config-api.txt

===============================
 gEDA Configuration System API
===============================

:Author: Peter Brett
:Status: Implemented

..
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  to generate HTML or LaTeX.  However, the markup is designed to be
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.. contents::
..
    1  Introduction
    2  Error Handling
      2.1  Errors in C functions
      2.2  Errors in Scheme functions
    3  Configuration Contexts
      3.1  Obtaining a Context
        3.1.1  Default context
        3.1.2  System context
        3.1.3  User context
        3.1.4  Cache context
      3.2  Loading configuration files
      3.3  Saving configuration files
      3.4  Context parents
      3.5  Context trust
    4  Configuration Parameters
      4.1  Groups
      4.2  Keys
      4.3  Configuration inheritance
      4.4  Values
        4.4.1  Getting configuration values
        4.4.2  Setting configuration values
    5  Configuration Events
      5.1  Event propagation
      5.2  Handlers
      5.3  Delaying events
    6  References

Introduction
============

This document outlines the API provided by liblepton for configuration
of both liblepton and applications based on it.  The API is designed for
not only saving and loading static configuration data to and from
permanent storage, but also to allow on-line event-driven updates to
the configuration state.  The API is designed to be equally usable
from C code and from Scheme extensions.  It is heavily inspired by the
``GKeyFile`` API from GLib, with a few changes and enhancements
[GLIB]_.

Error Handling
==============

Errors in C functions
---------------------

Recoverable errors that occur in C functions are signalled to the
caller using the "GError" mechanism [GLIB]_.

- File errors (e.g. "Access denied" or "File not found") are indicated
  using the ``G_IO_ERROR`` error domain.  See the GIO manual for
  information on error codes.  Functions that may cause a
  ``G_IO_ERROR`` are clearly noted as such.

- All other errors are indicated using the ``EDA_CONFIG_ERROR`` error
  domain.  The possible error codes are:

  ``EDA_CONFIG_ERROR_UNKNOWN_ENCODING``
    The text being parsed was in an unknown encoding.

  ``EDA_CONFIG_ERROR_PARSE``
    The configuration data was ill-formed.

  ``EDA_CONFIG_ERROR_KEY_NOT_FOUND``
    A requested configuration key was not found.

  ``EDA_CONFIG_ERROR_GROUP_NOT_FOUND``
    A requested configuration group was not found.

  ``EDA_CONFIG_ERROR_INVALID_VALUE``
    A configuration value could not be parsed into the requested
    format.

Errors in Scheme functions
--------------------------

All errors in Scheme functions are reported by raising Guile errors
[GUILE]_.  There are two possible error keys that may arise:

- File errors (e.g. "Access denied" or "File not found") are indicated
  with the ``system-error`` key.

- All other errors are indicated using the ``config-error`` key.  The
  ``data`` part of the error arguments is a list containing one of the
  following symbols:

  ``unknown-encoding``
    The text being parsed was in an unknown encoding.

  ``parse``
    The configuration data was ill-formed.

  ``key-not-found``
    A requested configuration key was not found.

  ``group-not-found``
    A requested configuration group was not found.

  ``invalid-value``
    A configuration value could not be parsed into the requested
    format.

Configuration Contexts
======================

A configuration parameter is always evaluated within a *configuration
context*.  Each context is associated with a configuration file
(although the file does not necessarily need to exist).

Each configuration context may have a *parent context*.  If, when
looking up a parameter, it has no value set in the selected context,
the parent context is checked, and so on.

Configuration contexts are reference counted.  You can increment and
decrement their reference counters using ``g_object_ref()`` and
``g_object_unref()`` [GOBJECT]_.

Four special contexts are always automatically defined: the `default
context`_, the `system context`_, the `user context`_ and the
`cache context`_.  The user context is the default parent context for
newly-created configuration contexts.

Obtaining a Context
-------------------

C function::

  EdaConfig *eda_config_get_context_for_path (const gchar *path,
                                              GError *error)

Scheme function::

  path-config-context path

Normally, you shouldn't create a configuration context directly; you
should obtain the configuration context associated with a ``path``.
If a context matching the path does not yet exist, it is created.

