Experimental: Add Flatbuffers support

[pmai]

This draft PR is a placeholder to highlight the ongoing work for adding support for Flatbuffers encoding of OSI messages. This work is performed as part of the Performance & Packaging WP.

Currently it supports generation of the code for Flatbuffers encoding, by auto-mapping of OSI3 proto files. It does not yet include examples making use of this encoding. Also note that use of Flatbuffers encoding is not on-the-wire compatible with OSI3 using ProtoBuffers for obvious reasons.

[Kohda-adc4g]

Checking and testing OSI #427

Environment

Linux (Ubuntu 20.04)
Open Simulation Interface v3.3.1
ProtocolBuffers v3.14.0
FlatBuffers v2.0

How to register flatbuffers as a submodule

Move to the directory where OSI is stored and register flatbuffers with the following command.

$ cd open-simulation-interface
$ git submodule add https://github.com/google/flatbuffers.git flatbuffers

.gitmodules file is automatically created and contains the contents.
It also creates clones of flatbuffers.

Modify CMakeList.txt When I create a Makefile using CMakeList.txt presented in
#427 and compile it, the following error occurs in my environment.

$ mkdir build
$ cd build
$ cmake -D BUILD_FLATBUFFER=ON ..

error: /home/OSI/test/osi_version.proto:5: 42: error: unable to load include file: google/protobuf/descriptor.proto
make[2]: *** [CMakeFiles/open_simulation_interface_fbs_build.dir/build.make:925: osi_version.fbs] error 1
make[1]: *** [CMakeFiles/Makefile2:253: CMakeFiles/open_simulation_interface_fbs_build.dir/all] error 2
make: *** [Makefile:130: all] error 2

The reason for this error is that the Protocol Buffers installation directory cannot be found when compiling with flatc.
Therefore, modify CMakeList.txt as follows and specify the location of Potocol Buffers in flatc.
Specify the location of Protocol Buffers using the flatc option flag "-I".

Add parameter:

-I "${PROTOBUF_IMPORT_DIRS}"

*** CMakeLists.txt 2021-05-12 09:34:47.159944775 +0900
--- CMakeLists.txt.pmai 2021-05-12 09:35:32.078618500 +0900

---

*** 102,108 ****
set(fbs "${proto_base}.fbs")
add_custom_command(
OUTPUT "${fbs}"
! COMMAND $<TARGET_FILE:flatc> -I "${PROTOBUF_IMPORT_DIRS}" -o "${CMAKE_CURRENT_BINARY_DIR}" --proto "${CMAKE_CURRENT_SOURCE_DIR}/${proto}"
DEPENDS "${proto}" flatc
WORKING_DIRECTORY "${CMAKE_CURRENT_BINARY_DIR}"
COMMENT "Convert ${proto} to ${fbs} using flatc"
--- 102,108 ----
set(fbs "${proto_base}.fbs")
add_custom_command(
OUTPUT "${fbs}"
! COMMAND $<TARGET_FILE:flatc> -o "${CMAKE_CURRENT_BINARY_DIR}" --proto "${CMAKE_CURRENT_SOURCE_DIR}/${proto}"
DEPENDS "${proto}" flatc
WORKING_DIRECTORY "${CMAKE_CURRENT_BINARY_DIR}"
COMMENT "Convert ${proto} to ${fbs} using flatc"

---

*** 116,122 ****
set(fbh "${flat_base}_generated.h")
add_custom_command(
OUTPUT "include/${fbh}"
! COMMAND $<TARGET_FILE:flatc> -I "${PROTOBUF_IMPORT_DIRS}" -o "${CMAKE_CURRENT_BINARY_DIR}/include" --cpp --gen-mutable --gen-name-strings --scoped-enums "${fbs}"
DEPENDS "${FLAT_FBS}" flatc
WORKING_DIRECTORY "${CMAKE_CURRENT_BINARY_DIR}"
COMMENT "Process ${fbs} to ${fbh} using flatc"
--- 116,122 ----
set(fbh "${flat_base}_generated.h")
add_custom_command(
OUTPUT "include/${fbh}"
! COMMAND $<TARGET_FILE:flatc> -o "${CMAKE_CURRENT_BINARY_DIR}/include" --cpp --gen-mutable --gen-name-strings --scoped-enums "${fbs}"
DEPENDS "${FLAT_FBS}" flatc
WORKING_DIRECTORY "${CMAKE_CURRENT_BINARY_DIR}"
COMMENT "Process ${fbs} to ${fbh} using flatc"

I was able to generate the fbs file with this without any problems.

[stefancyliax]

OSI CCB:

• @pmai please look at last comment. This should be part of 3.4 if we actually want to considering the switch to flatbuffers.

[pmai]

@stefancyliax this should now be up-to-date with master, and could be merged, but is missing a reviewer (can't self-review). If it builds, can be merged as is.

[stefancyliax]

@stefancyliax this should now be up-to-date with master, and could be merged, but is missing a reviewer (can't self-review). If it builds, can be merged as is.

Thanks. Will do!

[stefancyliax]

Looks good to me