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Add Python freethreading build #22443

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Merged
guitargeek merged 4 commits into from
Jun 6, 2026
Merged

Add Python freethreading build #22443

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965d757
[roottest] Ignore warning about Python GIL fallback in custom diff
guitargeek May 29, 2026
abfad9b
[ci] Make Python environment path configurable
guitargeek Jun 1, 2026
bf9aadc
[Python] Don't use limited Python API if Python was built without GIL
guitargeek Jun 6, 2026
9fbd5b0
[ci] Use Python freethreading+debug in Fedora Rawhide build
guitargeek Jun 1, 2026
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10 changes: 6 additions & 4 deletions .github/workflows/root-ci.yml
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Expand Up @@ -157,7 +157,7 @@
INCREMENTAL: ${{ !contains(github.event.pull_request.labels.*.name, 'clean build') && !matrix.platform == 'mac15' && !matrix.platform == 'mac26'}}
GITHUB_PR_ORIGIN: ${{ github.event.pull_request.head.repo.clone_url }}
OVERRIDES: ${{ join( matrix.overrides, ' ') }}
run: |

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GitHub Actions / lint-action-files 

"github.event.pull_request.head.ref" is potentially untrusted. avoid using it directly in inline scripts. instead, pass it through an environment variable. see https://docs.github.com/en/actions/reference/security/secure-use#good-practices-for-mitigating-script-injection-attacks for more details
[ -d "${VIRTUAL_ENV_DIR}" ] && source ${VIRTUAL_ENV_DIR}/bin/activate
echo "Python is now $(which python3) $(python3 --version)"
src/.github/workflows/root-ci-config/build_root.py \
Expand Down Expand Up @@ -287,7 +287,7 @@
INCREMENTAL: ${{ !contains(github.event.pull_request.labels.*.name, 'clean build') }}
GITHUB_PR_ORIGIN: ${{ github.event.pull_request.head.repo.clone_url }}
shell: cmd
run: "C:\\setenv.bat ${{ matrix.target_arch }} &&

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GitHub Actions / lint-action-files 

"github.event.pull_request.head.ref" is potentially untrusted. avoid using it directly in inline scripts. instead, pass it through an environment variable. see https://docs.github.com/en/actions/reference/security/secure-use#good-practices-for-mitigating-script-injection-attacks for more details
python .github/workflows/root-ci-config/build_root.py
--buildtype ${{ matrix.config }}
--platform windows10
Expand Down Expand Up @@ -415,10 +415,11 @@
is_special: true
property: "clang Ninja builtins auto-registration off"
overrides: ["CMAKE_CXX_STANDARD=20"]
# Fedora Rawhide with Python debug build
# Fedora Rawhide with Python freethreading+debug build
- image: rawhide
python_venv: "/py-venv-3.14td/ROOT-CI"
is_special: true
property: "Fedora pydebug"
property: "Fedora pydebug no GIL"
overrides: ["CMAKE_CXX_STANDARD=23"]
# Minimal build
- image: alma10
Expand Down Expand Up @@ -447,7 +448,7 @@
- self-hosted
- linux
- ${{ matrix.architecture == null && 'x64' || matrix.architecture }}
- ${{ matrix.extra-runs-on == null && 'cpu' || matrix.extra-runs-on }}

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property "extra-runs-on" is not defined in object type {architecture: string; image: string; is_special: bool; overrides: array<string>; platform_config: string; property: string; python_venv: string}

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property "extra-runs-on" is not defined in object type {architecture: string; image: string; is_special: bool; overrides: array<string>; platform_config: string; property: string; python_venv: string}

name: |
${{ matrix.image }} ${{ matrix.property }}
Expand All @@ -466,14 +467,15 @@
BUILD_DIR: /github/home/ROOT-CI/build
INSTALL_DIR: /github/home/ROOT-CI/install
POST_INSTALL_DIR: /github/home/ROOT-CI/PostInstall
PYTHON_VENV: ${{ matrix.python_venv || '/py-venv/ROOT-CI' }}

steps:
- name: Set up environment
run: |
ccache -o max_size=${{ matrix.is_special && '5G' || '1.5G' }}
ccache -s || true
if [ -d /py-venv/ROOT-CI/bin/ ]; then
. /py-venv/ROOT-CI/bin/activate && echo PATH=$PATH >> $GITHUB_ENV;
if [ -d "$PYTHON_VENV/bin/" ]; then
. "$PYTHON_VENV/bin/activate" && echo PATH=$PATH >> $GITHUB_ENV;
fi
if ${{ contains(matrix.property, 'auto-registration off') }}; then
echo "Disabling ROOT's object auto registration for this job"
Expand Down Expand Up @@ -513,7 +515,7 @@
INCREMENTAL: ${{ !contains(github.event.pull_request.labels.*.name, 'clean build') }}
GITHUB_PR_ORIGIN: ${{ github.event.pull_request.head.repo.clone_url }}
OVERRIDES: ${{ join( matrix.overrides, ' ') }}
run: ".github/workflows/root-ci-config/build_root.py

