Rework dynamic RMW

Defer list of available RMWs to build time of downstream packages so you can build a dependent package even if RMW_IMPLEMENTATION is set to a value not available when the rmw_implementation package was built.

Remove CMake Config flag "RMW_IMPLEMENTATION_FORCE_DYNAMIC_LOADING".
Add CMake Config flag "RMW_IMPLEMENTATION_BAKED" so you can statically link a middleware, even if multiple present at build time. This value can be explicitly set, but by default preserves the behavior of "statically link if one middleware found".

Downgrade missing rmw library to a warning, since some RMW's like rmw_cyclonedds_cpp and rmw_fastrtps_dynamic_cpp (?) do not need to inject build dependencies.
Add a scary message if the user tries to change the implementation and built with RMW_IMPLEMENTATION_BAKED=ON

Signed-off-by: Dan Rose <dan@digilabs.io>

rmw_implementation/CMakeLists.txt

@@ -15,65 +15,50 @@ find_package(ament_cmake REQUIRED)
 
 find_package(rcutils REQUIRED)
 
-# provides FindPoco.cmake and Poco on platforms without it
-find_package(poco_vendor)
-find_package(Poco COMPONENTS Foundation)
 find_package(rmw_implementation_cmake REQUIRED)
 
 if(BUILD_TESTING)
   find_package(ament_lint_auto REQUIRED)
   ament_lint_auto_find_test_dependencies()
 endif()
 
+find_package(rmw REQUIRED)
 get_default_rmw_implementation(RMW_IMPLEMENTATION)
 
-get_available_rmw_implementations(RMW_IMPLEMENTATIONS)
-message(STATUS "")
-message(STATUS "Found these rmw implementations:")
-foreach(impl IN LISTS RMW_IMPLEMENTATIONS)
-  message(STATUS "  '${impl}'")
-endforeach()
-
 message(STATUS "")
 message(STATUS "Default rmw implementation: '${RMW_IMPLEMENTATION}'")
 message(STATUS "")
 
-option(RMW_IMPLEMENTATION_FORCE_DYNAMIC_LOADING
-  "Force use of dynamic loading (via poco), even if there is only one implementation available"
-  OFF)
-
-# Nominally use dynamic loading if poco is available.
-set(RMW_IMPLEMENTATION_SUPPORTS_POCO ${Poco_FOUND})
-# Determine if we're going to actually support dynamic loading.
-if(RMW_IMPLEMENTATION_FORCE_DYNAMIC_LOADING)
-  # Force usage.
-  if(NOT Poco_FOUND)
-    message(FATAL_ERROR "Dynamic loading forced, but poco is not found!")
-  endif()
-  set(RMW_IMPLEMENTATION_SUPPORTS_POCO TRUE)
-elseif(NOT RMW_IMPLEMENTATIONS MATCHES ";")
-  set(RMW_IMPLEMENTATION_SUPPORTS_POCO FALSE)
+get_available_rmw_implementations(available_rmw_implementations)
+list(LENGTH available_rmw_implementations count_available_rmw_implementations)
+if(count_available_rmw_implementations EQUAL 1)
+  set(rmw_implementation_baked_default ON)
+else()
+  set(rmw_implementation_baked_default OFF)
 endif()
 
-if(NOT RMW_IMPLEMENTATION_SUPPORTS_POCO)
-  message(STATUS "Dynamic loading disabled; directly referencing '${RMW_IMPLEMENTATION}'")
-  message(STATUS "")
+option(RMW_IMPLEMENTATION_BAKED "\
+  Use only the RMW implementation specified at build time, \
+  ignoring runtime value of the 'RMW_IMPLEMENTATION' environment variable \
+  and the 'RMW_IMPLEMENTATION' CMake option"
+  ${rmw_implementation_baked_default})
 
+if(RMW_IMPLEMENTATION_BAKED)
+  message(STATUS "Dynamic loading disabled; directly referencing '${RMW_IMPLEMENTATION}'")
 else()
-  message(STATUS "Dynamic loading enabled with any of the available rmw implementations")
-  message(STATUS "")
+  # provides FindPoco.cmake and Poco on platforms without it
+  find_package(poco_vendor REQUIRED)
+  find_package(Poco REQUIRED COMPONENTS Foundation)
 
-  find_package(rmw REQUIRED)
+  message(STATUS "Dynamic loading enabled")
 
-  link_directories(${Poco_LIBRARY_DIR})
   add_library(${PROJECT_NAME} SHARED
     src/functions.cpp)
   ament_target_dependencies(${PROJECT_NAME}
     "Poco"
     "rmw")
   target_compile_definitions(${PROJECT_NAME}
     PUBLIC "DEFAULT_RMW_IMPLEMENTATION=${RMW_IMPLEMENTATION}")
-  target_link_libraries(${PROJECT_NAME})
   configure_rmw_library(${PROJECT_NAME})
 
   ament_export_libraries(${PROJECT_NAME})
@@ -91,8 +76,6 @@ register_rmw_implementation(
   "cpp:rosidl_typesupport_cpp"
 )
 
