gh-152384: pixi-packages: use flags to define variants

Author: lucascolley

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 `abi_tag` and `version` fields 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

@@ -1,6 +1,3 @@
-# NOTE: Please always only modify default/recipe.yaml and then run clone-recipe.sh to
-# propagate the changes to the other variants.
-
 context:
   # Keep up to date
   freethreading_tag: ${{ "t" if "freethreading" in variant else "" }}
@@ -9,7 +6,7 @@ recipe:
   name: python
 
 source:
-  - path: ../../..
+  - path: ../..
 
 outputs:
 - package:
@@ -25,12 +22,14 @@ outputs:
     name: python
     version: ${{ version }}
   build:
+    flags:
+      - ${{ variant }}
     string: "0_${{ abi_tag }}"
     files:
       exclude:
         - "*.o"
     script:
-      file: ../build.sh
+      file: build.sh
       env:
         PYTHON_VARIANT: ${{ variant }}
     python: