From 894c67d4e76a13ee4ad8ec017c627425c0159785 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?R=C3=A9gis=20Desgroppes?= <rdesgroppes@gmail.com>
Date: Tue, 24 Mar 2026 10:38:28 +0100
Subject: [PATCH] Provide `PYTHON_FOR_BUILD` to CPython's Windows build

### What does this PR do?
- add `@python_3_12//:python3` (current `rules_python`'s hermetic
  interpreter) to `python_win`'s srcs and expose it as
  [PYTHON_FOR_BUILD](https://github.com/python/cpython/blob/3.12/PCbuild/readme.txt#L230),
- resolve it to an absolute path in `build_python.bat` before
  `build.bat` is called, so MSBuild can use it regardless of its
  working directory

### Motivation
CPython's `PCbuild/find_python.bat` looks for a Python interpreter to
perform pre-compilation code generation (opcodes, grammar, token
tables) and to orchestrate PGO training runs.

When `PYTHON_FOR_BUILD` is not set,
[find_python.bat](https://github.com/python/cpython/blob/3.12/PCbuild/find_python.bat)
falls back through the following chain, each of which fails in a Bazel
action running under `--incompatible_strict_action_env` (Bazel 9's
default BTW):
1. `VIRTUAL_ENV`,
2. `externals` directory,
3. `HOST_PYTHON`,
4. `py.exe` launcher,
5. `NuGet` download: **failed for me**, which surfaced the hermeticity
   issue.

`PYTHON_FOR_BUILD` is CPython's dedicated variable for the interpreter
used to **build** CPython itself, as opposed to the one **being built**.

### Describe how you validated your changes
Confirmed on a local Windows VM that `find_python.bat` reports
"found as PYTHON_FOR_BUILD" and proceeds past code generation.

### Additional Notes
Found while making progress towards #48206; split out so it can be
reviewed/merged/reverted independently.
---
 deps/cpython.BUILD.bazel      | 2 ++
 deps/cpython/build_python.bat | 1 +
 2 files changed, 3 insertions(+)

diff --git a/deps/cpython.BUILD.bazel b/deps/cpython.BUILD.bazel
index ca535dcdf58018..4b02cde422ccfc 100644
--- a/deps/cpython.BUILD.bazel
+++ b/deps/cpython.BUILD.bazel
@@ -65,6 +65,7 @@ run_binary(
             "libffi_win_dir",
             "openssl-bin_win_dir",
             "tcltk_win_dir",
+            "@python_3_12//:python3",
             "@visual_studio//:msbuild",
         ]
     ),
@@ -84,6 +85,7 @@ run_binary(
         "TCLTK_DIR": r"$(location tcltk_win_dir)\amd64",
         "TCL_VERSION": python_externals["tcltk"],
         "MSBUILD": "$(location @visual_studio//:msbuild)",
+        "PYTHON_FOR_BUILD": "$(location @python_3_12//:python3)",
     },
     outs = [
         "build/{}".format(file)
diff --git a/deps/cpython/build_python.bat b/deps/cpython/build_python.bat
index da10f8722e43da..9aa52a28eb2848 100644
--- a/deps/cpython/build_python.bat
+++ b/deps/cpython/build_python.bat
@@ -3,6 +3,7 @@ for %%F in (%SRCFILE%) do set sourcedir=%%~dpF
 :: Make sure input paths are proper Windows paths as needed
 for %%F in (%OUTDIR%) do set destdir=%%~fF\\
 for %%F in (%MSBUILD%) do set MSBUILD=%%~fF
+for %%F in (%PYTHON_FOR_BUILD%) do set PYTHON_FOR_BUILD=%%~fF
 
 set build_outdir=%sourcedir%\PCbuild\amd64