CONTRIBUTING.md

Contributing

Feedback and contributions are very welcome!

Here's help on how to make contributions, divided into the following sections:

The quick-read part:

• general information,
• vulnerability reporting,
• code conventions overview,
• documentation.

The long-read part:

• code changes,
• how to check proposed changes before submitting them,
• reuse of other libraries, frameworks etc.

General information

For specific proposals, please provide them as pull requests or issues For general discussion, feel free to use our Matrix room: #quotient:matrix.org.

If you're new to the project (or FLOSS in general), here's the list of issues that we consider simple enough for starters - they don't require much knowledge about the project and should be relatively straightforward (but not necessarily quick to fix). Welcome aboard!

Pull requests and different branches recommended

Contributions are preferred in the form of pull requests at GitHub. See the respective articles about pull requests to learn how to deal with them.

We recommend creating different branches for different (logical) changes, and creating a pull request when you're done; the development integration branch is dev. See the GitHub documentation on creating branches and using pull requests.

How we handle proposals

We use GitHub to track all changes via its issue tracker and pull requests. Specific changes are proposed using those mechanisms. Issues are assigned to an individual who works on it and then marks it complete. If there are questions or objections, the conversation area of that issue or pull request is used to resolve it.

License

Unless a contributor explicitly specifies otherwise, we assume contributors to agree that all contributed code is released either under LGPL v2.1 or later. This is more than just LGPL v2.1 libQuotient now uses because the project plans to switch to LGPL v3 for library code in the near future.

Any components proposed for reuse should have a license that permits releasing a derivative work under LGPL v3 or later (that includes licenses permitting LGPL v2.1 or later but not LGPL v2.1 only). In any case, the component should be redistributable under a license from the list approved by OSI, no exceptions.

We use SPDX conventions for copyright statements. Please follow them when making a sizable contribution: add your name and year to the top of the file. New files should begin with the following preamble:

// SPDX-FileCopyrightText: 2021 Your Name <your@email.address>
// SPDX-License-Identifier: LGPL-2.1-or-later

Vulnerability reporting (security issues) - see SECURITY.md

Code changes

The code should strive to be DRY (don't repeat yourself), clear, and syntactically correct (i.e. buildable). Some technical debt is inevitable but glaring inconsistencies, duplications etc. will most likely cause a request for changes at the pull request review. Refactoring is always welcome; if you do it within another feature development, please arrange the refactoring before the feature, to make the review easier.

C++ feature set

As of Quotient 0.8, the C++ standard for newly written code is C++20. Since none of the supported compilers (GCC 11, Clang 11, Apple Clang 12, MSVC 19.30 - see also the pre-requisites in README) can handle the entire C++20 feature set, we have to stick with a subset. Most notably:

• std::bind_front, ranges, std::format, std::source_location can't be used yet;
• while concepts and constraints as a language feature are there, most library concepts are not available as of Apple Clang 12;
• No constexpr containers except std::array (but you can, and should, use QLatin1String and Quotient::operator""_ls that creates it, for constexpr Latin-1 strings).

The compiler support page, of cppreference.com, combined with the list of compiler versions above, can be used to check whether a given feature is there. Be mindful that Clang build configuration on Linux does not use Clang libc++ but rather the GNU standard library (i.e. you should look at Clang column for core language features but GCC libstdc++ for library features).

Code style and formatting

The code style is defined by .clang-format, and in general, all C++ files should follow it. Reasonable deviations from the defined style are allowed; use // clang-format off and // clang-format on to protect them.

Most fundamental things from .clang-format:

• We (mostly) use Webkit style: 4-space indents, no tabs, no trailing spaces, no last empty lines. If you spot code that doesn't follow this, fix it on the spot, thank you.
• Prefer keeping lines within 100 characters. Slight overflows are ok if that helps readability. Ideally, just use clang-format to format lines.

