Improve rosdep: 1) Friendlier to beginners, 2) Clarifying rosdep is R…

…OS-agnostic (#3719)

* Improve: rosdep

- Avoid giving an impression that `rosdep` is ros-dependent. It is actually widely applicable than ROS projects.
   - This matters IMO where the development involves non-ROS environment e.g. corporate, where people tend to end up inventing their own dependency management framework that runs alongside of `rosdep`, resulted in `rosdep` not being contributed.
- Trying to make it friendlier to non-expert readers by clarifyig `rosdistro` website and some jargons.
- Explanation of `<*depend>` is missing in ros2 doc set. Something detailed like http://docs.ros.org/en/melodic/api/catkin/html/howto/format2/catkin_library_dependencies.html
   - Thought I'd add those in here but ran out of time. I don't know if rosdep page is the best for such info anyways.

* Improve: rosdep: Add link to detail about the types of dependency

Co-authored-by: Tomoya Fujita <[email protected]>
Co-authored-by: Chris Lalancette <[email protected]>

source/Tutorials/Intermediate/Rosdep.rst

@@ -2,82 +2,114 @@
 
     Tutorials/Rosdep
 
-.. _rosdep:
-
 Managing Dependencies with rosdep
 =================================
 
+.. contents:: Table of Contents
+   :depth: 2
+   :local:
+
 **Goal:** Manage external dependencies using ``rosdep``.
 
 **Tutorial level:** Intermediate
 
 **Time:** 5 minutes
 
-.. contents:: Contents
-   :depth: 2
-   :local:
-
-Author: Steve Macenski
-
 This tutorial will explain how to manage external dependencies using ``rosdep``.
 
 What is rosdep?
 ---------------
 
-``rosdep`` is ROS's dependency management utility that can work with ROS packages and external libraries.
-``rosdep`` is a command-line utility for identifying and installing dependencies to build or install a package.
-It can be or is invoked when:
+``rosdep`` is a dependency management utility that can work with packages and external libraries.
+It is a command-line utility for identifying and installing dependencies to build or install a package.
+``rosdep`` is *not* a package manager in its own right; it is a meta-package manager that uses its own knowledge of the system and the dependencies to find the appropriate package to install on a particular platform.
+The actual installation is done using the system package manager (e.g. ``apt`` on Debian/Ubuntu, ``dnf`` on Fedora/RHEL, etc).
 
-- Building a workspace and needing appropriate dependencies to build the packages within
-- Install packages (e.g. ``sudo apt install ros-{DISTRO}-demo-nodes-cpp``) to check the dependencies needed for it to execute
-- and more!
+It is most often invoked before building a workspace, where it is used to install the dependencies of the packages within that workspace.
 
 It has the ability to work over a single package or over a directory of packages (e.g. workspace).
 
+.. note::
+
+    While the name suggests it is for ROS, ``rosdep`` is semi-agnostic to ROS.
+    You can utilize this powerful tool in non-ROS software projects by installing it as a standalone Python package.
+    Successfully running ``rosdep`` relies on ``rosdep keys`` to be available, which can be downloaded from a public git repository with a few simple commands.
+
 A little about package.xml files
 --------------------------------
 
-A package's ``package.xml`` file contains a set of dependencies.
-The dependencies in this file are generally referred to as "rosdep keys".
-These are represented in the tags ``<depend>``, ``<test_depend>``, ``<exec_depend>``, ``<build_depend>``, and ``<build_export_depend>``.
-They specify in what situation each of the dependencies are required in.
-
-- For dependencies only used in testing the code (e.g. ``gtest``), use ``test_depend``.
-- For dependencies only used in building the code, use ``build_depend``.
-- For dependencies needed by headers the code exports, use ``build_export_depend``.
-- For dependencies only used when running the code, use ``exec_depend``.
-- For mixed purposes, use ``depend``, which covers build, export, and execution time dependencies.
+The ``package.xml`` is the file in your software where ``rosdep`` finds the set of dependencies.
+It is important that the list of dependencies in the ``package.xml`` is complete and correct, which allows all of the tooling to determine the packages dependencies.
+Missing or incorrect dependencies can lead to users not being able to use your package, to packages in a workspace being built out-of-order, and to packages not being able to be released.
 
+The dependencies in the ``package.xml`` file are generally referred to as "rosdep keys".
 These dependencies are manually populated in the ``package.xml`` file by the package's creators and should be an exhaustive list of any non-builtin libraries and packages it requires.
 
+These are represented in the following tags (see `REP-149 <https://ros.org/reps/rep-0149.html>`__ for the full specification):
+
+``<depend>``
+^^^^^^^^^^^^
+
+These are dependencies that should be provided at both build time and run time for your package.
+For C++ packages, if in doubt, use this tag.
+Pure Python packages generally don't have a build phase, so should never use this and should use ``<exec_depend>`` instead.
+
+``<build_depend>``
+^^^^^^^^^^^^^^^^^^
+
+If you only use a particular dependency for building your package, and not at execution time, you can use the ``<build_depend>`` tag.
+
+With this type of dependency, an installed binary of your package does not require that particular package to be installed.
+
+However, that can create a problem if your package exports a header that includes a header from this dependency.
+In that case you also need a ``<build_export_depend>``.
+
+``<build_export_depend>``
+^^^^^^^^^^^^^^^^^^^^^^^^^
+
+If you export a header that includes a header from a dependency, it will be needed by other packages that ``<build_depend>`` on yours.
+This mainly applies to headers and CMake configuration files.
+Library packages referenced by libraries you export should normally specify ``<depend>``, because they are also needed at execution time.
+
+``<exec_depend>``
+^^^^^^^^^^^^^^^^^
+
+This tag declares dependencies for shared libraries, executables, Python modules, launch scripts and other files required when running your package.
+
+``<test_depend>``
+^^^^^^^^^^^^^^^^^
+
+This tag declares dependencies needed only by tests.
+Dependencies here should *not* be duplicated with keys specified by ``<build_depend>``, ``<exec_depend>``, or ``<depend>``.
+
 How does rosdep work?
 ---------------------
 