``path-config-context`` looks for a configuration file named
``geda.conf``.  If ``path`` is not a directory, it is truncated, and
then a file named ``geda.conf`` is looked for in that directory.  If
none is found, the parent directory is checked, and so on until a
configuration file is found or the filesystem root is reached.  If no
configuration file was found, the returned context will be associated
with a ``geda.conf`` in the same directory as ``path``.

.. warning:: Do not assume that the configuration file associated with
             the context returned by ``path-config-context`` is
             located in the directory specified by ``path``.

The configuration context returned by
``eda_config_get_context_for_path()`` is owned by the library, and
should not be disposed of with ``g_object_unref()``.


Default context
~~~~~~~~~~~~~~~

C function::

  EdaConfig *eda_config_get_default_context ()

Scheme function::

  default-config-context

The default context is not associated with any physical path or
on-disk configuration file, and has no parent context.  It contains
the default configuration used when no configuration file can be
loaded.  Applications should normally populate the default context
with their built-in default configuration settings on start-up, before
loading any further configuration files.

The configuration context returned by
``eda_config_get_default_context()`` is owned by the library, and
should not be disposed of with ``g_object_unref()``.

System context
~~~~~~~~~~~~~~

C function::

  EdaConfig *eda_config_get_system_context ()

Scheme function::

  system-config-context

The system context is used for system-wide configuration.  It is located:

1. By searching ``${XDG_CONFIG_DIRS}`` for a ``gEDA/geda-system.conf``
   configuration file.

2. By checking ``${sysconfdir}/gEDA`` for a configuration file.

Its parent context is the `default context`_.

The configuration context returned by
``eda_config_get_system_context()`` is owned by the library, and
should not be disposed of with ``g_object_unref()``.

User context
~~~~~~~~~~~~

C function::

  EdaConfig *eda_config_get_user_context ()

Scheme function::

  user-config-context

The user context is used for user-specific configuration, and is
loaded from ``${XDG_CONFIG_HOME}/gEDA/geda-user.conf`` [XDGDIRS]_.
Its parent context is the `system context`_.

The configuration context returned by
``eda_config_get_user_context()`` is owned by the library, and should
not be disposed of with ``g_object_unref()``.

Cache context
~~~~~~~~~~~~

C function::

  EdaConfig *eda_config_get_cache_context ()

Scheme function::

  None yet

The cache context is used for program-specific configuration, and is
loaded from ``${XDG_CACHE_HOME}/lepton-eda/gui.conf`` [XDGDIRS]_.
It has no parent context.

The configuration context returned by
``eda_config_get_cache_context()`` is owned by the library, and should
not be disposed of with ``g_object_unref()``.

Loading configuration files
---------------------------

Other than the `default context`_, all configuration contexts are
associated with an on-disk configuration file.

C function::

  const gchar *eda_config_get_filename (EdaConfig *cfg)

Scheme function::

  config-filename cfg

Return the filename of the configuration file associated with the
context ``cfg``.  For some contexts (including the `default
context`_), this will return ``NULL`` (in C) or ``#f`` (in Scheme).

C function::

  gboolean eda_config_load (EdaConfig *cfg, GError **err)

Scheme function::

  config-load! cfg

Attempt to load configuration parameters for the context ``cfg`` from
its associated file.  This function may generate a ``GIOError`` (in
C) or a ``system-error`` (in Scheme).

C function::

  gboolean eda_config_is_loaded (EdaConfig *cfg)

Scheme function::

  config-loaded? cfg

Determine whether the context ``cfg`` has been successfully loaded
from disk.

Saving configuration files
--------------------------

C function::

  gboolean eda_config_save (EdaConfig *cfg, GError *error)

Scheme function::

  config-save! cfg

Attempt to save configuration parameters for the context ``cfg`` to
its associated file.  This function may generate a ``GIOError`` (in
C) or a ``system-error`` (in Scheme).

C function::

  gboolean eda_config_is_changed (EdaConfig *cfg)

Scheme function::

  config-changed? cfg

Determine whether the context ``cfg`` has been altered since it was
last synchronised with the on-disk version by loading or saving it.

Context parents
---------------

C function::

  EdaConfig *eda_config_get_parent (EdaConfig *cfg)

Scheme function::

  config-parent cfg

Return the parent context of the context ``cfg``, if it has one.