API conventions

All non-inline symbols (functions, classes/structs, even namespace-level static variables) that are intended for use in client code must be declared with QUOTIENT_API macro (the macro itself is defined in the dedicated quotient_export.h file). This is concerned with symbols visibility in dynamic/shared libraries: the macro marks these symbols for exporting in the library symbol table. If you forget to use this macro where needed you will get linkage errors if you're lucky, obscure runtime errors otherwise (such as split-brained singleton instances). You only need to use the macro on the namespace level; inner symbols (member functions, e.g.) are exported if their class is exported.

Some header files of the library are not intended to be (directly) included by clients - these header files have names ending with _p.h (e.g. connection_p.h). All other header files are considered a part of the library official API (and, respectively, ABI). Calls, data structures and other symbols not intended for usage in client code should not be exposed in public header files, unless there's no way to keep them out. In particular, this includes private members (functions, typedefs, or variables) in public classes; use pimpl idiom to hide implementation details as much as possible. For cases when a definition has to be in the header file but should not be used in client code, use _impl namespace - anything in that namespace is not covered by API guarantees either (but may still contribute to the ABI surface, so tread carefully).

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. Make sure to read CODE_GENERATION.md before trying to change anything there.

Documentation changes

Most of the documentation is in Markdown format. All Markdown files use the .md filename extension.

Where reasonable, limit yourself to Markdown that will be accepted by different markdown processors (e.g., what is specified by CommonMark or the original Markdown). In practice, as long as libQuotient is hosted at GitHub, GFM (GitHub-flavoured Markdown) is used to show those files in a browser, so it's fine to use its extensions. In particular, you can mark code snippets with the programming language used; blank lines separate paragraphs, newlines inside a paragraph do not force a line break.

Be mindful that this is not the same (albeit similar) markdown algorithm used by GitHub when it renders issue or pull comments; in those cases newlines in paragraph-like content are considered as real line breaks; unfortunately this other algorithm is also called GitHub-flavoured markdown. (Yes, it would be better if there were different names for different things.)

In your markdown, please don't use tab characters and avoid "bare" URLs. In a hyperlink, the link text and URL should be on the same line. Both in C/C++ code comments and Markdown documents, try to keep your lines within the 100-character limit except hyperlinks (wrapping breaks them). Some historical text may not follow that rule - feel free to reformat those parts when you edit them.

Do not use trailing two spaces for line breaks, since these cannot be seen and may be silently removed by some tools. If, for whatever reason, a blank line is not an option, use <br/> (an HTML line break).

End of TL;DR

If you don't plan/have substantial contributions, you can stop reading here. Further sections are for those who's going to actively hack on the library code.

Code changes

More on code style and formatting

• Do not use struct when you have protected or private members; only use it to define plain-old-data structures, with maybe just a function or two among public members but no substantial behaviour. If you need access control or specific logic tightly coupled to the data structure, make it a class instead and consider if you still want to keep its member variables public.

• For newly created classes, keep to the rule of 3/5/0.

• Qt containers are generally preferred to STL containers, with exceptions listed below:

  • std::array and std::deque have no direct counterparts in Qt and are unconditionally accepted in the backend code. They can't be mapped to QML though; see another bullet below for implications of that.
  • Because of COW semantics, Qt containers cannot hold uncopyable classes. Classes without a default constructor are a problem too. Examples of that are SyncRoomData and EventsArray<>. Again, you can use STL containers for structures having those but consider the implications.
  • So, the implications. Because QML doesn't know about most of STL containers and cannot pull data out of them, you're only free to use STL containers in backend code (in the simplest case, within one .cpp file). The API exposing these containers can only be used from C++ code, with std::vector being a notable exception that QML knows about (but you still can't read uncopyable vectors such as EventsArray<>, as the previous bullet already said). For these cases you have to provide external means to iterate through the container and consume data from it; exposing a Qt item model is most natural to Qt code. If you don't provide such other means, expect questions at your pull request.
  • Notwithstanding the above (you're not going to use smart pointers with QML anyway), prefer std::unique_ptr<> over QScopedPointer<> as it gives stronger guarantees; also, some features of QScopedPointer are deprecated in Qt 6.