 ``rosdep`` will check for ``package.xml`` files in its path or for a specific package and find the rosdep keys stored within.
 These keys are then cross-referenced against a central index to find the appropriate ROS package or software library in various package managers.
 Finally, once the packages are found, they are installed and ready to go!
 
-The central index is known as ``rosdistro``, which `may be found here <https://github.com/ros/rosdistro>`_.
+``rosdep`` works by retrieving the central index on to your local machine so that it doesn't have to access the network every time it runs (on Debian/Ubuntu the configuration for it is stored in ``/etc/ros/rosdep/sources.list.d/20-default.list``).
+
+The central index is known as ``rosdistro``, which `may be found online <https://github.com/ros/rosdistro>`_.
 We'll explore that more in the next section.
 
 How do I know what keys to put in my package.xml?
 -------------------------------------------------
 
 Great question, I'm glad you asked!
 
-For ROS packages (e.g. ``nav2_bt_navigator``), you may simply place the name of the package.
-You can find a list of all released ROS packages in ``rosdistro`` at ``<distro>/distribution.yaml`` for your given ROS distribution.
+* If the package you want to depend in your package is ROS-based, AND has been released into the ROS ecosystem [1]_, e.g. ``nav2_bt_navigator``, you may simply use the name of the package. You can find a list of all released ROS packages in https://github.com/ros/rosdistro at ``<distro>/distribution.yaml`` (e.g. ``humble/distribution.yaml``) for your given ROS distribution.
+* If you want to depend on a non-ROS package, something often called "system dependencies", you will need to find the keys for a particular library. In general, there are two files of interest:
 
-For non-ROS package system dependencies, we will need to find the keys for a particular library.
-In general, there are two files of interest: ``rosdep/base.yaml`` and ``rosdep/python.yaml``.
-``base.yaml`` in general contains the ``apt`` system dependencies.
-``python.yaml`` in general contains the ``pip`` python dependencies.
+  * `rosdep/base.yaml <https://github.com/ros/rosdistro/blob/master/rosdep/base.yaml>`_ contains the ``apt`` system dependencies
+  * `rosdep/python.yaml <https://github.com/ros/rosdistro/blob/master/rosdep/python.yaml>`_ contains the Python dependencies
 
-To find a key, search for your library in this file and find the name in ``yaml`` that contains it.
+To find a key, search for your library in these files and find the name.
 This is the key to put in a ``package.xml`` file.
 
 For example, imagine a package had a dependency on ``doxygen`` because it is a great piece of software that cares about quality documentation (hint hint).
-We would search ``base.yaml`` for ``doxygen`` and come across:
+We would search ``rosdep/base.yaml`` for ``doxygen`` and come across:
 
 .. code-block:: yaml
 
@@ -105,10 +137,35 @@ Pull requests for rosdistro are typically merged well within a week.
 `Detailed instructions may be found here <https://github.com/ros/rosdistro/blob/master/CONTRIBUTING.md#rosdep-rules-contributions>`_ for how to contribute new rosdep keys.
 If for some reason these may not be contributed openly, it is possible to fork rosdistro and maintain a alternate index for use.
 
-
 How do I use the rosdep tool?
 -----------------------------
 
+rosdep installation
+^^^^^^^^^^^^^^^^^^^
+
+If you are using ``rosdep`` with ROS, it is conveniently packaged along with the ROS distribution.
+This is the recommended way to get ``rosdep``.
+You can install it with:
+
+.. code-block:: bash
+
+    apt-get install python3-rosdep
+
+.. note::
+
+    On Debian and Ubuntu, there is another, similarly named package called ``python3-rosdep2``.
+    If that package is installed, make sure to remove it before installing ``python3-rosdep``.
+
+If you are using ``rosdep`` outside of ROS, the system package may not be available.
+In that case, you can install it directly from https://pypi.org:
+
+.. code-block:: bash
+
+    pip install rosdep
+
+rosdep operation
+^^^^^^^^^^^^^^^^
+
 Now that we have some understanding of ``rosdep``, ``package.xml``, and ``rosdistro``, we're ready to use the utility itself!
 Firstly, if this is the first time using ``rosdep``, it must be initialized via:
 
@@ -135,4 +192,6 @@ Breaking that down:
 - ``--ignore-src`` means to ignore installing dependencies, even if a rosdep key exists, if the package itself is also in the workspace.
 
 There are additional arguments and options available.
-Use ``rosdep -h`` to see them.
+Use ``rosdep -h`` to see them, or look at the more complete documentation for rosdep at http://docs.ros.org/en/independent/api/rosdep/html/ .
+
+.. [1] "released into the ROS ecosystem" means the package is listed in one or more of the ``<distro>/distribution.yaml`` directories in the `rosdistro database <https://github.com/ros/rosdistro>`_.