CMake: introduce USE_<foo>

When there are multiple modules doing a thing and it really only
makes sense to have one of them in a given Calamares compilation,
the USE_<foo> variables allow you to select one, while ignoring
all the other implementations. If USE_<foo> is not set, all
implementations are included (as usual).

CMakeLists.txt

@@ -43,6 +43,22 @@ option( WITH_PYTHON "Enable Python modules API (requires Boost.Python)." ON )
 option( WITH_PYTHONQT "Enable next generation Python modules API (experimental, requires PythonQt)." ON )
 option( WITH_KF5Crash "Enable crash reporting with KCrash." ON )
 
+### USE_*
+#
+# By convention, when there are multiple modules that implement similar
+# functionality, and it only makes sense to have **at most one** of them
+# enabled at any time, those modules are named <foo>-<implementation>.
+# For example, services-systemd and services-openrc.
+#
+# Setting up SKIP_MODULES to ignore "the ones you don't want" can be
+# annoying and error-prone (e.g. if a new module shows up). The USE_*
+# modules provide a way to do automatic selection. To pick exactly
+# one of the implementations from group <foo>, set USE_<foo> to the
+# name of the implementation. If USE_<foo> is unset, or empty, then
+# all the implementations are enabled (this just means they are
+# **available** to `settings.conf`, not that they are used).
+#
+# Currently, no USE_<foo> variables exist.
 
 ### Calamares application info
 #

src/modules/CMakeLists.txt

@@ -15,13 +15,31 @@ string( REPLACE " " ";" SKIP_LIST "${SKIP_MODULES}" )
 file( GLOB SUBDIRECTORIES RELATIVE ${CMAKE_CURRENT_SOURCE_DIR} "*" )
 list( SORT SUBDIRECTORIES )
 
+# Handle the USE_<foo> variables by looking for subdirectories
+# with a <foo>-<implementation> kind of name.
+foreach( SUBDIRECTORY ${SUBDIRECTORIES} )
+endforeach()
+
 foreach( SUBDIRECTORY ${SUBDIRECTORIES} )
     list( FIND SKIP_LIST ${SUBDIRECTORY} DO_SKIP )
 
+    set( _skip_reason "user request" )
+    if( SUBDIRECTORY MATCHES "^[a-zA-Z0-9_]+-" )
+        string( REGEX REPLACE "^[^-]+-" "" _implementation ${SUBDIRECTORY} )
+        string( REGEX REPLACE "-.*" "" _category ${SUBDIRECTORY} )
+        if( USE_${_category} )
+            if( NOT "${_implementation}" STREQUAL "${USE_${_category}}" )
+                list( APPEND SKIP_LIST ${SUBDIRECTORY} )
+                set( _skip_reason "USE_${_category}=${USE_${_category}}" )
+                set( DO_SKIP 1 )
+            endif()
+        endif()
+    endif()
+
     if( NOT DO_SKIP EQUAL -1 )
         message( "${ColorReset}-- Skipping module ${BoldRed}${SUBDIRECTORY}${ColorReset}." )
         message( "" )
-        list( APPEND LIST_SKIPPED_MODULES "${SUBDIRECTORY} (user request)" )
+        list( APPEND LIST_SKIPPED_MODULES "${SUBDIRECTORY} (${_skip_reason})" )
     elseif( ( IS_DIRECTORY "${CMAKE_CURRENT_SOURCE_DIR}/${SUBDIRECTORY}" ) AND
             ( DO_SKIP EQUAL -1 ) )
         set( SKIPPED_MODULES )