autotools: Implement CapnProtoTargets.cmake

This completes the autotools installation of CMake config files, minus version compatibility checking.

c++/Makefile.am

@@ -115,7 +115,9 @@ pkgconfigdir = $(libdir)/pkgconfig
 pkgconfig_DATA = $(CAPNP_PKG_CONFIG_FILES)
 
 cmakeconfigdir = $(libdir)/cmake/CapnProto
-cmakeconfig_DATA = $(CAPNP_CMAKE_CONFIG_FILES) cmake/CapnProtoMacros.cmake
+cmakeconfig_DATA = $(CAPNP_CMAKE_CONFIG_FILES)                 \
+  cmake/CapnProtoMacros.cmake                                  \
+  cmake/CapnProtoTargets.cmake
 
 noinst_HEADERS = \
   src/kj/miniposix.h

c++/cmake/CapnProtoTargets.cmake

+# This CMake script adds imported targets for each shared library and executable distributed by
+# Cap'n Proto's autotools build.
+#
+# This file IS NOT USED by the CMake build! The CMake build generates its own version of this script
+# from its set of exported targets. I used such a generated script as a reference when writing this
+# one.
+#
+# The set of library targets provided by this script is automatically generated from the list of .pc
+# files maintained in configure.ac. The set of executable targets is hard-coded in this file.
+#
+# You can request that this script print debugging information by invoking cmake with:
+#
+#   -DCapnProto_DEBUG=ON
+#
+# TODO(soon): Implement CapnProtoConfigVersion.cmake, too.
+#
+# TODO(someday): Distinguish between debug and release builds. I.e., set IMPORTED_LOCATION_RELEASE
+#   rather than IMPORTED_LOCATION, etc., if this installation was configured as a release build. But
+#   how do we tell? grep for -g in CXXFLAGS?
+
+if(CMAKE_VERSION VERSION_LESS 3.1)
+  message(FATAL_ERROR "CMake >= 3.1 required")
+endif()
+
+set(forwarded_config_flags)
+if(CapnProto_FIND_QUIETLY)
+  list(APPEND forwarded_config_flags QUIET)
+endif()
+if(CapnProto_FIND_REQUIRED)
+  list(APPEND forwarded_config_flags REQUIRED)
+endif()
+# If the consuming project called find_package(CapnProto) with the QUIET or REQUIRED flags, forward
+# them to calls to find_package(PkgConfig) and pkg_check_modules(). Note that find_dependency()
+# would do this for us in the former case, but there is no such forwarding wrapper for
+# pkg_check_modules().
+
+find_package(PkgConfig ${forwarded_config_flags})
+if(NOT ${PkgConfig_FOUND})
+  # If we're here, the REQUIRED flag must not have been passed, else we would have had a fatal
+  # error. Nevertheless, a diagnostic for this case is probably nice.
+  if(NOT CapnProto_FIND_QUIETLY)
+    message(WARNING "pkg-config cannot be found")
+  endif()
+  set(CapnProto_FOUND OFF)
+  return()
+endif()
+
+function(_capnp_import_pkg_config_target target)
+  # Add an imported library target named CapnProto::${target}, using the output of various
+  # invocations of `pkg-config ${target}`. The generated imported library target tries to mimic the
+  # behavior of a real CMake-generated imported target as closely as possible.
+  #
+  # Usage: _capnp_import_pkg_config_target(target <all Cap'n Proto targets>)
+
+  set(all_targets ${ARGN})
+
+  pkg_check_modules(${target} ${forwarded_config_flags} ${target})
+
+  if(NOT ${${target}_FOUND})
+    if(NOT CapnProto_FIND_QUIETLY)
+      message(WARNING "CapnProtoConfig.cmake was configured to search for ${target}.pc, but pkg-config cannot find it. Ignoring this target.")