C function::

  void eda_config_set_parent (EdaConfig *cfg, EdaConfig *parent)

Scheme function::

  set-config-parent! cfg parent

Sets ``parent`` as the parent context of ``cfg``.  If ``parent`` is
NULL or ``#f``, sets ``cfg`` as having no parent context.

.. note:: Normally, application code should avoid using this function;
          keeping to the default configuration inheritance structure
          is recommended in order to ensure consistent behaviour of
          all liblepton applications.

Context trust
-------------

Some configuration parameters are dangerous; in particular, parameters
that may lead to arbitrary code execution need to be handled
carefully.  Such settings might include:

- Preferred PDF reader
- Preferred web browser
- Search path for Scheme plugins

Configuration contexts can be flagged as being 'trusted'.  This allows
code that needs to access such dangerous parameters to determine
whether the value has been obtained from a safe source.

By default, the `default context`_, `system context`_, `user
context`_ and `cache context`_ are trusted, and all other contexts untrusted.

C function::

  gboolean eda_config_is_trusted (EdaConfig *cfg)

Scheme function::

  config-trusted? cfg

Test whether ``cfg`` is a trusted configuration context.

C function::

  void eda_config_set_trusted (EdaConfig *cfg, gboolean trusted)

Scheme function::

  set-config-trusted! cfg trusted

Set whether the configuration context ``cfg`` should be trusted as a
source for dangerous configuration parameters.

.. warning:: You should not set a configuration context as trusted
             unless you are certain that it originated from a safe
             source (e.g. by interacting with the user to verify it).

C function::

  EdaConfig *eda_config_get_trusted_context (EdaConfig *cfg)

Scheme function::

  config-trusted-context cfg

If ``cfg`` is trusted, returns ``cfg``; otherwise, returns the first
parent context of the configuration context ``cfg`` that is a trusted
context.  If no trusted context could be found, returns NULL or
``#f``.

Configuration Parameters
========================

A gEDA/gaf *configuration parameter* consists of three components:

Group
  A UTF-8 string which identifies the general category in which the
  parameter lies (e.g. which application).

Name
  A UTF-8 string which specifically identifies the parameter within
  the group.

Value
  The value of the parameter.  This is stored as a UTF-8 string, but
  can be converted to a number of possible scalar and list types.

Groups, names and values are all case-sensitive.

Groups
------

C function::

  gchar **eda_config_get_groups (EdaConfig *cfg, gsize *length)

Scheme function::

  config-groups cfg

Returns a list of all groups available in ``cfg`` and its parent
contexts.  The value returned by ``eda_config_get_groups()`` is a
newly-allocated NULL-terminated array of strings.  Use
``g_strfreev()`` to free it [GLIB_].  The ``length`` argument is an
optional return location for the number of groups returned.

C function::

  gboolean eda_config_has_group (EdaConfig *cfg, const gchar *group)

Scheme function::

  config-has-group? cfg group

Test whether ``cfg`` or its parent contexts contain the specified
``group``.

Keys
----

C function::

  gchar **eda_config_get_keys (EdaConfig *cfg, const gchar *group,
                               gsize *length, GError **error)

Scheme function::

  config-keys cfg group

Returns a list of all keys available in the specified ``group`` in
``cfg`` and its parent contexts.  The value returned by
``eda_config_get_keys()`` is a newly-allocated NULL-terminated array
of strings.  Use ``g_strfreev()`` to free it [GLIB_].  The ``length``
argument is an optional return location for the number of keys
returned.  If neither ``cfg`` nor any of its parent contexts contain
``group``, raises an ``EdaConfigError`` (in C) or a ``config-error``
(in Scheme).

C function::

  gboolean eda_config_has_key (EdaConfig *cfg, const gchar *group,
                               const gchar *key, GError **err)

Scheme function::

  config-has-key? cfg group key

Test whether ``cfg`` or its parent contexts contains ``key`` in the
specified ``group``.

Configuration inheritance
-------------------------

C function::

  gboolean eda_config_is_inherited (EdaConfig *cfg, const gchar *group,
                                    const gchar *key, GError **err)

Scheme function::

  config-inherited? cfg group key

Tests whether the value of the configuration parameter with the given
``group`` and ``key`` is specified in the context ``cfg``, or whether
it is inherited from a parent context of ``cfg``.

