rippled/BUILD.md

## A crash course in CMake and Conan

To better understand how to use Conan,
we should first understand _why_ we use Conan,
and to understand that,
we need to understand how we use CMake.


### CMake

Technically, you don't need CMake to build this project.
You could manually compile every translation unit into an object file,
using the right compiler options,
and then manually link all those objects together,
using the right linker options.
However, that is very tedious and error-prone,
which is why we lean on tools like CMake.

We have written CMake configuration files
([`CMakeLists.txt`](./CMakeLists.txt) and friends)
for this project so that CMake can be used to correctly compile and link
all of the translation units in it.
Or rather, CMake will generate files for a separate build system
(e.g. Make, Ninja, Visual Studio, Xcode, etc.)
that compile and link all of the translation units.
Even then, CMake has parameters, some of which are platform-specific.
In CMake's parlance, parameters are specially-named **variables** like
[`CMAKE_BUILD_TYPE`][build_type] or
[`CMAKE_MSVC_RUNTIME_LIBRARY`][runtime].
Parameters include:

- what build system to generate files for
- where to find the compiler and linker
- where to find dependencies, e.g. libraries and headers
- how to link dependencies, e.g. any special compiler or linker flags that
    need to be used with them, including preprocessor definitions
- how to compile translation units, e.g. with optimizations, debug symbols,
    position-independent code, etc.
- on Windows, which runtime library to link with

For some of these parameters, like the build system and compiler,
CMake goes through a complicated search process to choose default values.
For others, like the dependencies,
_we_ had written in the CMake configuration files of this project
our own complicated process to choose defaults.
For most developers, things "just worked"... until they didn't, and then
you were left trying to debug one of these complicated processes, instead of
choosing and manually passing the parameter values yourself.

You can pass every parameter to CMake on the command line,
but writing out these parameters every time we want to configure CMake is
a pain.
Most humans prefer to put them into a configuration file, once, that
CMake can read every time it is configured.
For CMake, that file is a [toolchain file][toolchain].


### Conan

These next few paragraphs on Conan are going to read much like the ones above
for CMake.

Technically, you don't need Conan to build this project.
You could manually download, configure, build, and install all of the
dependencies yourself, and then pass all of the parameters necessary for
CMake to link to those dependencies.
To guarantee ABI compatibility, you must be sure to use the same set of
compiler and linker options for all dependencies _and_ this project.
However, that is very tedious and error-prone, which is why we lean on tools
like Conan.

We have written a Conan configuration file ([`conanfile.py`](./conanfile.py))
so that Conan can be used to correctly download, configure, build, and install
all of the dependencies for this project,
using a single set of compiler and linker options for all of them.
It generates files that contain almost all of the parameters that CMake
expects.
Those files include:

- A single toolchain file.
- For every dependency, a CMake [package configuration file][pcf],
    [package version file][pvf], and for every build type, a package
    targets file.
    Together, these files implement version checking and define `IMPORTED`
    targets for the dependencies.

The toolchain file itself amends the search path
([`CMAKE_PREFIX_PATH`][prefix_path]) so that [`find_package()`][find_package]
will [discover][search] the generated package configuration files.

**Nearly all we must do to properly configure CMake is pass the toolchain
file.**
What CMake parameters are left out?
You'll still need to pick a build system generator,
and if you choose a single-configuration generator,
you'll need to pass the `CMAKE_BUILD_TYPE`,
which should match the `build_type` setting you gave to Conan.

Even then, Conan has parameters, some of which are platform-specific.
In Conan's parlance, parameters are either settings or options.
**Settings** are shared by all packages, e.g. the build type.
**Options** are specific to a given package, e.g. whether to build and link
OpenSSL as a shared library.

For settings, Conan goes through a complicated search process to choose
defaults.
For options, each package recipe defines its own defaults.

You can pass every parameter to Conan on the command line,
but it is more convenient to put them in a [profile][profile].
**All we must do to properly configure Conan is edit and pass the profile.**
By default, Conan will use the profile named "default".
You can let Conan create the default profile with this command:

```
conan profile new default --detect
```


## Branches

For a stable release, choose the `master` branch or one of the [tagged
releases](https://github.com/ripple/rippled/releases).

```
git checkout master
```

For the latest release candidate, choose the `release` branch.

```
git checkout release
```

If you are contributing or want the latest set of untested features,
then use the `develop` branch.

```
git checkout develop
```


## Platforms

rippled is written in the C++20 dialect and includes the `<concepts>` header.
The [minimum compiler versions][2] that can compile this dialect are given
below:

| Compiler | Minimum Version
|---|---
| GCC         | 10
| Clang       | 13
| Apple Clang | 13.1.6
| MSVC        | 19.23

We do not recommend Windows for rippled production use at this time.
As of January 2023, the Ubuntu platform has received the highest level of
quality assurance, testing, and support.
Additionally, 32-bit Windows development is not supported.