+    endif()
+    return()
+  endif()
+
+  if(CapnProto_DEBUG)
+    # Dump the information pkg-config discovered.
+    foreach(var VERSION LIBRARY_DIRS LIBRARIES LDFLAGS_OTHER INCLUDE_DIRS CFLAGS_OTHER)
+      message(STATUS "${target}_${var} = ${${target}_${var}}")
+    endforeach()
+  endif()
+
+  if(NOT ${${target}_VERSION} VERSION_EQUAL ${CapnProto_VERSION})
+    if(NOT CapnProto_FIND_QUIETLY)
+      message(WARNING "CapnProtoConfig.cmake was configured to search for version ${CapnProto_VERSION}, but ${target} version ${${target}_VERSION} was found. Ignoring this target.")
+    endif()
+    return()
+  endif()
+
+  # Make an educated guess as to what the target's .so and .a filenames must be.
+  set(target_name_shared
+      ${CMAKE_SHARED_LIBRARY_PREFIX}${target}-${CapnProto_VERSION}${CMAKE_SHARED_LIBRARY_SUFFIX})
+  set(target_name_static
+      ${CMAKE_STATIC_LIBRARY_PREFIX}${target}${CMAKE_STATIC_LIBRARY_SUFFIX})
+
+  # Find the actual target's file. find_library() sets a cache variable, so I made the variable name
+  # unique-ish.
+  find_library(CapnProto_${target}_IMPORTED_LOCATION
+    NAMES ${target_name_shared} ${target_name_static}  # prefer libfoo-version.so over libfoo.a
+    PATHS ${${target}_LIBRARY_DIRS}
+    NO_DEFAULT_PATH
+  )
+  if(NOT CapnProto_${target}_IMPORTED_LOCATION)
+    # Not an error if the library doesn't exist -- we may have found a lite mode installation.
+    if(CapnProto_DEBUG)
+      message(STATUS "${target} library does not exist")
+    endif()
+    return()
+  endif()
+
+  # Record some information about this target -- shared versus static, location and soname -- which
+  # we'll use to build our imported target later.
+
+  set(target_location ${CapnProto_${target}_IMPORTED_LOCATION})
+  get_filename_component(target_name "${target_location}" NAME)
+
+  set(target_type STATIC)
+  set(imported_soname_property)
+  if(target_name STREQUAL ${target_name_shared})
+    set(target_type SHARED)
+    set(imported_soname_property IMPORTED_SONAME ${target_name})
+  endif()
+
+  # Each library dependency of the target is either the target itself, a sibling Cap'n Proto
+  # library, or a system library. We ignore the first case by removing this target from the
+  # dependencies. The remaining dependencies are either passed through or, if they are a sibling
+  # Cap'n Proto library, prefixed with `CapnProto::`.
+  set(dependencies ${${target}_LIBRARIES})
+  list(REMOVE_ITEM dependencies ${target})
+  set(target_interface_libs)
+  foreach(dependency ${dependencies})
+    list(FIND all_targets ${dependency} target_index)
+    # TODO(cleanup): CMake >= 3.3 lets us write: `if(NOT ${dependency} IN_LIST all_targets)`
+    if(target_index EQUAL -1)
+      list(APPEND target_interface_libs ${dependency})
+    else()
+      list(APPEND target_interface_libs CapnProto::${dependency})
+    endif()
+  endforeach()
+
+  add_library(CapnProto::${target} ${target_type} IMPORTED)
+  set_target_properties(CapnProto::${target} PROPERTIES
+    ${imported_soname_property}
+    IMPORTED_LOCATION "${target_location}"
+    INTERFACE_COMPILE_FEATURES "cxx_constexpr"
+    INTERFACE_COMPILE_OPTIONS "${${target}_CFLAGS_OTHER}"
+    INTERFACE_INCLUDE_DIRECTORIES "${${target}_INCLUDE_DIRS}"