-# necessary since get_available_rmw_implementations excludes rmw_implementation
-list(APPEND RMW_IMPLEMENTATIONS "rmw_implementation")
 ament_package(
   CONFIG_EXTRAS "rmw_implementation-extras.cmake.in"
 )

rmw_implementation/rmw_implementation-extras.cmake.in

@@ -15,69 +15,57 @@
 # generated from rmw_implementation/rmw_implementation-extras.cmake.in
 
 find_package(rmw_implementation_cmake REQUIRED)
-set(default_rmw_implementation "@RMW_IMPLEMENTATION@")
-set(available_rmw_implementations "@RMW_IMPLEMENTATIONS@")
 
 if(NOT "${RMW_IMPLEMENTATION}" STREQUAL "")
-  set(requested_rmw_implementation ${RMW_IMPLEMENTATION})
-  if(@RMW_IMPLEMENTATION_SUPPORTS_POCO@)
-    if(NOT requested_rmw_implementation IN_LIST available_rmw_implementations)
-      message(FATAL_ERROR
-        "The RMW implementation has been specified as "
-        "'${requested_rmw_implementation}' through CMake, "
-        "however it is not in the list of supported rmw implementations, "
-        "which was specified when the 'rmw_implementation' package was built.")
-    endif()
-  else()
-    if(NOT requested_rmw_implementation STREQUAL default_rmw_implementation)
-      message(FATAL_ERROR
-        "The RMW implementation has been specified as "
-        "'${requested_rmw_implementation}' through CMake, "
-        "however this needs to match the RMW implementation "
-        "'${default_rmw_implementation}', "
-        "which was specified when the 'rmw_implementation' package was built.")
-    endif()
-  endif()
+  set(requested_rmw_implementation "${RMW_IMPLEMENTATION}")
+  set(requested_rmw_implementation_from "CMake")
+elseif(NOT "$ENV{RMW_IMPLEMENTATION}" STREQUAL "")
+  set(requested_rmw_implementation "$ENV{RMW_IMPLEMENTATION}")
+  set(requested_rmw_implementation_from
+    "environment variable 'RMW_IMPLEMENTATION'")
+else()
+  set(requested_rmw_implementation "@RMW_IMPLEMENTATION@")
+  set(requested_rmw_implementation_from
+    "the default when @PROJECT_NAME@ wasbuilt")
 endif()
 
-if(NOT "$ENV{RMW_IMPLEMENTATION}" STREQUAL "")
-  set(requested_rmw_implementation $ENV{RMW_IMPLEMENTATION})
-  if(@RMW_IMPLEMENTATION_SUPPORTS_POCO@)
-    if(NOT requested_rmw_implementation IN_LIST available_rmw_implementations)
-      message(FATAL_ERROR
-        "The RMW implementation has been specified as "
-        "'${requested_rmw_implementation}' through the environment variable "
-        "'RMW_IMPLEMENTATION', "
-        "however it is not in the list of supported rmw implementations, "
-        "which was specified when the 'rmw_implementation' package was built.")
-    endif()
-  else()
-    if(NOT requested_rmw_implementation STREQUAL default_rmw_implementation)
-      message(FATAL_ERROR
-        "The RMW implementation has been specified as "
-        "'${requested_rmw_implementation}' "
-        "through the environment variable 'RMW_IMPLEMENTATION', "
-        "however this needs to match the RMW implementation "
-        "'${default_rmw_implementation}', "
-        "which was specified when the 'rmw_implementation' package was built.")
-    endif()
+if(@RMW_IMPLEMENTATION_BAKED@)
+  message(STATUS "Using RMW implementation '@RMW_IMPLEMENTATION@'")
+  if(NOT requested_rmw_implementation STREQUAL @RMW_IMPLEMENTATION@)
+    message(ERROR
+      "The RMW implementation has been specified as "
+      "'${requested_rmw_implementation}' via "
+      "${requested_rmw_implementation_from}, but rmw_implementation was built "
+      "without dynamic linking enabled. We will use '@RMW_IMPLEMENTATION@'."
+    )
+  endif()
+  find_package("@RMW_IMPLEMENTATION@" REQUIRED)
+
+  # TODO should never need definitions and include dirs?
+  list(APPEND rmw_implementation_DEFINITIONS
+    ${@RMW_IMPLEMENTATION@_DEFINITIONS})
+  list(APPEND rmw_implementation_INCLUDE_DIRS
+    ${@RMW_IMPLEMENTATION@_INCLUDE_DIRS})
+  list(APPEND rmw_implementation_LIBRARIES
+    ${@RMW_IMPLEMENTATION@_LIBRARIES})
+else()
+
+  get_available_rmw_implementations(available_rmw_implementations)
+
+  if(NOT requested_rmw_implementation IN_LIST available_rmw_implementations)
+    message(WARNING
+      "The RMW implementation has been specified as "
+      "'${requested_rmw_implementation}' via "
+      "${requested_rmw_implementation_from}, but it is not available at this "
+      "time.\n Make sure it does not expect a typesupport or other build "
+      "dependencies.\n\n Currently available middlewares: "
+      "'${available_rmw_implementations}'")
   endif()
-endif()
 
-if(@RMW_IMPLEMENTATION_SUPPORTS_POCO@)
   set(selected_rmw_implementation "@PROJECT_NAME@")
   # no need to find_package @PROJECT_NAME@
   # since this code is already part of a find_package call of that package
-else()
-  set(selected_rmw_implementation "${default_rmw_implementation}")
-  message(STATUS "Using RMW implementation '${selected_rmw_implementation}' as default")
-  find_package("${selected_rmw_implementation}" REQUIRED)
-
-  # TODO should never need definitions and include dirs?
-  list(APPEND rmw_implementation_DEFINITIONS ${${selected_rmw_implementation}_DEFINITIONS})
-  list(APPEND rmw_implementation_INCLUDE_DIRS ${${selected_rmw_implementation}_INCLUDE_DIRS})
-  list(APPEND rmw_implementation_LIBRARIES ${${selected_rmw_implementation}_LIBRARIES})
-endif()
 
-find_package(Threads REQUIRED)
-list(APPEND rmw_implementation_LIBRARIES "${CMAKE_THREAD_LIBS_INIT}")
+  find_package(Threads REQUIRED)
+  list(APPEND rmw_implementation_LIBRARIES "${CMAKE_THREAD_LIBS_INIT}")
+endif()