Default Component ACL Rules (3.x)

Default Component ACL Rules
===========================

Joomla Tracker: http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=30550 (submitted for both Joomla 2.5 and 3.x)


Background / Motivation
-----------------------

There is currently no easy way to specify the default action permission rules
for a component that has component-specific custom permission rules.  If
component developers want default ACL rules for their custom ACL rules, they
must create code that modifies the root asset.  This is extra work for
component developers and a potential source of errors and problems.

Proposed Fix
------------

The proposed fix addresses this problem by extending the syntax of component
'access.xml' file that lists the component-specific ACL rules.  The goal is to
allow the component developer to specify the groups that should be allowed to
perform the ACL action as part of the XML definition for each ACL rule.  This
would require updates to Joomla (the platform) to interpret the additions to
the 'access.xml' and add the appropriate permissions to the system.  

The proposed function provides new functions to install or remove the default
permissions specified in the 'access.xml' file.  Normally this would be done
in by the installer by having the component installer call the new function
during installation.  In the following description, assume the new function is
called by the installer.

These additions will be backwards compatible since the additional XML
attributes will be ignored by versions of Joomla that have not implemented
this fix.

Since there are many different ways that a particular component can implement
ACL rules, a few guiding principles are necessary:

  1. If the default for a custom ACL rule is not specified, the default root
     ACL rules will not be modified.  This is the current behavior.

  2. If the group with the specified permssions in the default rules does not
     exist in the Joomla installation, no root permission will be set and an
     information message will be generated.  Some Joomla installations have
     been known to delete all common groups (such as Public, Registered) and
     create new groups. So there is no guarantee that normally expected groups
     will exist.

  3. The component developer will specify a component (usually com_content)
     and an action to check and the installer will find a group with that
     permission.  The component developer may give a suggestion for which
     group to check first.  If the group is found and has the required
     permissions, then it will be used.  Otherwise the installer will search
     for a suitable group with the required permissions.  All groups will be
     checked and the one with the least authority will be used (eg, if both
     Author and Publisher have the required permission, Author will be chosen
     since Publisher has more authority).

  4. This new feature would only be applied to any custom component-specific
     rule in a components 'access.xml'.  This approach cannot be used to
     modify the core rules in the root rule.


Example 'access.xml' syntax additions
-------------------------------------

The following syntax change is proposed.  In the following examples, the
component for the 'access.xml' file is 'com_example':

```xml

```

Notice the new 'default' attribute at the end.  The groups will be searched to
find the least authoritative group that has the 'core.delete' permission for
com_content (for instance, delete example items they created).  This will mean
that all derived groups also have this permission.

Note that this could be a comma-separated list of group names.

The component developer could also specify a hint on which group to check
first:

```xml

```

In this case, the installer would first check to see if the 'Author' group has
the required permission.  If it does, it would be used.  If not, no warning
message would be given and the search for a group with that permission would
continue.

Note that the test component could be something other than 'com_content':

```xml

```

Which would find a group that has permission to do 'core.delete' for the
'com_xyz123' component.  This could be used to synchronize default permissions
between two related components.


Testing the new functionality
-----------------------------

A test component 'com_permtest' is available on github:

   https://github.com/jmcameron/component_permissions_test

This includes a zipped up version of the component that is ready to install.

   comp_permtest.zip

This test component should work on both Joomla 2.5.x and 3.x.  Obviously the
functions will not do anything if the fixes to Joomla described here are not
implemented.

To try these fixes:

   * Install the fixes on your Joomla 2.5 or 3.x site (by doing 'git pull' or
     using a patch file)

   * Install the component in the regular way and then go to the
     'component-permissions-test' component under the 'Components' menu in the
     back end.

   * Read the  instructions there on how to test it.

Note that these fixes include unit tests for the new capabilities (on the
Joomal 3.x version).  See the bottom of the this document.


Implementation Notes
--------------------

The functionality above is implemented by adding functions to JAccess and
JRules:


**JRules::removeActions($action_regexp)**

 * Remove all actions from the rule whose action name matches the regular
   expression.  This a general function and could be used to remove core
   actions from the root rule, if the calling code requested it (not
   recommended).


**JAccess::getGroupId($groupname)**

 * Returns a group ID for the given group name.

 * This could be moved elsewhere, but there did not seem to be a suitable
   place.  We may want to add a JUserGroup class (in
   libraries/joomla/user/group.php) and move this function into that class.


**JAccess::removeDescendentGroups($groupIds)**

 * Removes all groups in the provided set of groups that have ancestors in the
   set of groups.  This is part of the code that helps select the least
   authoritative group since it eliminates children of groups that are already
   in the set since the child group is likely to have more permissions than
   its parent group.


**JAccess::leastAuthoritativeGroup($groupIds, $asset)**

 * Selects the least authoritative group out of a set of groups.  
   Read the code for a clearer idea of how this works.


**JAccess::installComponentDefaultRules($component, $file = null)**

 * Parse the rules for custom rules in component's 'access.xml' file and
   install new permissions in the root asset, as needed.

 * Does not allow modifying core rules.

 * If a group with the required permission is not found or the group name
   specified in the hint part is unknown by the Joomla installation, the
   default specified will be ignored.  Currently a log message is being added.
   This should probably be a JApplication::enqueueMessage() call but I could
   not get that to work in the unit tests.

 * This function can be called any time, but it would make sense to call this
   during component installation in the postflight section.

 * The optional $file can be used to specify an alternate location for the
   'acess.xml' file (for testing).


**JAccess::purgeComponentDefaultRules($component)**

 * Removes all custom rules for $component defined in the root asset.

 * Core rules will not be modified.

 * This function can be called at any time, but it would make sense to call
   this during the uninstallation of a component (during preflight or the
   uninstall function).


Unit tests
----------

 * Unit tests for Joomla 3 that cover the features above have been added to
   the *tests/unit/suites/libraries/access* folder.

 * The custom unit test configuration file
   **tests/system/servers/configdef.php** was added to the toplevel .gitignore
   since it could contain private data and should be ignored (like .gitignore
   already ignores the top-level phpunit config file **phpunit.xml**).


Jonathan Cameron  
April 28, 2013