build: generate flatbuffers headers at build time

[tete17]

Summary

• Generate flatbuffers C++ headers at CMake build time from the .fbs schemas instead of committing pre-generated headers
• Remove the flatbuffers version pin (was 25.9.23) from vcpkg.json — any compatible version now works
• Delete the generated/ directory (−1,800 lines of generated code)

Why

The pre-committed *_generated.h files contain a static_assert checking for an exact flatbuffers version match. This means every flatbuffers upgrade requires manually re-running flatc and committing the results — a maintenance burden and source of version drift. With build-time generation, upgrading flatbuffers is just a vcpkg baseline bump.

How it works

Uses flatbuffers_generate_headers() from flatbuffers' own BuildFlatBuffers.cmake module (shipped by vcpkg). This creates:

• A custom target GENERATE_osrm_fbs that runs flatc when .fbs schemas change
• An INTERFACE library osrm_fbs whose include directory points to the generated output
• Headers land in ${CMAKE_BINARY_DIR}/osrm_fbs/engine/api/flatbuffers/ — same include path as before, no source changes needed

Note: This PR is stacked on #7487 (vcpkg migration). Should be merged after that PR lands.

Test plan

• CLion build with flatbuffers 25.12.19 (unpinned) — all 5 headers generated, full build succeeds
• Generated headers contain version 25.12.19 assertions (was 25.9.23), confirming the pin is gone
• CI green

🤖 Generated with Claude Code

[nilsnolde]

is there a fallback in case BuildFlatBuffers.cmake in not available?

[tete17]

Hey @nilsnolde not realy but I wouldn't worry.

Any distribution of flatbuffers includes this cmake file https://packages.debian.org/trixie/amd64/libflatbuffers-dev/filelist#:~:text=/usr/lib/x86_64%2Dlinux%2Dgnu/cmake/flatbuffers/BuildFlatBuffers.cmake

It is required as many projects depend on it to autogenerate the targets based on their definitions. You probablly have it in your system if you install flatbuffers through your package manager

[nilsnolde]

ah perfect, thanks @tete17 ! yeah, I was mainly worried about debian like distros. hmm, seems bookworm doesn't have it: https://packages.debian.org/bookworm/amd64/libflatbuffers-dev/filelist. it's EOL in a few months. since trixie is just a few weeks old, I'd strongly suggest to introduce a fall back.

[DennisOSRM]

If this is pulled from vcpkg then there shouldn't be an issue on Debian bookworm, right? Or am I missing something here?

[nilsnolde]

IMO it's a bit nicer generally to not require vcpkg installations (always builds from source last I checked) and instead be able to use binary system packages for dependencies (except for win of course). I didn't really have the time to take a deeper look, I asked here if vcpkg is optional or required, but didn't get an answer yet.

[tete17]

Hey @nilsnolde sorry for ignoring the questions.

So regarding bookworm I have a strong suspecious, but I can't test it that, the flatbuffers team just restructure the cmake files and are now in a different place. If we wanted to support a debian based system based on bookworm (Ubutnu 22.04 for that matter as well) we would need to introduce an if clause in the cmake to include the right folder.

Then again this would be to support an EOL operating system for native packages when they could just turn on the vcpkg integration and not worry about anything. Yes the initial cmake configure is annoyiong as it would take 1h in a medium system not going to lie but is a worth price to pay to ensure everybody uses the same library versions.

[DennisOSRM]

I wouldn't worry too much about bookworm personally. It's EOL in this long term support phase in six weeks and since the beginning we had the policy of supporting the latest debian release only.

[nilsnolde]

yeah sorry, I read the trixie release wrong. and since vcpkg remains optional (as conan was), there's no real problem anyways.

[DennisOSRM]

Once #7487 is done, this needs a rebase. One idea to make this reviewable independently could be to use the other PR's branch as the base. And then after it's done to change the base branch to master.