build: make the tree compile on gcc 16, and add a CI lane so it stays that way - #945

What this fixes

The project does not currently build with gcc 16, which is what current Linux distributions ship. Five files use getpid() without including the header that declares it. They compiled anyway on older compilers, because some other standard header happened to include it for them. Newer compilers do not, so the build stops:

error: '::getpid' has not been declared; did you mean 'getpt'?

One of those five is src/vllm/entrypoints/openai/server_main.cpp. That is the OpenAI server's own entry point, so this is not just a test problem. A binary the project ships cannot be built on a current distribution.

Why it keeps happening

This is the second time. Three files were fixed a week ago after a report on #41. The same mistake then appeared in five new files, because each new model-loader test copies the same helper.

The reason nobody notices is that CI cannot see it. There are eighteen CI jobs, including Windows and ARM, but every Linux job installs the distribution's default g++, which on ubuntu-latest is gcc 13. No lane uses anything newer. So the mistake is invisible to CI and to anyone developing on Ubuntu, and it only shows up for people on newer distributions.

Fixing the five files again would not stop a third time. So this adds the missing coverage instead.

What is in this PR

1. The five missing includes. One line each.

2. A new CI job, build-newest-gcc. It builds the project in the official gcc:16 container. It only compiles, it does not run the tests, because the existing build-test-cpu job already runs them and this one exists to catch compile problems. Cost is roughly one extra build per pull request. It follows the same pattern as the existing CUDA job, which also runs in a container.

3. One warning is downgraded, on gcc 16 and newer only. Without this the new job cannot pass. gcc 16 reports an array-bounds problem inside the standard library and the bundled JSON library, on code that is correct. The warning still appears, it just no longer stops the build. On gcc 15 and older nothing changes at all, and a genuine out-of-bounds still fails the build there.

Worth knowing about item 3: this is not something the code can be rewritten to avoid. The warning appears after the compiler merges the shared-pointer cleanup function, which is identical for every shared pointer type, and then blames one type's destructor for another type's size. The standard library itself silences this same warning in the same place. The upstream bug is tree-optimization/122197, and Eigen, assimp and CMSSW all handle it the same way.

4. A Host compilers section in docs/USAGE.md. Not originally planned. check-doc-checkpoint classifies src/vllm/entrypoints/ as a user-facing surface and has no bypass by design, so rather than write a filler documentation update, this states the truthful user-facing fact: which compilers build the tree, and what the gcc 16 warning policy is.

What I checked, and what I did not

Checked on gcc 16.1.1 (Arch Linux):

• Full CPU build with tests enabled, no command-line workarounds, zero errors.
• The server binary builds and runs: vllm.cpp 0.0.3 c-abi=20.
• check-doc-checkpoint, check-public-doc-tables, check-readme-structure and check-env-doc all pass.

Not checked: the CI file itself. There is no way to run a GitHub Actions job locally, so this pull request's own CI run will be the first time that file executes. If it needs adjusting, say so and I will fix it.

If you would prefer something different

Item 3 is a policy decision about the -Werror rule, so it is yours to make. Alternatives, and I am happy to switch to any of them:

• Silence the warning narrowly with pragmas at each affected call site instead. That mirrors what the standard library does, but scatters compiler-specific pragmas through ordinary code and returns whenever someone adds a call site.
• Keep -Werror untouched and mark the new job as non-blocking, so it reports the problem without failing the build.
• Take only the five includes and drop items 2 and 3 entirely.

Note

This change was prepared with the help of Claude AI and reviewed by the contributor before submission.