New incompatible flags disable ctx.fragments.py, ctx.fragments.bazel_py

`--incompatible_remove_ctx_py_fragment` removes `ctx.fragments.py`.

`--incompatible_remove_ctx_bazel_py_fragment` removes `ctx.fragments.bazel_py`.

`rules_python` can use this to determine if it should read Python flags from Starlark definitions or Bazel-native:

```py
if not hasattr(ctx.fragments, "py"):
  print("I should read Python flags from Starlark.")
````

For bazel-contrib/rules_python#3252.

**Caveats:**
This approach has the downside that it's all-or-nothing: removing an entire fragment doesn't permit partial Starlarkification. Which is hypothetically an issue if we have to keep certain flags native because of strange inner Bazel dependencies.

I think for Python this isn't a huge risk. Worst-case we can redefine the flags to restrict fragment access instead of completely removing it.

Closes #27056.

PiperOrigin-RevId: 811388173
Change-Id: I8e148e65fce8d007f0724ebac6752319e0c86c54

Author: gregestren

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

@@ -64,6 +64,18 @@ public static final class Options extends FragmentOptions {
                 + "`import myreponame.mytoplevelpackage.package.module` is valid. The latter form "
                 + "is less likely to experience import name collisions.")
     public boolean experimentalPythonImportAllRepositories;
+
+    @Option(
+        name = "incompatible_remove_ctx_bazel_py_fragment",
+        defaultValue = "false",
+        documentationCategory = OptionDocumentationCategory.UNDOCUMENTED,
+        effectTags = {OptionEffectTag.BUILD_FILE_SEMANTICS},
+        metadataTags = {OptionMetadataTag.INCOMPATIBLE_CHANGE},
+        help =
+            "When true, Python build flags are defined with Python rules (in BUIILD files) and the"
+                + " ctx.fragments.bazel_py attribute is not present. This is a migration flag to"
+                + " move all Python flags from core Bazel to Python rules.")
+    public boolean disablePyFragment;
   }
 
   private final Options options;
@@ -77,6 +89,11 @@ public BazelPythonConfiguration(BuildOptions buildOptions) throws InvalidConfigu
     }
   }
 
+  @Override
+  public boolean shouldInclude() {
+    return !options.disablePyFragment;
+  }
+
   @StarlarkConfigurationField(
       name = "python_top",
       doc = "Deprecated. Always returns None. Use toolchain resolution instead.")

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

@@ -55,6 +55,7 @@ public class PythonConfiguration extends Fragment implements StarlarkValue {
   private final boolean defaultToExplicitInitPy;
   @Nullable private final Label nativeRulesAllowlist;
   private final boolean disallowNativeRules;
+  private final boolean disablePyFragment;
 
   public PythonConfiguration(BuildOptions buildOptions) {
     PythonOptions pythonOptions = buildOptions.get(PythonOptions.class);
@@ -68,6 +69,12 @@ public PythonConfiguration(BuildOptions buildOptions) {
     this.nativeRulesAllowlist = pythonOptions.nativeRulesAllowlist;
     this.disallowNativeRules = pythonOptions.disallowNativeRules;
     this.includeLabelInLinkstamp = pythonOptions.includeLabelInPyBinariesLinkstamp;
+    this.disablePyFragment = pythonOptions.disablePyFragment;
+  }
+
+  @Override
+  public boolean shouldInclude() {
+    return !disablePyFragment;
   }
 
   @Override
@@ -140,14 +147,11 @@ public void processForOutputPathMnemonic(Fragment.OutputDirectoriesContext ctx)
       structField = true,
       doc = "The effective value of --build_python_zip")
   public boolean buildPythonZip() {
-    switch (buildPythonZip) {
-      case YES:
-        return true;
-      case NO:
-        return false;
-      default:
-        return OS.getCurrent() == OS.WINDOWS;
-    }
+    return switch (buildPythonZip) {
+      case YES -> true;
+      case NO -> false;
+      default -> OS.getCurrent() == OS.WINDOWS;
+    };
   }
 
   /**

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

@@ -174,6 +174,18 @@ public String getTypeDescription() {
   // Helper field to store hostForcePython in exec configuration
   private PythonVersion defaultPythonVersion = null;
 
+  @Option(
+      name = "incompatible_remove_ctx_py_fragment",
+      defaultValue = "false",
+      documentationCategory = OptionDocumentationCategory.UNDOCUMENTED,
+      effectTags = {OptionEffectTag.BUILD_FILE_SEMANTICS},
+      metadataTags = {OptionMetadataTag.INCOMPATIBLE_CHANGE},
+      help =
+          "When true, Python build flags are defined with Python rules (in BUIILD files) and"
+              + " ctx.fragments.py is undefined. This is a migration flag to move all Python flags "
+              + " from core Bazel to Python rules.")
+  public boolean disablePyFragment;
+
   /**
    * Returns the Python major version ({@code PY2} or {@code PY3}) that targets that do not specify
    * a version should be built for.

src/main/starlark/builtins_bzl/common/builtin_exec_platforms.bzl

@@ -195,6 +195,7 @@ bazel_fragments["BazelPythonConfiguration$Options"] = fragment(
     propagate = [
         "//command_line_option:python_path",
         "//command_line_option:experimental_python_import_all_repositories",
+        "//command_line_option:incompatible_remove_ctx_bazel_py_fragment",
     ],
 )
 
@@ -445,6 +446,7 @@ bazel_fragments["PythonOptions"] = fragment(
         "//command_line_option:incompatible_py3_is_default",
         "//command_line_option:incompatible_python_disallow_native_rules",
         "//command_line_option:python_native_rules_allowlist",
+        "//command_line_option:incompatible_remove_ctx_py_fragment",
     ],
     outputs = [
         "//command_line_option:python_version",