On Windows, have the autodetecting Python toolchain revert to reading…

… --python_path

This works around the fact that the autodetecting toolchain doesn't run under Windows (#7844). We'll have to add a Windows toolchain in the future, and then remove --python_path (and also address the EnsurePythonPathOption behavior in blaze_util_windows.cc).

RELNOTES: None
PiperOrigin-RevId: 241051815

src/main/java/com/google/devtools/build/lib/rules/python/PyCommon.java

@@ -459,6 +459,19 @@ private static PyRuntimeInfo initRuntimeFromToolchain(
               "The Python toolchain does not provide a runtime for Python version %s",
               version.name()));
     }
+
+    // Hack around the fact that the autodetecting Python toolchain, which is automatically
+    // registered, does not yet support windows. In this case, we want to return null so that
+    // BazelPythonSemantics falls back on --python_path. See toolchain.bzl.
+    // TODO(#7844): Remove this hack when the autodetecting toolchain has a windows implementation.
+    if (py2RuntimeInfo != null
+        && py2RuntimeInfo.getInterpreterPathString() != null
+        && py2RuntimeInfo
+            .getInterpreterPathString()
+            .equals("/_magic_pyruntime_sentinel_do_not_use")) {
+      return null;
+    }
+
     return result;
   }
 

tools/python/BUILD.tools

@@ -75,4 +75,5 @@ constraint_setting(name = "py3_interpreter_path")
 define_autodetecting_toolchain(
     name = "autodetecting_toolchain",
     pywrapper_template = "pywrapper_template.txt",
+    windows_config_setting = "@bazel_tools//src/conditions:windows",
 )

tools/python/toolchain.bzl

@@ -116,7 +116,10 @@ register_toolchains("//my_pkg:my_toolchain")
 # toolchain, based on the "py" wrapper (e.g. "py -2" and "py -3"). Use select()
 # in the template attr of the _generate_*wrapper targets.
 
-def define_autodetecting_toolchain(name, pywrapper_template):
+def define_autodetecting_toolchain(
+        name,
+        pywrapper_template,
+        windows_config_setting):
     """Defines the autodetecting Python toolchain.
 
     For use only by @bazel_tools//tools/python:BUILD; see the documentation
@@ -127,6 +130,9 @@ def define_autodetecting_toolchain(name, pywrapper_template):
             "autodetecting_toolchain". This param is present only to make the
             BUILD file more readable.
         pywrapper_template: The label of the pywrapper_template.txt file.
+        windows_config_setting: The label of a config_setting that matches when
+            the platform is windows, in which case the toolchain is configured
+            in a way that triggers a workaround for #7844.
     """
     if native.package_name() != "tools/python":
         fail("define_autodetecting_toolchain() is private to " +
@@ -163,9 +169,26 @@ def define_autodetecting_toolchain(name, pywrapper_template):
         visibility = ["//visibility:private"],
     )
 
+    # 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.
+    native.py_runtime(
+        name = "_sentinel_py2_runtime",
+        interpreter_path = "/_magic_pyruntime_sentinel_do_not_use",
+        python_version = "PY2",
+        visibility = ["//visibility:private"],
+    )
+
     py_runtime_pair(
         name = "_autodetecting_py_runtime_pair",
-        py2_runtime = ":_autodetecting_py2_runtime",
+        py2_runtime = select({
+            # If we're on windows, inject the sentinel to tell native rule logic
+            # that we attempted to use the autodetecting toolchain and need to
+            # switch back to legacy behavior.
+            # TODO(#7844): Remove this hack.
+            windows_config_setting: ":_sentinel_py2_runtime",
+            "//conditions:default": ":_autodetecting_py2_runtime",
+        }),
         py3_runtime = ":_autodetecting_py3_runtime",
         visibility = ["//visibility:public"],
     )