calamares/src/modules/README.md

# Calamares modules

Calamares modules are plugins that provide features like installer pages,
batch jobs, etc. An installer page (visible to the user) is called a "view",
while other modules are "jobs".

Each Calamares module lives in its own directory.

All modules are installed in `$DESTDIR/lib/calamares/modules`.

There are two **types** of Calamares module:
* viewmodule, for user-visible modules. These may be in C++, or PythonQt.
* jobmodule, for not-user-visible modules. These may be done in C++,
  Python, or as external processes.

A viewmodule exposes a UI to the user. The PythonQt-based modules
are considered experimental (and as of march 2019 may be on the
way out again as never-used-much and PythonQt is not packaged
on Debian anymore).

There are three (four) **interfaces** for Calamares modules:
* qtplugin (viewmodules, jobmodules),
* python (jobmodules only),
* pythonqt (viewmodules, jobmodules, optional),
* process (jobmodules only).

## Module directory

Each Calamares module lives in its own directory. The contents
of the directory depend on the interface and type of the module.

### Module descriptor

A Calamares module must have a *module descriptor file*, named
`module.desc`. For C++ (qtplugin) modules using CMake as a build-
system and using the calamares_add_plugin() function -- this is the
recommended way to create such modules -- the module descriptor
file is optional, since it can be generated by the build system.
For other module interfaces, the module descriptor file is required.

The module descriptor file must be placed in the module's directory.
The module descriptor file is a YAML 1.2 document which defines the
module's name, type, interface and possibly other properties. The name
of the module as defined in `module.desc` must be the same as the name
of the module's directory.

Module descriptors **must** have the following keys:
- *name* (an identifier; must be the same as the directory name)
- *type* ("job" or "view")
- *interface* (see below for the different interfaces; generally we
  refer to the kinds of modules by their interface)

Module descriptors for Python and PythonQt modules **must** have the following key:
- *script* (the name of the Python script to load, nearly always `main.py`)

Module descriptors **may** have the following keys:
- *requiredModules* (a list of modules which are required for this module
  to operate properly)
- *emergency* (a boolean value, set to true to mark the module
  as an emergency module)

### Required Modules

A module may list zero (if it has no requirements) or more modules
by name. As modules are loaded from the global sequence in `settings.conf`,
each module is checked that all of the modules it requires are
already loaded before it. This ensures that if a module needs
another one to fill in globalstorage keys, that happens before
it needs those keys.

### Emergency Modules

Only C++ modules and job modules may be emergency modules. If, during an
*exec* step in the sequence, a module fails, installation as a whole fails
and the install is aborted. If there are emergency modules in the **same**
exec block, those will be executed before the installation is aborted.
Non-emergency modules are not executed.

If an emergency-module fails while processing emergency-modules for
another failed module, that failure is ignored and emergency-module
processing continues.

Use the EMERGENCY keyword in the CMake description of a C++ module
to generate a suitable `module.desc`.

A module that is marked as an emergency module in its module.desc
must **also** set the *emergency* key to *true* in its configuration file
(see below). If it does not, the module is not considered to be an emergency
module after all (this is so that you can have modules that have several
instances, only some of which are actually needed for emergencies).

### Module-specific configuration

A Calamares module **may** read a module configuration file,
named `<modulename>.conf`. If such a file is present in the
module's directory, it can be shipped as a *default* configuration file.
This only happens if the CMake-time option `INSTALL_CONFIG` is on.

The sample configuration files may work and may be suitable for
your distribution, but no guarantee is given about their stability
beyond syntactic correctness.

The module configuration file, if it exists, is a YAML 1.2 document
which contains a YAML map of anything.

All sample module configuration files are installed in
`$DESTDIR/share/calamares/modules` but can be overridden by
files with the same name placed manually (or by the packager)
in `/etc/calamares/modules`.



## C++ modules

Currently the recommended way to write a module which exposes one or more
installer pages (viewmodule) is through a C++ and Qt plugin. Viewmodules must
implement `Calamares::ViewStep`. They can also implement `Calamares::Job`
to provide jobs.

To add a Qt plugin module, put it in a subdirectory and make sure it has
a `CMakeLists.txt` with a `calamares_add_plugin` call. It will be picked
up automatically by our CMake magic. The `module.desc` file is optional.



## Python modules

Modules may use one of the python interfaces, which may be present
in a Calamares installation (but also may not be). These modules must have
a `module.desc` file. The Python script must implement one or more of the
Python interfaces for Calamares -- either the python jobmodule interface,
or the experimental pythonqt job- and viewmodule interfaces.

To add a Python or process jobmodule, put it in a subdirectory and make sure
it has a `module.desc`. It will be picked up automatically by our CMake magic.
For all kinds of Python jobs, the key *script* must be set to the name of
the main python file for the job. This is almost universally `main.py`.

`CMakeLists.txt` is *not* used for Python and process jobmodules.

Calamares offers a Python API for module developers, the core Calamares
functionality is exposed as `libcalamares.job` for job data,
`libcalamares.globalstorage` for shared data and `libcalamares.utils` for
generic utility functions. Documentation is inline.

All code in Python job modules must obey PEP8, the only exception are
`libcalamares.globalstorage` keys, which should always be
camelCaseWithLowerCaseInitial to match the C++ identifier convention.

For testing and debugging we provide the `testmodule.py` script which
fakes a limited Calamares Python environment for running a single jobmodule.

### Python Jobmodule

A Python jobmodule is a Python program which imports libcalamares and has a
function `run()` as entry point. The function `run()` must return `None` if
everything went well, or a tuple `(str,str)` with an error message and
description if something went wrong.

### Python API

**TODO:** this needs documentation



## PythonQt modules

The PythonQt modules are considered experimental and may be removed again
due to low uptake. Their documentation is also almost completely lacking.

### PythonQt Jobmodule

A PythonQt jobmodule implements the experimental Job interface by defining
a subclass of something.

### PythonQt Viewmodule

A PythonQt viewmodule implements the experimental View interface by defining
a subclass of something.

### Python API

**TODO:** this needs documentation



## Process jobmodules

A process jobmodule runs a (single) command. The interface is *process*,
while the module type must be *job* or *jobmodule*.

The module-descriptor key *command* should have a string as value, which is
passed to the shell -- remember to quote it properly. It is generally
recommended to use a *shellprocess* job module instead (less configuration,
easier to have multiple instances).