+
+    # I'm dumping LDFLAGS_OTHER in with the libraries because there exists no
+    # INTERFACE_LINK_OPTIONS. See https://gitlab.kitware.com/cmake/cmake/issues/16543.
+    INTERFACE_LINK_LIBRARIES "${target_interface_libs};${${target}_LDFLAGS_OTHER}"
+  )
+
+  if(CapnProto_DEBUG)
+    # Dump all the properties we generated for the imported target.
+    foreach(prop
+        IMPORTED_LOCATION
+        IMPORTED_SONAME
+        INTERFACE_COMPILE_FEATURES
+        INTERFACE_COMPILE_OPTIONS
+        INTERFACE_INCLUDE_DIRECTORIES
+        INTERFACE_LINK_LIBRARIES)
+      get_target_property(value CapnProto::${target} ${prop})
+      message(STATUS "CapnProto::${target} ${prop} = ${value}")
+    endforeach()
+  endif()
+endfunction()
+
+# ========================================================================================
+# Imported library targets
+
+# Build a list of targets to search for from the list of .pc files.
+# I.e. [somewhere/foo.pc, somewhere/bar.pc] -> [foo, bar]
+set(library_targets)
+foreach(filename ${CAPNP_PKG_CONFIG_FILES})
+  get_filename_component(target ${filename} NAME_WE)
+  list(APPEND library_targets ${target})
+endforeach()
+
+# Try to add an imported library target CapnProto::foo for each foo.pc distributed with Cap'n Proto.
+foreach(target ${library_targets})
+  _capnp_import_pkg_config_target(${target} ${library_targets})
+endforeach()
+
+# Handle lite-mode and no libraries found cases. It is tempting to set a CapnProto_LITE variable
+# here, but the real CMake-generated implementation does no such thing -- we'd need to set it in
+# CapnProtoConfig.cmake.in itself.
+if(TARGET CapnProto::capnp AND TARGET CapnProto::kj)
+  if(NOT TARGET CapnProto::capnp-rpc)
+    if(NOT CapnProto_FIND_QUIETLY)
+      message(STATUS "Found an installation of Cap'n Proto lite. Executable and library targets beyond libkj and libcapnp will be unavailable.")
+    endif()
+    # Lite mode doesn't include the executables, so return here.
+    return()
+  endif()
+else()
+  # If we didn't even find capnp or kj, then we didn't find anything usable.
+  set(CapnProto_FOUND OFF)
+  return()
+endif()
+
+# ========================================================================================
+# Imported executable targets
+
+get_filename_component(_IMPORT_PREFIX "${CMAKE_CURRENT_LIST_FILE}" PATH)
+get_filename_component(_IMPORT_PREFIX "${_IMPORT_PREFIX}" PATH)
+get_filename_component(_IMPORT_PREFIX "${_IMPORT_PREFIX}" PATH)
+get_filename_component(_IMPORT_PREFIX "${_IMPORT_PREFIX}" PATH)
+
+# Add executable targets for the capnp compiler and plugins. This list must be kept manually in sync
+# with the rest of the project.
+
+add_executable(CapnProto::capnp_tool IMPORTED)
+set_target_properties(CapnProto::capnp_tool PROPERTIES
+  IMPORTED_LOCATION "${_IMPORT_PREFIX}/bin/capnp${CMAKE_EXECUTABLE_SUFFIX}"
+)
+
+add_executable(CapnProto::capnpc_cpp IMPORTED)
+set_target_properties(CapnProto::capnpc_cpp PROPERTIES
+  IMPORTED_LOCATION "${_IMPORT_PREFIX}/bin/capnpc-c++${CMAKE_EXECUTABLE_SUFFIX}"
+)
+
+add_executable(CapnProto::capnpc_capnp IMPORTED)
+set_target_properties(CapnProto::capnpc_capnp PROPERTIES
+  IMPORTED_LOCATION "${_IMPORT_PREFIX}/bin/capnpc-capnp${CMAKE_EXECUTABLE_SUFFIX}"
+)