Fix 1985

.audit/oberstet_fix_1985.md

@@ -0,0 +1,8 @@
+- [ ] I did **not** use any AI-assistance tools to help create this pull request.
+- [x] I **did** use AI-assistance tools to *help* create this pull request.
+- [x] I have read, understood and followed the project's AI_POLICY.md when creating code, documentation etc. for this pull request.
+
+Submitted by: @oberstet
+Date: 2026-10-18
+Related issue(s): #1985
+Branch: oberstet:fix_1985

docs/changelog.rst

@@ -22,6 +22,7 @@ Changelog
 * Adopt the contribution workflow shared by all WAMP projects: ``CONTRIBUTING.md``, the pull request template and ``.audit/README.md`` are now deployed byte-identically from wamp-cicd (GitHub issue first, red → green tests, AI-assistance disclosure) and kept in sync by a CI drift check; everything specific to Autobahn|Python moved to a new ``DEVELOPMENT.md`` (also included in the docs). ``.cicd`` is pinned to wamp-cicd ``38ded6a``, the same commit across the WAMP fleet (#1956)
 * Remove the opt-in entropy depletion tests (``autobahn/test/test_rng.py``, enabled by ``AUTOBAHN_CI_ENABLE_RNG_DEPLETION_TESTS``, never run in CI): they read ``/proc/sys/kernel/random/entropy_avail``, which has been a constant 256 since Linux 5.18, and ``/dev/random`` no longer blocks since Linux 5.6, so one test passed whatever happened and the other always failed (#1278)
 * Build the ARM64 Linux wheels natively on GitHub's ``ubuntu-24.04-arm`` runners instead of under QEMU emulation on x86_64 runners, as zlmdb does since #183. Under emulation the builds took around 40 minutes (up to 96) and segfaulted intermittently, so each build had 9 attempts of 120 minutes; the same container images and build script are used, and a build attempt now gets 60 minutes and 3 tries (#1940)
+* Build the bundled ``flatc`` on all cores: ``hatch_build.py`` ran ``cmake --build`` without ``--parallel``, so ``flatc`` was compiled on one core in every wheel build, editable install and ``just test-*`` run. It now passes ``--parallel <os.cpu_count()>``; an explicit ``CMAKE_BUILD_PARALLEL_LEVEL`` in the environment still wins. Measured on a 32-core host: 237 s serial, 25 s with 16 jobs. Skipping the build when ``flatc``'s inputs did not change is still open (#1985)
 * Add an ``import-smoke-lowest`` CI job (``just test-imports-lowest``) that runs the import smoke test on CPython 3.11 with every direct dependency resolved to the lowest version ``pyproject.toml`` allows (``uv --resolution lowest-direct``), so stale version floors are caught. The import smoke test now also covers the Twisted and asyncio bindings, each module imported in a fresh interpreter (#1955)
 
 **asyncio**

hatch_build.py

@@ -247,8 +247,9 @@ def _build_flatc(self, build_data):
             print(f"ERROR: cmake configure failed:\n{result.stderr}")
             return False
 
-        # Step 2: Build flatc
-        print("  -> Building flatc...")
+        # Step 2: Build flatc, on all cores (#1985). An explicit
+        # CMAKE_BUILD_PARALLEL_LEVEL in the environment wins: cmake reads it
+        # itself when --parallel is given without a job count.
         build_args = [
             cmake_path,
             "--build",
@@ -257,7 +258,15 @@ def _build_flatc(self, build_data):
             "Release",
             "--target",
             "flatc",
+            "--parallel",
         ]
+        jobs = os.environ.get("CMAKE_BUILD_PARALLEL_LEVEL")
+        if jobs:
+            print(f"  -> Building flatc ({jobs} jobs, CMAKE_BUILD_PARALLEL_LEVEL)...")
+        else:
+            jobs = str(os.cpu_count() or 1)
+            build_args.append(jobs)
+            print(f"  -> Building flatc ({jobs} parallel jobs)...")
 
         result = subprocess.run(
             build_args,