FindProtobuf: add flag to allow descriptor files to be generated

- The .desc files will be in the same folder as the generated .cc and .h files.
- Paths to generate .desc files are stored in a variable passed in
- This is only implemented for C++
- Remove legacy ARGS
- Add test that generates and uses C++ protobuf message
- Add test that checks that the generated .desc file can be instantiated
  with DynamicMessageFactory
- Add Help rst for new feature

1 files changed, 26 insertions, 3 deletions

diff --git a/Modules/FindProtobuf.cmake b/Modules/FindProtobuf.cmake
index c1444a1c96..7292aecfea 100644
--- a/Modules/FindProtobuf.cmake
+++ b/Modules/FindProtobuf.cmake
@@ -79,6 +79,7 @@
 #   include_directories(${CMAKE_CURRENT_BINARY_DIR})
 #   protobuf_generate_cpp(PROTO_SRCS PROTO_HDRS foo.proto)
 #   protobuf_generate_cpp(PROTO_SRCS PROTO_HDRS EXPORT_MACRO DLL_EXPORT foo.proto)
+#   protobuf_generate_cpp(PROTO_SRCS PROTO_HDRS DESCRIPTORS PROTO_DESCS foo.proto)
 #   protobuf_generate_python(PROTO_PY foo.proto)
 #   add_executable(bar bar.cc ${PROTO_SRCS} ${PROTO_HDRS})
 #   target_link_libraries(bar ${Protobuf_LIBRARIES})
@@ -92,12 +93,15 @@
 #
 #   Add custom commands to process ``.proto`` files to C++::
 #
-#     protobuf_generate_cpp (<SRCS> <HDRS> [EXPORT_MACRO <MACRO>] [<ARGN>...])
+#     protobuf_generate_cpp (<SRCS> <HDRS>
+#         [DESCRIPTORS <DESC>] [EXPORT_MACRO <MACRO>] [<ARGN>...])
 #
 #   ``SRCS``
 #     Variable to define with autogenerated source files
 #   ``HDRS``
 #     Variable to define with autogenerated header files
+#   ``DESCRIPTORS``
+#     Variable to define with auotgenerated descriptor files, if requested.
 #   ``EXPORT_MACRO``
 #     is a macro which should expand to ``__declspec(dllexport)`` or
 #     ``__declspec(dllimport)`` depending on what is being compiled.
@@ -116,7 +120,7 @@
 #     ``.proto`` filess
 
 function(PROTOBUF_GENERATE_CPP SRCS HDRS)
-  cmake_parse_arguments(protobuf "" "EXPORT_MACRO" "" ${ARGN})
+  cmake_parse_arguments(protobuf "" "EXPORT_MACRO;DESCRIPTORS" "" ${ARGN})
 
   set(PROTO_FILES "${protobuf_UNPARSED_ARGUMENTS}")
   if(NOT PROTO_FILES)
@@ -158,6 +162,10 @@ function(PROTOBUF_GENERATE_CPP SRCS HDRS)
 
   set(${SRCS})
   set(${HDRS})
+  if (protobuf_DESCRIPTORS)
+    set(${protobuf_DESCRIPTORS})
+  endif()
+
   foreach(FIL ${PROTO_FILES})
     get_filename_component(ABS_FIL ${FIL} ABSOLUTE)
     get_filename_component(FIL_WE ${FIL} NAME_WE)
@@ -173,11 +181,23 @@ function(PROTOBUF_GENERATE_CPP SRCS HDRS)
     list(APPEND ${SRCS} "${_protobuf_protoc_src}")
     list(APPEND ${HDRS} "${_protobuf_protoc_hdr}")
 
+    if(protobuf_DESCRIPTORS)
+      set(_protobuf_protoc_desc "${CMAKE_CURRENT_BINARY_DIR}/${FIL_WE}.desc")
+      set(_protobuf_protoc_flags "--descriptor_set_out=${_protobuf_protoc_desc}")
+      list(APPEND ${protobuf_DESCRIPTORS} "${_protobuf_protoc_desc}")
+    else()
+      set(_protobuf_protoc_desc "")
+      set(_protobuf_protoc_flags "")
+    endif()
+
     add_custom_command(
       OUTPUT "${_protobuf_protoc_src}"
              "${_protobuf_protoc_hdr}"
+             ${_protobuf_protoc_desc}
       COMMAND  protobuf::protoc
-      ARGS "--cpp_out=${DLL_EXPORT_DECL}${CMAKE_CURRENT_BINARY_DIR}" ${_protobuf_include_path} ${ABS_FIL}
+               "--cpp_out=${DLL_EXPORT_DECL}${CMAKE_CURRENT_BINARY_DIR}"
+               ${_protobuf_protoc_flags}
+               ${_protobuf_include_path} ${ABS_FIL}
       DEPENDS ${ABS_FIL} protobuf::protoc
       COMMENT "Running C++ protocol buffer compiler on ${FIL}"
       VERBATIM )
@@ -185,6 +205,9 @@ function(PROTOBUF_GENERATE_CPP SRCS HDRS)
 
   set(${SRCS} "${${SRCS}}" PARENT_SCOPE)
   set(${HDRS} "${${HDRS}}" PARENT_SCOPE)
+  if(protobuf_DESCRIPTORS)
+    set(${protobuf_DESCRIPTORS} "${${protobuf_DESCRIPTORS}}" PARENT_SCOPE)
+  endif()
 endfunction()
 
 function(PROTOBUF_GENERATE_PYTHON SRCS)