Merge bitcoin#30935: ci: Approximate MAKEJOBS in image build phase

fa71bed ci: Approximate MAKEJOBS in image build phase (MarcoFalke)

Pull request description:

  The `MAKEJOBS` env var is the default in image builds, which is fine, because it is only relevant when building msan (or iwyu) and only differs when setting MAKEJOBS to something other than `nproc` (currently used as an approximation).

  So the normal workflow of `MAKEJOBS="-j$(nproc)" FILE_ENV="./ci/test/00_setup_env_native_msan.sh" ./ci/test_run_all.sh` already works today.

  However, `MAKEJOBS="-j1" FILE_ENV="./ci/test/00_setup_env_native_msan.sh" ./ci/test_run_all.sh` does not.

  This is hard to fix, because making the env var a build arg means that changing it (and only it) requires a new (expensive and redundant) build.

  So add an option `HAVE_CGROUP_CPUSET`, which can be set to approximate `MAKEJOBS` a bit. Can be tested via:

  `HAVE_CGROUP_CPUSET=yo MAKEJOBS="-j_something"  FILE_ENV="./ci/test/00_setup_env_native_msan.sh" ./ci/test_run_all.sh`

ACKs for top commit:
  fanquake:
    ACK fa71bed

Tree-SHA512: 43ef194c71d726f4cfa3fe08a5894c7872150f37da7e4fa0c2d89e4572bc63acadb5dae3286a5e5cc14a8ce3e1ebcc14571f1a3541e8db2d18d2f7503764a2f3

ci/test/01_base_install.sh

@@ -15,6 +15,8 @@ if [ "$(git config --global ${CFG_DONE})" == "true" ]; then
   exit 0
 fi
 
+MAKEJOBS="-j$( nproc )"  # Use nproc, because MAKEJOBS is the default in docker image builds.
+
 if [ -n "$DPKG_ADD_ARCH" ]; then
   dpkg --add-architecture "$DPKG_ADD_ARCH"
 fi
@@ -45,7 +47,7 @@ if [[ ${USE_MEMORY_SANITIZER} == "true" ]]; then
     -DLLVM_ENABLE_RUNTIMES="compiler-rt;libcxx;libcxxabi;libunwind" \
     -S /msan/llvm-project/llvm
 
-  ninja -C /msan/clang_build/ "-j$( nproc )"  # Use nproc, because MAKEJOBS is the default in docker image builds
+  ninja -C /msan/clang_build/ "$MAKEJOBS"
   ninja -C /msan/clang_build/ install-runtimes
 
   update-alternatives --install /usr/bin/clang++ clang++ /msan/clang_build/bin/clang++ 100
@@ -64,7 +66,7 @@ if [[ ${USE_MEMORY_SANITIZER} == "true" ]]; then
     -DLIBCXX_HARDENING_MODE=debug \
     -S /msan/llvm-project/runtimes
 
-  ninja -C /msan/cxx_build/ "-j$( nproc )"  # Use nproc, because MAKEJOBS is the default in docker image builds
+  ninja -C /msan/cxx_build/ "$MAKEJOBS"
 
   # Clear no longer needed source folder
   du -sh /msan/llvm-project
@@ -74,7 +76,7 @@ fi
 if [[ "${RUN_TIDY}" == "true" ]]; then
   ${CI_RETRY_EXE} git clone --depth=1 https://github.com/include-what-you-use/include-what-you-use -b clang_"${TIDY_LLVM_V}" /include-what-you-use
   cmake -B /iwyu-build/ -G 'Unix Makefiles' -DCMAKE_PREFIX_PATH=/usr/lib/llvm-"${TIDY_LLVM_V}" -S /include-what-you-use
-  make -C /iwyu-build/ install "-j$( nproc )"  # Use nproc, because MAKEJOBS is the default in docker image builds
+  make -C /iwyu-build/ install "$MAKEJOBS"
 fi
 
 mkdir -p "${DEPENDS_DIR}/SDKs" "${DEPENDS_DIR}/sdk-sources"

ci/test/02_run_container.sh

@@ -15,12 +15,22 @@ if [ -z "$DANGER_RUN_CI_ON_HOST" ]; then
   python3 -c 'import os; [print(f"{key}={value}") for key, value in os.environ.items() if "\n" not in value and "HOME" != key and "PATH" != key and "USER" != key]' | tee "/tmp/env-$USER-$CONTAINER_NAME"
   # System-dependent env vars must be kept as is. So read them from the container.
   docker run --rm "${CI_IMAGE_NAME_TAG}" bash -c "env | grep --extended-regexp '^(HOME|PATH|USER)='" | tee --append "/tmp/env-$USER-$CONTAINER_NAME"
+
+  # Env vars during the build can not be changed. For example, a modified
+  # $MAKEJOBS is ignored in the build process. Use --cpuset-cpus as an
+  # approximation to respect $MAKEJOBS somewhat, if cpuset is available.
+  MAYBE_CPUSET=""
+  if [ "$HAVE_CGROUP_CPUSET" ]; then
+    MAYBE_CPUSET="--cpuset-cpus=$( python3 -c "import random;P=$( nproc );M=min(P,int('$MAKEJOBS'.lstrip('-j')));print(','.join(map(str,sorted(random.sample(range(P),M)))))" )"
+  fi
   echo "Creating $CI_IMAGE_NAME_TAG container to run in"
 
+  # shellcheck disable=SC2086
   DOCKER_BUILDKIT=1 docker build \
       --file "${BASE_READ_ONLY_DIR}/ci/test_imagefile" \
       --build-arg "CI_IMAGE_NAME_TAG=${CI_IMAGE_NAME_TAG}" \
       --build-arg "FILE_ENV=${FILE_ENV}" \
+      $MAYBE_CPUSET \
       --label="${CI_IMAGE_LABEL}" \
       --tag="${CONTAINER_NAME}" \
       "${BASE_READ_ONLY_DIR}"