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GitHub Actions / lint-action-files 

"github.event.pull_request.head.ref" is potentially untrusted. avoid using it directly in inline scripts. instead, pass it through an environment variable. see https://docs.github.com/en/actions/reference/security/secure-use#good-practices-for-mitigating-script-injection-attacks for more details
--buildtype RelWithDebInfo
--platform ${{ matrix.image }}
--platform_config ${{ matrix.platform_config }}
Expand Down
3 changes: 2 additions & 1 deletion bindings/pyroot/pythonizations/CMakeLists.txt
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Expand Up @@ -37,7 +37,8 @@ if (Python3_VERSION VERSION_GREATER_EQUAL "3.11")
# On Windows we can't use the stable ABI yet: it requires linking against a
# different libpython, so as long as we don't build all translation units in
# the ROOT Pythonization library with the stable ABI we should not use it.
if(NOT MSVC)
# Free-threaded (Py_GIL_DISABLED) interpreters don't support the limited API.
if(NOT MSVC AND NOT Python3_GIL_DISABLED)
target_compile_options(${libname} PRIVATE -DPy_LIMITED_API=0x030B0000)
endif()
endif()
Expand Down
3 changes: 2 additions & 1 deletion bindings/tpython/CMakeLists.txt
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Expand Up @@ -36,7 +36,8 @@ if (Python3_VERSION VERSION_GREATER_EQUAL "3.10")
# On Windows we can't use the stable ABI yet: it requires linking against a
# different libpython, so as long as we don't build all translation units in
# the ROOT Pythonization library with the stable ABI we should not use it.
if(NOT MSVC)
# Free-threaded (Py_GIL_DISABLED) interpreters don't support the limited API.
if(NOT MSVC AND NOT Python3_GIL_DISABLED)
target_compile_options(ROOTTPython PRIVATE -DPy_LIMITED_API=0x030A0000)
endif()
endif()
Expand Down
27 changes: 27 additions & 0 deletions cmake/modules/SearchInstalledSoftware.cmake
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Expand Up @@ -412,6 +412,33 @@ if(tmva-pymva OR tmva-sofie)
endif()
find_package(Python3 3.10 COMPONENTS ${python_components})

# Detect whether the found Python interpreter is a free-threaded build
# (Py_GIL_DISABLED is defined in pyconfig.h). The limited C API is not
# supported in free-threaded builds; including Python.h with Py_LIMITED_API
# defined produces a hard error there
# (https://docs.python.org/3/howto/free-threading-extensions.html).
# Checking the preprocessor symbol directly is more reliable than asking the
# interpreter (e.g. sysconfig.get_config_var may misreport, and
# sys._is_gil_enabled() can be overridden at runtime via PYTHON_GIL=1).
set(Python3_GIL_DISABLED FALSE)
if(Python3_Development_FOUND OR Python3_Development.Module_FOUND)
include(CheckCXXSourceCompiles)
set(_old_required_includes ${CMAKE_REQUIRED_INCLUDES})
set(CMAKE_REQUIRED_INCLUDES ${Python3_INCLUDE_DIRS})
check_cxx_source_compiles("
#include <Python.h>
#ifndef Py_GIL_DISABLED
#error \"GIL is not disabled\"
#endif
int main() { return 0; }
" ROOT_PYTHON_GIL_DISABLED)
set(CMAKE_REQUIRED_INCLUDES ${_old_required_includes})
if(ROOT_PYTHON_GIL_DISABLED)
set(Python3_GIL_DISABLED TRUE)
message(STATUS "Python ${Python3_VERSION} is a free-threaded build (Py_GIL_DISABLED defined); the limited C API will not be used")
endif()
endif()

#---Check for OpenGL installation-------------------------------------------------------
# OpenGL is required by various graf3d features that are enabled with opengl=ON,
# or by the Cocoa-related code that always requires it.
Expand Down
1 change: 1 addition & 0 deletions roottest/scripts/custom_diff.py
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Expand Up @@ -33,6 +33,7 @@ def __init__(self):
r"^In file included from input_line", # Wrapper input line
r"^[:space:]*$", # Lines which are empty apart from spaces
r"^Info in <TROOT>: Object auto registration", # ROOT 7 mode
r"^<frozen importlib\._bootstrap>:\d+: RuntimeWarning: The global interpreter lock \(GIL\) has been enabled", # Free-threaded Python re-enabling the GIL to load a C extension that isn't GIL-safe
]
]

Expand Down
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