_FORMAT=<path> to CMake, as mentioned in the next section.

Generating CS API contents

  1. Pass additional configuration to CMake when configuring libQuotient: -DMATRIX_SPEC_PATH=/path/to/matrix-spec/ -DGTAD_PATH=/path/to/gtad. Note that MATRIX_SPEC_PATH should lead to the repo (not the API folder) while GTAD_PATH should have the path to GTAD binary. If you need to specify where your clang-format is (see the previous section) add -DCLANG_FORMAT=/path/to/clang-format to the line above. If everything's right, the detected locations will be mentioned in CMake output and an additional build target called update-api will be configured.
  2. Generate the code: cmake --build <your build dir> --target update-api. Building this target will create (overwriting without warning) source files in Quotient/csapi, Quotient/identity, Quotient/application-service for all YAML files it can find in /path/to/matrix-spec/data/api/client-server and their dependencies.

Changing generated code

See the more detailed description of what GTAD is and how it works in the documentation on GTAD in its source repo. Only parts specific for libQuotient are described here.

GTAD uses the following three kinds of sources:

  1. OpenAPI files. Each file is treated as a separate source (unlike swagger-codegen, you do not need to have a single file for the whole API).
  2. A configuration file, in Quotient case it's gtad/gtad.yaml - common for all OpenAPI files GTAD is invoked on.
  3. Source code template files: gtad/*.mustache - are also common.

The Mustache files have a templated (not in C++ sense) definition of a network job class derived from BaseJob; if necessary, data structure definitions used by this job are put before the job class. Bigger Mustache files look a bit daunting for a newcomer; and the only known highlighter that can handle the combination of Mustache (originally a web templating language) and C++ can be found in CLion IDE. Fortunately, all our Mustache files are reasonably concise and well-formatted these days. To simplify things some reusable Mustache blocks are defined in gtad.yaml - see its mustache: section. Adventurous souls that would like to figure out what's going on in these files should speak up in the Quotient room - I (Kitsune) will be very glad to navigate you.

The types map in gtad.yaml defines a mapping from OpenAPI types to C++/Qt. It uses the following type attributes aside from pretty obvious imports::

  • avoidCopy - this attribute defines whether a const ref should be used instead of a value. For basic types like int this is obviously unnecessary; but compound types like QVector should rather be taken by reference when possible.
  • moveOnly - some types are not copyable at all and must be moved instead (an obvious example is any structure that uses, directly or indirectly, std::unique_ptr<>).
  • useOptional - wrap types that have no value with "null" semantics (i.e. number types and custom-defined data structures) into std::optional.
  • omittedValue - an alternative for useOptional, just provide a value used for an omitted parameter. This is used for bool parameters which normally are considered false if omitted (or they have an explicit default value, passed in the "official" GTAD's defaultValue variable).
  • initializer - this is a partial (see GTAD and Mustache documentation for explanations but basically it's a variable that is a Mustache template itself) that specifies how exactly a default value should be passed to the parameter. E.g., the default value for a QString parameter is transformed as u"{{defaultValue}}"_s.

Instead of relying on the event structure definition in the OpenAPI files, gtad.yaml uses pointers to libQuotient's event structures: EventPtr, RoomEventPtr and StateEventPtr. Respectively, arrays of events, when encountered in OpenAPI definitions, are converted to Events, RoomEvents and StateEvents containers. When there's no way to figure the type from the definition, an opaque QJsonObject is used, leaving the conversion to the library and/or client code.

Submitting API changes

Getting the API changes upstream requires coordination across a few Matrix projects (the API is a contract between the client and the server, after all). The recommended sequence is different, mainly depending on on whether or not you have to write a Matrix Spec Change aka MSC. Simply speaking, if your changes don't break compatibility with existing Matrix ecosystem (e.g. it's a documentation fix, or a fix in the API definition to align with the real use), or if you implement an existing approved MSC, you don't need to submit an MSC and it's a matter of two PRs: one to the official repo with the spec text and API definitions, that resides at https://github.com/matrix-org/matrix-spec and one to libQuotient.

If your changes require an MSC (e.g. you add a new API call or change an existing one beyond minor adjustments):

  1. Submit an MSC before submitting changes to the API definition files and libQuotient. See the link about MSCs above on what it should and should not have and how it should be submitted.
  2. The MSC gets reviewed by the Spec Core Team. This can be a lengthy process but it's necessary for the Matrix ecosystem integrity.
  3. When your MSC has at least some approvals (not necessarily a complete acceptance but at least some approvals should be there) the MSC process strongly recommends to show an implementation in existing projects. In the case of Client-Server API that usually means a homeserver and a client application. Submit PRs to the projects you took for that including libQuotient, referring to your MSC; for API definition files, use https://github.com/quotient-im/matrix-spec (the fork) instead of the official repo, as the official repo only accepts PRs on approved MSCs. You will have to show that your implementation is actually working to get your MSC approved.
  4. Once the MSC is accepted, you can officially submit your changes in API definitions as a "spec PR" to the official repo.

In any case, when submitting a PR for libQuotient, please make sure generated files are committed separately from non-generated ones (no need to make two PRs; just separate the files in different commits).