C function::

  EdaConfig *eda_config_get_source (EdaConfig *cfg, const gchar *group,
                                    const gchar *key, GError **err)

Scheme function::

  config-source group key

Returns the configuration context (either ``cfg`` or one of its parent
contexts) in which the configuration parameter with the given
``group`` and ``key`` has a value specified.


Values
------

Each value is stored as a UTF-8 string in the configuration file.
However, this string can be parsed as several different types.  All of
the following types are supported:

- UTF-8 strings
- Booleans
- Integers
- Double-precision floating point numbers

In addition, lists of all of the above are supported.

Getting configuration values
~~~~~~~~~~~~~~~~~~~~~~~~~~~~

In general, errors in obtaining a value using the C functions should
be checked for using a GError.  The list functions take a return
location for the ``length`` of list (for
``eda_config_get_string_list()`` this argument may be NULL, since the
returned array is NULL-terminated).  Returned strings should be freed
using ``g_free()``, and returned lists should be freed using
``g_free()`` or ``g_strfreev()`` as appropriate [GLIB]_.

C functions::

  gchar *eda_config_get_string (EdaConfig *cfg, const gchar *group, const gchar *key,
                                GError **err)

  gboolean eda_config_get_boolean (EdaConfig *cfg, const gchar *group, const gchar *key,
                                   GError **err)

  gint eda_config_get_int (EdaConfig *cfg, const gchar *group, const gchar *key,
                           GError **err)

  gdouble eda_config_get_double (EdaConfig *cfg, const gchar *group, const gchar *key,
                                 GError **err)

  gchar **eda_config_get_string_list (EdaConfig *cfg, const gchar *group, const gchar *key,
                                      gsize *length, GError **err)

  gboolean *eda_config_get_boolean_list (EdaConfig *cfg, const gchar *group, const gchar *key,
                                         gsize *length, GError **err)

  gint *eda_config_get_int_list (EdaConfig *cfg, const gchar *group, const gchar *key,
                                 gsize *length, GError **err)

  gdouble *eda_config_get_double_list (EdaConfig *cfg, const gchar *group, const gchar *key,
                                       gsize *length, GError **err)

Scheme functions::

  config-string cfg group key [default]
  config-boolean cfg group key [default]
  config-int cfg group key [default]
  config-real cfg group key [default]
  config-string-list cfg group key [default]
  config-boolean-list cfg group key [default]
  config-int-list cfg group key [default]
  config-real-list cfg group key [default]

If a ``default`` argument is specified, it is returned in lieu of
raising a ``config-error`` exception if an error occurs.

All values returned by ``config-real`` and ``config-real-list`` are
real and inexact [GUILE]_, even if the on-disk representation would
permit an exact representation.

Setting configuration values
~~~~~~~~~~~~~~~~~~~~~~~~~~~~

C functions::

  void eda_config_set_string (EdaConfig *cfg, const gchar *group, const gchar *key,
                              const gchar *value)

  void eda_config_set_boolean (EdaConfig *cfg, const gchar *group, const gchar *key,
                               gboolean value)

  void eda_config_set_int (EdaConfig *cfg, const gchar *group, const gchar *key,
                           gint value)

  void eda_config_set_double (EdaConfig *cfg, const gchar *group, const gchar *key,
                              gdouble value)

  void eda_config_set_string_list (EdaConfig *cfg, const char *group, const gchar *key,
                                   const gchar * const list[], gsize length)

  void eda_config_set_boolean_list (EdaConfig *cfg, const gchar *group, const gchar *key,
                                    gboolean list[], gsize length)

  void eda_config_set_int_list (EdaConfig *cfg, const gchar *group, const gchar *key,
                                gint list[], gsize length)

  void eda_config_set_double_list (EdaConfig *cfg, const gchar *group, const gchar *key,
                                   gdouble list[], gsize length)

Scheme functions::

  set-config! cfg group key value

The ``set-config!`` function infers the necessary type from the type
of its ``value`` argument.

Configuration Events
====================

When the value of a configuration parameter is altered, either by
loading a file or directly via the API, a *configuration event* is
generated.  Handlers can be registered to be notified when a
configuration event occurs.  Handlers can match on:

- A particular group and key;

- A particular group;

- or on all parameters.

