gh-152384: pixi-packages: use flags to define variants (#152385)

Co-authored-by: Hugo van Kemenade <1324225+hugovk@users.noreply.github.com>
Co-authored-by: Isuru Fernando <isuruf@gmail.com>
Co-authored-by: Nathan Goldbaum <nathan.goldbaum@gmail.com>

Author: 4 people

.github/workflows/reusable-san.yml

@@ -39,6 +39,8 @@ jobs:
       run: |
         sudo ./.github/workflows/posix-deps-apt.sh
         # On ubuntu-26.04 image, clang is clang-21 by default
+        # NOTE: when bumping to a new version of clang,
+        # please update the versions in `Tools/pixi-packages/variants.yaml` too.
         echo "CC=clang" >> "$GITHUB_ENV"
         echo "CXX=clang++" >> "$GITHUB_ENV"
     - name: TSan option setup

Misc/NEWS.d/next/Tools-Demos/2026-07-10-14-38-31.gh-issue-152384.uaPADM.rst

@@ -0,0 +1,8 @@
+The CPython Pixi packages are now all accessible at the same
+``Tools/pixi-packages`` subdirectory, rather than at
+``Tools/pixi-packages/{variant}`` as before. Variants are now selected not
+via subdirectory but via ``flags``; see
+https://pixi.prefix.dev/latest/concepts/package_specifications/#extras-and-flags
+for usage instructions. The ``tsan-freethreading`` variant has been renamed
+to ``tsan_freethreading``, while the ``default``, ``asan``, and
+``freethreading`` variants retain their previous names.

Tools/pixi-packages/README.md

@@ -10,34 +10,34 @@ in a [Pixi workspace](https://pixi.sh/latest/first_workspace/), like:
 ```toml
 [dependencies]
 python.git = "https://github.com/python/cpython"
-python.subdirectory = "Tools/pixi-packages/asan"
+python.subdirectory = "Tools/pixi-packages"
+python.flags = ["asan"]
 ```
 
 This is particularly useful when developers need to build CPython from source
 (for example, for an ASan or TSan-instrumented build), as it does not require any manual
 clone or build steps. Instead, Pixi will automatically handle both the build
 and installation of the package.
 
-Each package definition is contained in a subdirectory, but they share the build script
-`build.sh` in this directory. Currently defined package variants:
+Each package variant carries a 'flag' to enable selection of that variant — see
+[the Pixi docs](https://pixi.prefix.dev/latest/concepts/package_specifications/#extras-and-flags)
+for details of how to use this. Currently defined package variants:
 
 - `default`
 - `freethreading`
 - `asan`: ASan-instrumented build
-- `tsan-freethreading`: TSan-instrumented free-threading build
+- `tsan_freethreading`: TSan-instrumented free-threading build
 
 ## Maintenance
 
-- Keep the `abi_tag` and `version` fields in each `variants.yaml` up to date with the
+- Keep the `version` field in `variants.yaml` up to date with the
   Python version
 - Update `build.sh` for any breaking changes in the `configure` and `make` workflow
 
 ## Opportunities for future improvement
 
 - More package variants (such as UBSan)
 - Support for Windows
-- Using a single `pixi.toml` for all package variants is blocked on
-  [pixi#5248](https://github.com/prefix-dev/pixi/issues/5248)
 
 ## Troubleshooting
 

Tools/pixi-packages/asan/variants.yaml

@@ -7,7 +7,7 @@ if [[ "${PYTHON_VARIANT}" == "freethreading" ]]; then
 elif [[ "${PYTHON_VARIANT}" == "asan" ]]; then
     CONFIGURE_EXTRA="--with-address-sanitizer"
     export ASAN_OPTIONS="strict_init_order=true"
-elif [[ "${PYTHON_VARIANT}" == "tsan-freethreading" ]]; then
+elif [[ "${PYTHON_VARIANT}" == "tsan_freethreading" ]]; then
     CONFIGURE_EXTRA="--disable-gil --with-thread-sanitizer"
     export TSAN_OPTIONS="suppressions=${SRC_DIR}/Tools/tsan/suppressions_free_threading.txt"
 elif [[ "${PYTHON_VARIANT}" == "default" ]]; then
@@ -17,6 +17,14 @@ else
     exit 1
 fi
 
+VER_REF=$(grep "\[PYTHON_VERSION\]\, \[" configure.ac | sed -n 's/.*\[\([0-9.]*\)\].*/\1/p')
+VER=$(echo ${PKG_VERSION} | sed -E 's/^([0-9]+\.[0-9]+).*/\1/')
+
+if [[ "${VER_REF}" != "${VER}" ]]; then
+    echo "Unexpected version from conda package. Got ${VER}. Expected ${VER_REF}. Do you need to update 'version' in 'variants.yaml'?"
+    exit 1
+fi
+
 # rattler-build by default set a target of 10.9
 # override it to at least 10.12
 case ${MACOSX_DEPLOYMENT_TARGET:-10.12} in