• Always use QVector instead of QList unless Qt's own API uses it - see the great article by Marc Mutz on Qt containers for details. With Qt 6, these two become the same type matching what used to be QVector in Qt 5.

  (Note: unfortunately, QVector is a type alias in Qt 6 and that breaks templated code because type deduction doesn't work with aliases. This breakage will go away as compilers adopt C++23 sufficiently but it may take a couple more years, as of this writing; in the meantime, the fix boils down to specifying the template parameter of QVector explicitly.)

• When you write logs within the library always use logging categories defined in logging_categories_p.h instead of plain qDebug(), to avoid a log line being assigned the default category. qCDebug(CATEGORY) is the preferred form; qDebug(CATEGORY) (without C) is accepted as well. Do not add new logging categories without necessity; if you do, make sure to add the new category to logging_categories_p.h, so that there's a central reference for all of them (mentioned in README.md, by the way).

Comments

Whenever you add a new call to the library API that you expect to be used from client code, make sure to supply a proper doc-comment along with the call. Quotient uses the Doxygen C++-styled doc-comments (//!, \brief); some legacy code may use Javadoc (/** ... */, @brief) or C-styled Doxygen (/*! ... */) but that is not encouraged any more. Some parts are not documented at all; adding doc-comments to them and/or converting the existing ones to the assumed style is highly encouraged; it's also a nice and easy first-time contribution.

Use \brief for the summary, and follow with details after an empty doc-comment line, using \param, \return etc. as necessary.

When commenting in-code:

• Don't restate what's happening in the code unless it's not really obvious. We assume the readers to have some command of C++ and Qt. If your code is not obvious, consider making it clearer itself before commenting.
• That said, both C++ and Qt have their arcane/novel features and dark corners, and education of code readers is a great thing. Use your experience to figure what might be not that well-known, and comment such cases: leave references to web pages, Quotient wiki etc. Do not comment std:: calls just because they are less known - readers are expected to know about cppreference.com and look it up.
• More important than everything above - make sure to document not so much "what" but more "why" certain code is done the way it is. In the worst case, the logic of the code can be reverse-engineered; but you can almost never reverse-engineer the line of reasoning and the pitfalls avoided.

Automated tests

We gradually introduce autotests based on a combination of CTest and Qt Test frameworks - see autotests/ directory. There are very few of those, as we have just started adding those to the new code (you guessed it; adding more tests to the old code is very welcome and also is a good exercise to get to know the library).

On top of that, libQuotient comes with a command-line end-to-end test suite called Quotest. Any significant addition to the library API should be accompanied by a respective test in autotests/ and/or in Quotest.

To add a test to autotests:

• In a new .cpp file in autotests/ (you don't need a header file), define a test class derived from QObject and write tests as member functions in its private slots: section. See other autotests to get an idea of what it should look like.
• Add a quotient_add_test macro call with your test to autotests/CMakeLists.txt

To add a test to Quotest:

• In quotest.cpp, add a new test to the TestSuite class. Similar to Qt Test, each test in Quotest is a private slot; unlike Qt Test, you should use special macros, TEST_DECL() and TEST_IMPL(), to declare and define the test (those macros conceal passing the testing handle in thisTest variable to the test method).
• In the test function definition, add test logic using FINISH_TEST macro to check for the test outcome and complete the test (be mindful that FINISH_TEST always returns, not only in case of error). ALL (even failing) branches should conclude with a FINISH_TEST (or FAIL_TEST that is a shortcut for a failing FINISH_TEST) invocation, unless you want the test to fail with a "DID NOT FINISH" message in the logs under certain conditions.

The TestManager class sets up some basic test fixture to help you with testing; notably, the tests can rely on having an initialised Room object with loaded state for the test room in targetRoom member variable. Note that it's normal for tests to go async, which is not something Qt Test is easy with (and this is why Quotest doesn't directly use Qt Test but rather fetches a few ideas from it).

