CMake Plugins

Problem

• plugin names and plugin folders not always exactly match (resolver_*, crypto_*)
• plugin should be able to register new variants
• there should be only one place to define a new plugin
• multiple categories should be possible per plugin, defined in README.md
• on some OS some plugins won't work (simpleini)
• some unit tests depend on bindings

Constraints

• should work with supported CMake
• It should be easy to add trivial plugins (without dependency and variants)
• It should be possible to add complex plugins (with dependencies on plugins/unit tests and many variants)

Assumptions

Considered Alternatives

• have one more mapping that describes folder <-> plugins (bad coherence)
• directly use README.md to also describe CMake deps (too limited in expression)
• split up AddPlugin.cmake and CMakeLists.txt (does not work well with variants)
• simply adding all directories in src/plugins and decide within the add_plugin if we should drop the plugin (see below in Rationale)

Names for flag:

• DEPENDENCIES -> might be used for add_plugin
• ASSEMBLE_DEPS
• FIND_DEPS -> not only finding happens
• PROCESS_DEPS
• HANDLE_DEPS
• DEPS_MODE -> too generic, new terminology
• CHECK_DEPS -> not only checking happens
• DO_DEPS -> sounds funny
• FIND_PACKAGES -> not only packages are subject to this phase
• COLLECT -> needs NOT
• FIND_PHASE -> needs NOT
• DEPENDENCY -> ambiguous

Decision

Introduce a CMake process where all plugins are processed three times. Following CMake variables are used for the phases:

• COLLECTION_PHASE .. collect all add_plugins
• DEPENDENCY_PHASE .. resolve all dependencies, do add_plugins again
• ADDTESTING_PHASE .. (reserve for potential 3rd phase)

1. Collection phase (COLLECTION_PHASE is ON), add_plugin internally builds up:
   • ADDED_PLUGINS
   • REMOVED_PLUGINS
   • ADDED_DIRECTORIES
2. assemble dependency phase (DEPENDENCY_PHASE is ON, only considering ADDED_DIRECTORIES), with:
   • find_libraries, actually search for libraries on the system (only relevant libraries of plugins that are considered for inclusion)
   • add_plugin, with actually adding the plugins
3. assemble all unit tests (ADDTESTING_PHASE is ON), either
   • with ADD_TEST in add_plugin, or
   • with add_plugintest (for unittests that have dependencies to bindings)

Rationale

Solves all the issues without adding too much complexity for actually adding plugins.

Maintaining additional mappings is very time-consuming. Simply adding all plugins directories is problematic. It would:

• clutter the CMake output (especially in the case of errors)
• introduce more variables into the CMakeCache which are irrelevant for the user
• maybe even find libraries in wrong versions which are incompatible to what other plugins need

Implications

• need to adopt all CMakeLists.txt

Related Decisions

• Plugin Variants

Notes

Limitations

• ADDED_DIRECTORIES of variants will be kept.
• Typos in plugin names are currently not checked, strings that are not plugin names are simply ignored.