The events are also emitted as a GObject "config-changed" signal.

Event propagation
-----------------

Recall that configuration value look-ups "flow" from configuration
contexts to their parent contexts, i.e. up the configuration tree.  In
a similar manner, configuration events flow *down* the configuration
tree, from the context where the parameter was altered to any child
contexts which inherit that parameter's value.

For example, consider a context ``parent`` which has child contexts
``child1`` and ``child2``.  In ``parent``, the parameter ``foo`` has
value ``bar``; in ``child1``, its value is ``baz``; and in ``child2``,
it is not set.

::

                   parent  [foo=bar]
                     |
                    / \
   [foo=baz]  child1   child2  [foo not set]

If the ``foo`` parameter is altered in ``parent``, then the resulting
event will be dispatched in ``parent`` and ``child2``.

Handlers
--------

In C, configuration event handlers must have the
``EdaConfigEventHandlerFunc`` prototype::

  void handler (EdaConfig *cfg, const gchar *group, const gchar *key, gpointer user_data)

In Scheme, a configuration event handler must be a closure that
accepts three arguments::

  handler cfg group key

``cfg`` is always the configuration context that received the event,
and the ``group`` and ``key`` identify the configuration parameter
that was changed.  The ``user_data`` argument to the C handler is a
data structure provided by the API user at the time when the handler
was registered.

Adding and removing handlers is quite straightforward:

C function::

  void eda_config_event_add (EdaConfig *cfg, const gchar *group, const gchar *key, EdaConfigEventHandlerFunc func, gpointer user_data)

Scheme function::

  add-config-event! cfg func [group [key]]

Registers ``func`` to be called when the configuration parameter
identified by ``group`` and ``key`` is modified in the context
``cfg``.  If ``key`` is not specified (or NULL), matches on any
parameter in ``group``.  If group is not specified (or NULL), matches
on any parameter.  If an identical event handler was already
registered, does nothing.

C function::

  void eda_config_event_remove (EdaConfig *cfg, const gchar *group, const gchar *key, EdaConfigEventHandlerFunc func, gpointer user_data)

Scheme function::

  remove-config-event! cfg func [group [key]]

Unregisters the event handler ``func`` from the context ``cfg``.  The
``group``, ``key``, and (in C) ``user_data`` parameters must match
those specified when the event handler was registered.  If no matching
event handler was registered, does nothing.

C function::

  void eda_config_event_remove_all (EdaConfig *cfg, EdaConfigEventHandlerFunc func, gpointer user_data)

Scheme function::

  remove-config-event! cfg func #t

Unregisters all occurrences of the event handler ``func`` from the
context ``cfg``, no matter what events ``func`` was set to trigger on.
In C, the ``user_data`` parameter must match that specified when the
event handler was registered.


Delaying events
---------------

It is sometimes desirable to delay configuration event notifications.
For example, this might be necessary if you are planning to change a
large number of configuration values at the same time.

C functions::

  void eda_config_event_freeze (EdaConfig *cfg)
  void eda_config_event_thaw (EdaConfig *cfg)

When ``eda_config_event_freeze()`` is called on it, the context
``cfg`` stops dispatching event notifications.  Whenever an event
notification would normally be dispatched, it instead is added to an
internal queue.  When ``eda_config_event_thaw()`` is called, any
queued events are dispatched.  Multiple pairs of
``eda_config_event_freeze()`` and ``eda_config_event_thaw()`` calls
can be nested.

Scheme function::

  delay-config-events cfg thunk

Call ``thunk``, delaying configuration event notifications until
``thunk`` exits, either normally or non-locally.


References
==========

.. [GLIB]    The GNOME Project, "GLib Reference Manual".
             <http://library.gnome.org/devel/glib/stable/>

.. [GOBJECT] The GNOME Project, "GObject Reference Manual".
             <http://library.gnome.org/devel/glib/stable/>

.. [GUILE]   Free Software Foundation, "GNU Guile Reference Manual", Version
             1.8.
             <http://www.gnu.org/software/guile/manual/>

.. [XDGDIRS] W. Bastian, R. Lortie and L. Poettering, "XDG Base
             Directory Specification", Version 0.7.
             <http://www.freedesktop.org/wiki/Standards/basedir-spec>

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