Security and privacy

Pay attention to security, and work with, not against, the usual security hardening practices.

char * and similar unchecked C-style read/write arrays are forbidden - use Qt containers (QString/QLatin1String for strings, in particular) or std::array<>/std::span<> instead (E2EE-related code can also use handy byte_view_t and byte_span_t aliases). When dealing with QObjects, organise them in parent-child trees and let Qt manage object lifecycles for you instead explicit deletions. If that doesn't work in a given situation (no obvious parent, non-trivial lifecycle, the object doesn't derive from QObject, etc.), try to at least use std::unique_ptr<> and std::move() it when/where appropriate to make sure the object has clear ownership. Finally, shared pointers can be used when circular dependencies are not a concern. Avoid direct pointer arithmetic wherever possible and only use bare pointers within QObject parent-child trees and for non-owning/weak access to resources owned by std::unique_ptr<>. Consider using references instead of pointers, where applicable.

Exercise the principle of least privilege where reasonable and appropriate. Prefer internally cohesive code while avoiding too much external coupling.

Avoid dealing with any personal information (e.g. email addresses) and secrets (passwords, tokens, keys) unless it is the specific and exclusive purpose of the given code. Absolutely don't spill around this information in logs, and only display those in UI where really needed. Do not forget about the issue of local access (in particular, be very careful when storing something in temporary files, let alone permanent configuration or state).

Where possible, avoid mechanisms that could be used for user tracking (we do need to verify people are logged in but that's pretty much it), and ensure that third parties can't use interactions for tracking. Matrix protocols evolve towards decoupling personal information from user activity entirely - follow this trend.

Performance

We want the software to have decent performance for users even on weaker machines. At the same time we keep libQuotient single-threaded as much as possible, to keep the code simple. That means being cautious about operation complexity (read about big-O notation if you need a kickstart on the subject). This especially refers to operations on the whole timeline and the list of users - each of these can have tens of thousands of elements so even operations with linear complexity, if heavy enough (with I/O or complex processing), can produce noticeable GUI freezing or stuttering. When you don't see a way to reduce algorithmic complexity, either split processing into isolated pieces that can be individually scheduled as queued events (see the end of Connection::consumeRoomData() to get the idea) or uncouple the logic from GUI and execute it outside of the main thread with QtConcurrent facilities.

Having said that, there's always a trade-off between various attributes; in particular, readability and maintainability of the code is more important than squeezing every bit out of a clumsy algorithm. Beware of premature optimization and profile the code before diving into hardcore tweaking that might not give the benefits you think it would.

Speaking of profiling logs (see README.md on how to turn them on) - if you expect some code to take considerable (more than 10k "simple operations") time you might want to setup a QElapsedTimer and drop the elapsed time into logs under PROFILER logging category. See the existing code for examples - room.cpp has quite a few. In order to reduce small timespan logging spam, PROFILER log lines are usually guarded by a check that the timer counted big enough time (200 microseconds by default, 20 microseconds for tighter parts); this threshold can be altered at compile-time by defining PROFILER_LOG_USECS preprocessor symbol (i.e. passing -DPROFILE_LOG_USECS=<usecs> to the compiler if you're on Linux/macOS).

How to check proposed changes before submitting them

Checking the code on at least one configuration is essential; if you only have a hasty fix that doesn't even compile, better make an issue and put a link to your commit or gist into it (with an explanation what it is about and why).

Compiler warnings

The warnings configuration applied when using CMake can be found in CMakeLists.txt. Most warnings triggered by that configuration are not formally considered errors (the compiler will keep going) but please treat them as such.

If you want the IDE to be really picky about your code you can use the following line for the Clang analyzer code model to enable most compiler warnings while keeping the number of false positives at bay (that does not include clang-tidy/clazy warnings - see the next section on those): -Weverything -Werror=return-type -Wno-c++98-compat -Wno-c++98-compat-pedantic -Wno-c++20-compat -Wno-unused-macros -Wno-newline-eof -Wno-exit-time-destructors -Wno-global-constructors -Wno-gnu-zero-variadic-macro-arguments -Wno-documentation -Wno-missing-prototypes -Wno-shadow-field-in-constructor -Wno-padded -Wno-weak-vtables -Wno-unknown-attributes -Wno-comma -Wno-shadow-uncaptured-local -Wno-switch-enum -Wno-pragma-once-outside-header -Wno-range-loop-bind-reference -Wno-unsafe-buffer-usage

Static analysis tools

Many IDEs these days can automatically run your code through clang-tidy. The source code contains .clang-tidy file with the recommended set of checks that doesn't give too many false positives.

