[Avocado-devel] RFC: Plugin execution order

[Cleber Rosa]

As detailed in the following card:

  https://trello.com/c/oWzrV48E/837-execution-order-support-in-plugins

It should be possible to specify a custom order for plugins to be
executed by setting specific configuration.

The first observed approach would be to create a section
called `[plugins.<type>]` where the `<type>`  conforms to the
description on fully qualified plugin names given here:


https://github.com/avocado-framework/avocado/pull/1495/commits/193a10ce98cb5747395eefcb485dd452696b4b11#diff-0f4f89ace79fa15278d9b283c2d9d9b2R84

Then, by creating a key named `order`, containing the short names as a
list.  Enabled plugins not listed will be executed *after* plugins
listed, but in non-determined order.

For instance, consider the following entry points::

  'avocado.plugins.result' : [
     'xunit = avocado.plugins.xunit:XUnitResult',
     'json = avocado.plugins.jsonresult:JSONResult',
     'archive = avocado.plugins.archive:Archive',
     'mail = avocado.plugins.mail:Mail',
     'html = avocado_result_html:HTMLResult'
   ]

We can say that:

* The plugin type, according to the fully qualified plugin name
  definition here is `result`.

* The plugin fully qualified names are:
  - result.xunit
  - result.json
  - result.archive
  - result.mail
  - result.html

* The short names for plugins of type "result" are:
  - xunit
  - json
  - archive
  - mail
  - html

To make sure that the mail plugin is run after (and thus includes)
the HTML result, the following configuration entry can be set::

  [plugins.result]
  order = html, archive

The other result plugins, namely xunit, json and mail, will still
be run.  It's guaranteed they'll be run *after* the other result
plugins.  The order in which they'll run after the explicitly
ordered plugins is undefined.

Other possible approach
-----------------------

The other approach possible, would require a default order value
for plugins.  This would still preferably be done in configuration
rather than in code.  Then, the fully qualified name for a plugin could
be used as part of the configuration section.  Example::

  [plugin.result.archive]
  order = 50

  [plugin.result.html]
  order = 30

This would make the `html` plugin run before the `archive` plugin.
While more verbose, it would allow for external plugins to ship with
stock configuration files that would set, by default, its ordering.

Feedback is highly appreciated!

-- 
Cleber Rosa
[ Sr Software Engineer - Virtualization Team - Red Hat ]
[ Avocado Test Framework - avocado-framework.github.io ]