[protobuf][protoc][grpc] Better support when cross compiling

[ekilmer]

protoc tool should always be built and be compatible for the target
system.

Fix upb finding protoc executable to better support cross compilation.

Describe the pull request

• What does your PR fix?

  N/A

• Which triplets are supported/not supported? Have you updated the CI baseline?

  Yes

• Does your PR follow the maintainer guide?

  I believe so, yes

• If you have added/updated a port: Have you run ./vcpkg x-add-version --all and committed the result?

  Yes

If you are still working on the PR, open it as a Draft: https://github.blog/2019-02-14-introducing-draft-pull-requests/

[Neumann-A]

invalid. Use a host dependency

[ekilmer]

Use a host dependency

Are you talking about using the "--host-triplet" option and this document https://github.com/microsoft/vcpkg/blob/master/docs/users/host-dependencies.md?

I am executing the following:

./vcpkg install --triplet=arm64-osx --host-triplet=x64-osx-release grpc[codegen]

with the (maybe weird) intention of building a vcpkg installation directory that can be delivered to an arm64 machine as a pre-built dependency cache (basically vcpkg export). Without these changes, I cannot precompile an arm64 protoc executable due to the host and target triplet check and copying of the host (x64) protoc to the target (arm64) tools directory.

There are also references to x64-osx-release in the CMake config files of the arm64-osx packages, which won't work when an arm64 machine tries to run things (with the gRPC plugins being the primary offender). I don't quite understand why there are *-vcpkg-tools.cmake and protobuf-targets-vcpkg-protoc.cmake files or what they're used for.

I understand that arm64 Macs can run x86 binaries with Rosetta, but I am going for correctness here by ensuring correct architecture binaries in their respective directories and CMake configurations that work when faced with the above scenario.

I am also now realizing that I don't understand the whole picture of how vcpkg handles cross-compilation. Is there a better way to accomplish what I'm trying to do?

[github-actions]

You have modified or added at least one vcpkg.json where you should check the license field.

If you feel able to do so, please consider adding a "license" field to the following files:

• ports/cartographer/vcpkg.json
• ports/offscale-libetcd-cpp/vcpkg.json
• ports/srpc/vcpkg.json

Valid values for the license field can be found in the documentation

[Neumann-A]

can we not simply fix the target protobuf::protoc for crosscompiling instead of patching everyone downstream?

[ekilmer]

Good question. I'm unsure of the best solution, and I'm still trying to wrap my head around CMake cross compilation best practices and what vcpkg is doing.

I was under the impression that it's probably best to expect that any exported targets from a find_package call are all the same architecture, OS, etc... However, as I keep investigating all of this, I wonder how useful an exported executable target is if it needs to be replaced or goes unused during cross compilation. It seems like a cross-compilation-compatible solution would either:

1. Rely on find_program to find all host executables that will be run during the build
2. Separate config files for library and executable targets where the user would need to either specify an exact *_DIR path to the host config file or (after much testing) rely on find_package with NO_CMAKE_PATH, NO_CMAKE_FIND_ROOT_PATH, and HINTS ${CMAKE_SYSTEM_PREFIX_PATH} options to search only the host directories,¹ i.e.,

   find_package(Protobuf CONFIG HINTS ${CMAKE_SYSTEM_PREFIX_PATH} NO_CMAKE_PATH NO_CMAKE_FIND_ROOT_PATH)

fix the target protobuf::protoc

I believe what you're describing is attempted with these lines

vcpkg/ports/protobuf/vcpkg-cmake-wrapper.cmake

Line 14 in 1eb4c7f

find_program(Protobuf_PROTOC_EXECUTABLE NAMES protoc PATHS "${CMAKE_CURRENT_LIST_DIR}/../../tools/protobuf" NO_DEFAULT_PATH)

vcpkg/ports/protobuf/protobuf-targets-vcpkg-protoc.cmake

Lines 4 to 8 in 1eb4c7f

	# Import target "protobuf::protoc" for configuration "Release"
	set_property(TARGET protobuf::protoc APPEND PROPERTY IMPORTED_CONFIGURATIONS RELEASE)
	set_target_properties(protobuf::protoc PROPERTIES
	IMPORTED_LOCATION_RELEASE "${Protobuf_PROTOC_EXECUTABLE}"
	)

I've removed these files in my PR, but I should investigate this again as you suggest. However, these files, being only in vcpkg, would hide downstream projects' issues if they tried to cross compile outside of vcpkg without also fixing the target. What I mean is that I think there is room for a patch in upstream protobuf for better cross compilation support, and I'm wondering what a good solution is and want to test it here.

list(INSERT CMAKE_SYSTEM_PREFIX_PATH 0 "${VCPKG_INSTALLED_DIR}/${VCPKG_HOST_TRIPLET}") # CMake 3.15 is required for list(PREPEND ...).

Footnotes

1. Vcpkg currently doesn't touch CMAKE_SYSTEM_PREFIX_PATH, so we'd want to add the path to the host triplet installation root. I've done very light testing locally by adding the following to scripts/buildsystem/vcpkg.cmake: ↩

[JonLiu1993]

Pinging @ekilmer for response. Is work still being done for this PR?

[ekilmer]

@JonLiu1993 I think I need to approach this differently. I haven't had much time to investigate, so I'll close this for now.

Maybe I'll reopen this or another PR in the future when I have more time.