Qt Creator in addition knows about clazy, a Qt-aware static analysis tool that hunts for Qt-specific issues that are easy to overlook otherwise, such as possible unintended copying of a Qt container. Most of clazy checks are relevant to our code; here's the configuration line the author of this text is using: level2,no-foreach,no-non-pod-global-static,no-range-loop-reference,no-ctor-missing-parent-argument,no-missing-qobject-macro,no-jni-signatures,no-qt-keywords,no-qt4-qstring-from-array

Continuous Integration

We use GitHub Actions to check buildability and smoke-testing on Linux (GCC, Clang), MacOS (Clang), and Windows (MSVC). Every PR will go through these, and you'll see the traffic lights from them on the PR page. If your PR fails on any platform double-check that it's not your code causing it - and fix (or ask how to fix if you don't know) if it is. Generally, pull requests are not accepted until they build on all platforms.

Git commit messages

When writing git commit messages, try to follow the guidelines in How to Write a Git Commit Message:

1. Separate subject from body with a blank line.
2. Be reasonable on the subject line length, because this is what we see in commit logs that are constrained in horizontal space. Try to fit in 50 characters whenever possible.
3. Capitalize the subject line.
4. Do not end the subject line with a period.
5. Aim to use the imperative mood in the subject line (command form).
6. Use the body to explain what and why vs. how (git tracks how it was changed in detail, don't repeat that). Sometimes a quick overview of "how" is acceptable if a commit is huge - but maybe split a commit into smaller ones, to begin with?

Reuse (libraries, frameworks, etc.)

SDK/package management is unfortunately messy in C++, and we try to keep building the library as easy as possible. Besides, every additional dependency means additional attack surface and more effort to stay on most recent versions of all deps - not just upstream but within each ecosystem (Linux distros, Homebrew, etc.). Because of these considerations we are very conservative about adding dependencies to libQuotient. That mainly relates to libraries external to Qt; you can use most of non-visual Qt components as needed (with minor caveats mentioned below). Fortunately, even the Qt components now in use (Qt Core and Qt Network) are very feature-rich and provide plenty of ready-made stuff.

Some cases need additional explanation:

• Don't reinvent the wheel - look through documentation on Qt and C++ standard library and use existing facilities as much as possible. C++ standard in particular has grown considerably in recent years, providing many useful algorithms and primitives out of the box.
• libQuotient is a library to build Qt applications; for that reason, components from KDE Frameworks should be really lightweight and useful to be accepted as a dependency. If the intention is to better integrate libQuotient into KDE environment there's nothing wrong in building another library on top of libQuotient. Consider people who run LXDE or even GNOME and normally don't have KDE frameworks installed (some actively avoid installing those) - libQuotient caters to them too.
• Never forget that libQuotient is an offscreen library; it only depends on QtGui to handle QImage objects (entirely offscreen still). While there's a bunch of visual code (in C++ and QML) shared between Quotient-enabled applications, this is likely to end up in a separate (Quotient-backed) library, rather than in libQuotient itself.
• Also be mindful that libQuotient strives to be cross-platform: at least across the three most popular platforms (Linux, macOS, Windows) are supported officially, but we also try to stay friendly to other platforms, such as Android, BSD, or Haiku. If the introduced dependency (even from inside Qt - e.g. DBus on Linux) is unavailable on one of the platforms already supported, it can only be added to enable a platform-specific feature and should not break or disable functionality on other platforms.

Attribution

This text was originally based on CONTRIBUTING.md from CII Best Practices Badge project, which is a collective work of its contributors (many thanks!). The text is licensed under CC-BY-4.0.