Visual Studio 2022 is not yet supported.
This is because rippled is not compatible with [Boost][] versions 1.78 or 1.79,
but Conan cannot build Boost versions released earlier than them with VS 2022.
We expect that rippled will be compatible with Boost 1.80, which should be
released in August 2022.
Until then, we advise Windows developers to use Visual Studio 2019.

[Boost]: https://www.boost.org/


## Prerequisites

To build this package, you will need Python (>= 3.7),
[Conan][] (>= 1.55), and [CMake][] (>= 3.16).

> **Warning**
> The commands in this document are not meant to be blindly copied and pasted.
> This document is written for multiple audiences,
> meaning that your particular circumstances may require some commands and not
> others.
> You should never run any commands without understanding what they do
> and why you are running them.
>
> These instructions assume a basic familiarity with Conan and CMake.
> If you are unfamiliar with Conan,
> then please read the [crash course](#a-crash-course-in-cmake-and-conan)
> at the beginning of this document,
> or the official [Getting Started][3] walkthrough.

[Conan]: https://conan.io/downloads.html
[CMake]: https://cmake.org/download/

You'll need to compile in the C++20 dialect:

```
conan profile update settings.compiler.cppstd=20 default
```

Linux developers will commonly have a default Conan [profile][] that compiles
with GCC and links with libstdc++.
If you are linking with libstdc++ (see profile setting `compiler.libcxx`),
then you will need to choose the `libstdc++11` ABI:

```
conan profile update settings.compiler.libcxx=libstdc++11 default
```

We find it necessary to use the x64 native build tools on Windows.
An easy way to do that is to run the shortcut "x64 Native Tools Command
Prompt" for the version of Visual Studio that you have installed.

Windows developers must build rippled and its dependencies for the x64
architecture:

```
conan profile update settings.arch=x86_64 default
```

If you have multiple compilers installed on your platform,
then you'll need to make sure that Conan and CMake select the one you want to
use.
This setting will set the correct variables (`CMAKE_<LANG>_COMPILER`) in the
generated CMake toolchain file:

```
conan profile update 'conf.tools.build:compiler_executables={"c": "<path>", "cpp": "<path>"}' default
```

It should choose the compiler for dependencies as well,
but not all of them have a Conan recipe that respects this setting (yet).
For the rest, you can set these environment variables:

```
conan profile update env.CC=<path> default
conan profile update env.CXX=<path> default
```


## How to build and test

Let's start with a couple of examples of common workflows.
The first is for a single-configuration generator (e.g. Unix Makefiles) on
Linux or MacOS:

```
conan export external/snappy snappy/1.1.9@
mkdir .build
cd .build
conan install .. --output-folder . --build missing --settings build_type=Release
cmake -DCMAKE_TOOLCHAIN_FILE:FILEPATH=build/generators/conan_toolchain.cmake -DCMAKE_BUILD_TYPE=Release ..
cmake --build .
./rippled --unittest
```

The second is for a multi-configuration generator (e.g. Visual Studio) on
Windows:

```
conan export external/snappy snappy/1.1.9@
mkdir .build
cd .build
conan install .. --output-folder . --build missing --settings build_type=Release --settings compiler.runtime=MT
conan install .. --output-folder . --build missing --settings build_type=Debug --settings compiler.runtime=MTd
cmake -DCMAKE_TOOLCHAIN_FILE:FILEPATH=build/generators/conan_toolchain.cmake ..
cmake --build . --config Release
cmake --build . --config Debug
./Release/rippled --unittest
./Debug/rippled --unittest
```

Now to explain the individual steps in each example:

1. Export our [Conan recipe for Snappy](./external/snappy).

    It does not explicitly link the C++ standard library,
    which allows us to statically link it.

1. Create a build directory (and move into it).

    You can choose any name you want.

    Conan will generate some files in what it calls the "install folder".
    These files are implementation details that you don't need to worry about.
    By default, the install folder is your current working directory.
    If you don't move into your build directory before calling Conan,
    then you may be annoyed to see it polluting your project root directory
    with these files.
    To make Conan put them in your build directory,
    you'll have to add the option
    `--install-folder` or `-if` to every `conan install` command.

1. Generate CMake files for every configuration you want to build.

    For a single-configuration generator, e.g. `Unix Makefiles` or `Ninja`,
    you only need to run this command once.
    For a multi-configuration generator, e.g. `Visual Studio`, you may want to
    run it more than once.

    Each of these commands should have a different `build_type` setting.
    A second command with the same `build_type` setting will just overwrite
    the files generated by the first.
    You can pass the build type on the command line with `--settings
    build_type=$BUILD_TYPE` or in the profile itself, under the section
    `[settings]`, with the key `build_type`.

    If you are using a Microsoft Visual C++ compiler, then you will need to
    ensure consistency between the `build_type` setting and the
    `compiler.runtime` setting.
    When `build_type` is `Release`, `compiler.runtime` should be `MT`.
    When `build_type` is `Debug`, `compiler.runtime` should be `MTd`.

