refactor: replace the magic sentinel string in the runtime env toolchain with a powershell wrapper

python/private/runtime_env_toolchain.bzl

@@ -54,12 +54,9 @@ def define_runtime_env_toolchain(name):
         supports_build_time_venv = supports_build_time_venv,
     )
 
-    # This is a dummy runtime whose interpreter_path triggers the native rule
-    # logic to use the legacy behavior on Windows.
-    # TODO(#7844): Remove this target.
     py_runtime(
-        name = "_magic_sentinel_runtime",
-        interpreter_path = "/_magic_pyruntime_sentinel_do_not_use",
+        name = "_runtime_env_py3_runtime_windows",
+        interpreter = "//python/private:runtime_env_toolchain_interpreter.ps1",
         python_version = "PY3",
         visibility = ["//visibility:private"],
         tags = ["manual"],
@@ -69,11 +66,7 @@ def define_runtime_env_toolchain(name):
     py_runtime_pair(
         name = "_runtime_env_py_runtime_pair",
         py3_runtime = select({
-            # If we're on windows, inject the sentinel to tell native rule logic
-            # that we attempted to use the runtime_env toolchain and need to
-            # switch back to legacy behavior.
-            # TODO(#7844): Remove this hack.
-            "@platforms//os:windows": ":_magic_sentinel_runtime",
+            "@platforms//os:windows": ":_runtime_env_py3_runtime_windows",
             "//conditions:default": ":_runtime_env_py3_runtime",
         }),
         visibility = ["//visibility:public"],

python/private/runtime_env_toolchain_interpreter.ps1

@@ -0,0 +1,51 @@
+# PowerShell equivalent of runtime_env_toolchain_interpreter.sh
+
+$ErrorActionPreference = "Stop"
+
+function Die {
+    param([string]$message)
+    $header = ("Error occurred while attempting to use the deprecated Python "
+        + "toolchain `n(@rules_python//python/runtime_env_toolchain:all).")
+    Write-Error "$header`n$message"
+    exit 1
+}
+
+# Try the "python3" command name first, then fall back on "python".
+$pythonBin = Get-Command python3 -ErrorAction SilentlyContinue
+if (-not $pythonBin) {
+    $pythonBin = Get-Command python -ErrorAction SilentlyContinue
+}
+
+if (-not $pythonBin) {
+    Die ("Neither 'python3' nor 'python' were found on the target platform's "
+        + "PATH, which is:`n`n$($env:PATH)`n`nPlease ensure an interpreter "
+        + "is available on this platform (and marked executable), or else "
+        + "register an appropriate Python toolchain as per the "
+        + "documentation for py_runtime_pair "
+        + "(https://github.com/bazel-contrib/rules_python/blob/master/"
+        + "docs/python.md#py_runtime_pair).")
+}
+
+# Because this is a wrapper script that invokes Python, it prevents Python
+# from detecting virtualenvs like normal (i.e. using the venv symlink to
+# find the real interpreter). To work around this, we have to manually
+# detect the venv, then trick the interpreter into understanding we're in a
+# virtual env.
+$selfDir = $PSScriptRoot
+$venvPath = Join-Path $selfDir "pyvenv.cfg"
+$venvParentPath = Join-Path $selfDir "..\pyvenv.cfg"
+
+if ((Test-Path $venvPath) -or (Test-Path $venvParentPath)) {
+    $venvBin = $MyInvocation.MyCommand.Path
+    if (-not (Test-Path $pythonBin.Source)) {
+        Die "ERROR: Python interpreter does not exist: $($pythonBin.Source)"
+    }
+    # PYTHONEXECUTABLE is also used because switching argv0 doesn't fully
+    # trick the pyenv wrappers.
+    # NOTE: The PYTHONEXECUTABLE envvar only works for non-Mac starting in
+    # Python 3.11
+    $env:PYTHONEXECUTABLE = $venvBin
+}
+# NOTE: Windows doesn't have an exec equivalent. The call operator (&)
+# creates a sub-process, which is the closest equivalent.
+& $pythonBin.Source $args