Plugin Variants

Problem

Some plugins are generic in the sense that they cannot fully define their contract statically. (Note: if they can decide it statically, you should prefer compilation variants.) Instead their contract is based on their configuration. We will call every combination of plugins+configuration, where we get a different contract plugin variant.

The current issue is that there is no way to enumerate possible plugin variants as needed to list all functionality of Elektra.

Constraints

• User Customizability:
  • Users must be able to add new plugin variants
  • Users should be able to suppress plugin variants
• Modularity:
  • The plugins themselves yield a list of plugin variants
  • The code defining variants is part of the plugin's code
  • To get a full list of all variants of all plugins, one simply has to iterate over all plugins and ask the plugins for possible variants.
• libtool should provide a convenience layer for easy access of all plugin variants. In this case there should also be a performance optimization, that plugins do not need to be reopened required multiple times.

Assumptions

• The number of variants needs to be bounded: plugin variants need to be fully enumerable

Considered Alternatives

A specification of the plugin's configuration and a tool that can enumerate all possible essential configurations. Problems here are:

• Plugins might need to self-reference (a validation plugin might have plugin variants, too)
• Elektra’s specification language was not complete nor consistent at point of writing
• Does not fit with the checkconf (see here) approach.

Decision

Implementation delayed after 1.0.

Best implementation candidate was:

1. Provide a function int genconf (KeySet * ks, Key * errorKey) where ks is filled with a list of all variants with the essential configuration (subkeys config) and the changed parts of the contract (subkeys infos).
2. Keys defined in system:/elektra/plugins/<plugin>/variants/<variantname> have the same content, but take precedence. If a variant with the same name is defined, only config or infos from genconf are considered if they are not mentioned in system:/elektra/plugins/variants.
3. If the bool key override (for a plugin or a variant) is true, it will be overwritten (content of genconf ignored, but instead a plugin or variant as given is created).
4. If the bool key disable (for a plugin or a variant) is true the plugin or a variant of the plugin will not be available.
5. Empty config and infos mean:
   • config: The "default variant" (without any parameter) should be also available (has useful functionality)
   • infos: It is not possible to determine the changes of the contract, the user need to instantiate the plugin and enquiry the contract.

Example

genconf for augeas yields:

system:/access
system:/access/config
system:/access/config/lens = Access.lns
system:/access/infos
system:/access/infos/provides = storage/access
system:/aliases
system:/aliases/config
system:/aliases/config/lens = Aliases.lns
system:/aliases/infos
system:/aliases/infos/provides = storage/aliases

genconf for python might yield:

user:/configparser/config
user:/configparser/config/script = python_configparser.py

The user:/admin specifies:

system:/elektra/plugins/jni/disable = 1
system:/elektra/plugins/augeas/variants/access
system:/elektra/plugins/augeas/variants/access/disable = 1
system:/elektra/plugins/augeas/variants/aliases
system:/elektra/plugins/augeas/variants/aliases/infos
system:/elektra/plugins/augeas/variants/aliases/infos/status = 10000
system:/elektra/plugins/python/variants/configparser
system:/elektra/plugins/python/variants/configparser/override = 1
system:/elektra/plugins/python/variants/configparser/config
system:/elektra/plugins/python/variants/configparser/config/script = mybetter_configparser.py

As result we get:

1. access of plugin augeas is not available
2. aliases of plugin augeas as defined from genconf, but with changes in contract (infos/status)
3. configparser of plugin python is completely redefined (result from genconf will not be considered) but it will be considered as specified.
4. the plugin jni will not be available

To have a space-separated simpleini one would use:

system:/elektra/plugins/simpleini/variants/spacesep
system:/elektra/plugins/simpleini/variants/spacesep/config
system:/elektra/plugins/simpleini/variants/spacesep/config/format = "% %"

Rationale

• The genconf API was chosen to be consistent with checkconf. The pluginhandle might be missing for some scenarios. If it is needed, we should change both APIs for consistency reasons.
• The very verbose (deeply nested) subkeys (and names instead of arrays) provide better customizability and extensibility.

Implications

• genconf needs to be implemented for the plugins:
  • augeas
  • jni
  • lua
  • python
• PluginDatabase needs an extension to list all plugin variants
• kdb plugin-list should be able to list all variants, e.g. like: augeas lens=Access.lns so that a user can copy and paste this for the kdb mount command.

Current state

Currently only overrides of plugin configuration (config/...) is possible, overrides of other plugin information (contract information) does not work yet.

It is also not possible to add additional information to a variant, only overrides work. E.g.

system:/elektra/plugins/augeas/variants/aliases
system:/elektra/plugins/augeas/variants/aliases/override = 1
system:/elektra/plugins/augeas/variants/aliases/config
system:/elektra/plugins/augeas/variants/aliases/config/lens = Aliases.lns
system:/elektra/plugins/augeas/variants/aliases/config/otherparam = 0

works, while

system:/elektra/plugins/augeas/variants/aliases
system:/elektra/plugins/augeas/variants/aliases/config
system:/elektra/plugins/augeas/variants/aliases/config/otherparam = 0

gets ignored.

Related Decisions

• CMake Plugins

Notes

Discussions took place here.