CMake function which bundles multiple static libraries into one

bundle_static_library.cmake

# MIT License
# 
# Copyright (c) 2019 Cristian Adam
# 
# Permission is hereby granted, free of charge, to any person obtaining a copy
# of this software and associated documentation files (the "Software"), to deal
# in the Software without restriction, including without limitation the rights
# to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
# copies of the Software, and to permit persons to whom the Software is
# furnished to do so, subject to the following conditions:
# 
# The above copyright notice and this permission notice shall be included in all
# copies or substantial portions of the Software.

# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
# IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
# FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
# AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
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# OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
# SOFTWARE.

function(bundle_static_library tgt_name bundled_tgt_name)
  list(APPEND static_libs ${tgt_name})

  function(_recursively_collect_dependencies input_target)
    set(_input_link_libraries LINK_LIBRARIES)
    get_target_property(_input_type ${input_target} TYPE)
    if (${_input_type} STREQUAL "INTERFACE_LIBRARY")
      set(_input_link_libraries INTERFACE_LINK_LIBRARIES)
    endif()
    get_target_property(public_dependencies ${input_target} ${_input_link_libraries})
    foreach(dependency IN LISTS public_dependencies)
      if(TARGET ${dependency})
        get_target_property(alias ${dependency} ALIASED_TARGET)
        if (TARGET ${alias})
          set(dependency ${alias})
        endif()
        get_target_property(_type ${dependency} TYPE)
        if (${_type} STREQUAL "STATIC_LIBRARY")
          list(APPEND static_libs ${dependency})
        endif()

        get_property(library_already_added
          GLOBAL PROPERTY _${tgt_name}_static_bundle_${dependency})
        if (NOT library_already_added)
          set_property(GLOBAL PROPERTY _${tgt_name}_static_bundle_${dependency} ON)
          _recursively_collect_dependencies(${dependency})
        endif()
      endif()
    endforeach()
    set(static_libs ${static_libs} PARENT_SCOPE)
  endfunction()

  _recursively_collect_dependencies(${tgt_name})

  list(REMOVE_DUPLICATES static_libs)

  set(bundled_tgt_full_name 
    ${CMAKE_BINARY_DIR}/${CMAKE_STATIC_LIBRARY_PREFIX}${bundled_tgt_name}${CMAKE_STATIC_LIBRARY_SUFFIX})

  if (CMAKE_CXX_COMPILER_ID MATCHES "^(Clang|GNU)$")
    file(WRITE ${CMAKE_BINARY_DIR}/${bundled_tgt_name}.ar.in
      "CREATE ${bundled_tgt_full_name}\n" )
        
    foreach(tgt IN LISTS static_libs)
      file(APPEND ${CMAKE_BINARY_DIR}/${bundled_tgt_name}.ar.in
        "ADDLIB $<TARGET_FILE:${tgt}>\n")
    endforeach()
    
    file(APPEND ${CMAKE_BINARY_DIR}/${bundled_tgt_name}.ar.in "SAVE\n")
    file(APPEND ${CMAKE_BINARY_DIR}/${bundled_tgt_name}.ar.in "END\n")

    file(GENERATE
      OUTPUT ${CMAKE_BINARY_DIR}/${bundled_tgt_name}.ar
      INPUT ${CMAKE_BINARY_DIR}/${bundled_tgt_name}.ar.in)

    set(ar_tool ${CMAKE_AR})
    if (CMAKE_INTERPROCEDURAL_OPTIMIZATION)
      set(ar_tool ${CMAKE_CXX_COMPILER_AR})
    endif()

    add_custom_command(
      COMMAND ${ar_tool} -M < ${CMAKE_BINARY_DIR}/${bundled_tgt_name}.ar
      OUTPUT ${bundled_tgt_full_name}
      COMMENT "Bundling ${bundled_tgt_name}"
      VERBATIM)
  elseif(MSVC)
    find_program(lib_tool lib)

    foreach(tgt IN LISTS static_libs)
      list(APPEND static_libs_full_names $<TARGET_FILE:${tgt}>)
    endforeach()

    add_custom_command(
      COMMAND ${lib_tool} /NOLOGO /OUT:${bundled_tgt_full_name} ${static_libs_full_names}
      OUTPUT ${bundled_tgt_full_name}
      COMMENT "Bundling ${bundled_tgt_name}"
      VERBATIM)
  else()
    message(FATAL_ERROR "Unknown bundle scenario!")
  endif()

  add_custom_target(bundling_target ALL DEPENDS ${bundled_tgt_full_name})
  add_dependencies(bundling_target ${tgt_name})

  add_library(${bundled_tgt_name} STATIC IMPORTED)
  set_target_properties(${bundled_tgt_name} 
    PROPERTIES 
      IMPORTED_LOCATION ${bundled_tgt_full_name}
      INTERFACE_INCLUDE_DIRECTORIES $<TARGET_PROPERTY:${tgt_name},INTERFACE_INCLUDE_DIRECTORIES>)
  add_dependencies(${bundled_tgt_name} bundling_target)

endfunction()

I have added some logic to handle two cases we found in our project. Would love some feedback from folks (especially since I can only really test these in our project)
1 Strip ::@.* patterns from (INTERFACE_)LINK_LIBRARIES that are added by CMAKE when target_link_libraries is used in a different directory than where the library was created.
2 Enhancing to include object library support for clang/gcc only

https://gist.github.com/4wrxb/ce9bbe30f5a082e70944bdfb7463e600

Doesn't work for imported library targets because the public_dependencies property is not exported during installation.

Since the recursion happens at configure time, I'm having issues with ordering.

If I have:

add_subdirectory(a)
add_subdirectory(b)
add_subdirectory(c)

and in b/CMakeLists.txt I have

add_library(b STATIC a c)
bundle_static_library(b b_bundled)

Then c is not yet a target when the recursive function runs, so it's not added.

Is there a way to make it run as a POST_BUILD command somehow instead?

As the recursion happens at configure time, it can't handle dependencies that are specified with generator expression. This is unfortunately triggered by conan 2 with its CMakeDeps generator, so I can't bundle static libraries pulled in by conan with this function.

I'm not sure if generator expressions could be handled though, replicating the recursive collection logic with generator expressions seems impossible.