1. Configure CMake once.

    For all choices of generator, pass the toolchain file generated by Conan.
    It will be located at
    `$OUTPUT_FOLDER/build/generators/conan_toolchain.cmake`.
    If you are using a single-configuration generator, then pass the CMake
    variable [`CMAKE_BUILD_TYPE`][build_type] and make sure it matches the
    `build_type` setting you chose in the previous step.

    This step is where you may pass build options for rippled.

1. Build rippled.

    For a multi-configuration generator, you must pass the option `--config`
    to select the build configuration.
    For a single-configuration generator, it will build whatever configuration
    you passed for `CMAKE_BUILD_TYPE`.

5. Test rippled.

    The exact location of rippled in your build directory
    depends on your choice of CMake generator.
    You can run unit tests by passing `--unittest`.
    Pass `--help` to see the rest of the command line options.


### Options

The `unity` option allows you to select between [unity][5] and non-unity
builds.
Unity builds may be faster for the first build (at the cost of much
more memory) since they concatenate sources into fewer translation
units.
Non-unity builds may be faster for incremental builds, and can be helpful for
detecting `#include` omissions.

Below are the most commonly used options,
with their default values in parentheses.

- `assert` (OFF): Enable assertions.
- `reporting` (OFF): Build the reporting mode feature.
- `tests` (ON): Build tests.
- `unity` (ON): Configure a [unity build][5].
- `san` (): Enable a sanitizer with Clang. Choices are `thread` and `address`.


### Troubleshooting

#### Conan

If you find trouble building dependencies after changing Conan settings,
then you should retry after removing the Conan cache:

```
rm -rf ~/.conan/data
```


#### no std::result_of

If your compiler version is recent enough to have removed `std::result_of` as
part of C++20, e.g. Apple Clang 15.0,
then you might need to add a preprocessor definition to your bulid:

```
conan profile update 'env.CFLAGS="-DBOOST_ASIO_HAS_STD_INVOKE_RESULT"' default
conan profile update 'env.CXXFLAGS="-DBOOST_ASIO_HAS_STD_INVOKE_RESULT"' default
conan profile update 'tools.build:cflags+=["-DBOOST_ASIO_HAS_STD_INVOKE_RESULT"]' default
conan profile update 'tools.build:cxxflags+=["-DBOOST_ASIO_HAS_STD_INVOKE_RESULT"]' default
```


#### recompile with -fPIC

```
/usr/bin/ld.gold: error: /home/username/.conan/data/boost/1.77.0/_/_/package/.../lib/libboost_container.a(alloc_lib.o):
  requires unsupported dynamic reloc 11; recompile with -fPIC
```

If you get a linker error like the one above suggesting that you recompile
Boost with position-independent code, the reason is most likely that Conan
downloaded a bad binary distribution of the dependency.
For now, this seems to be a [bug][1] in Conan just for Boost 1.77.0 compiled
with GCC for Linux.
The solution is to build the dependency locally by passing `--build boost`
when calling `conan install`:

```
conan install --build boost ...
```


## How to add a dependency

If you want to experiment with a new package, here are the steps to get it
working:

1. Search for the package on [Conan Center](https://conan.io/center/).
1. In [`conanfile.py`](./conanfile.py):
    1. Add a version of the package to the `requires` property.
    1. Change any default options for the package by adding them to the
    `default_options` property (with syntax `'$package:$option': $value`)
1. In [`CMakeLists.txt`](./CMakeLists.txt):
    1. Add a call to `find_package($package REQUIRED)`.
    1. Link a library from the package to the target `ripple_libs` (search for
    the existing call to `target_link_libraries(ripple_libs INTERFACE ...)`).
1. Start coding! Don't forget to include whatever headers you need from the
   package.


[1]: https://github.com/conan-io/conan-center-index/issues/13168
[2]: https://en.cppreference.com/w/cpp/compiler_support/20
[3]: https://docs.conan.io/en/latest/getting_started.html
[5]: https://en.wikipedia.org/wiki/Unity_build
[build_type]: https://cmake.org/cmake/help/latest/variable/CMAKE_BUILD_TYPE.html
[runtime]: https://cmake.org/cmake/help/latest/variable/CMAKE_MSVC_RUNTIME_LIBRARY.html
[toolchain]: https://cmake.org/cmake/help/latest/manual/cmake-toolchains.7.html
[pcf]: https://cmake.org/cmake/help/latest/manual/cmake-packages.7.html#package-configuration-file
[pvf]: https://cmake.org/cmake/help/latest/manual/cmake-packages.7.html#package-version-file
[find_package]: https://cmake.org/cmake/help/latest/command/find_package.html
[search]: https://cmake.org/cmake/help/latest/command/find_package.html#search-procedure
[prefix_path]: https://cmake.org/cmake/help/latest/variable/CMAKE_PREFIX_PATH.html
[profile]: https://docs.conan.io/en/latest/reference/profiles.html