Move merge_runfiles_with_generated_inits_empty_files_supplier to common

PyBuiltins.

This particular function relies on the Bazel/Google-specific PythonSemantics
init py filter functions, so required a bit of refactoring to pass around
the appropriate function to the helpers.

Work towards bazelbuild#15897

PiperOrigin-RevId: 488742400
Change-Id: I1e7b2ce3c69b3e89415aa0ce65457f12c1923011

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

@@ -17,4 +17,8 @@
 import com.google.devtools.build.lib.rules.python.PyBuiltins;
 
 /** PyBuiltins with Bazel-specific functionality. */
-public final class BazelPyBuiltins extends PyBuiltins {}
+public final class BazelPyBuiltins extends PyBuiltins {
+  public BazelPyBuiltins() {
+    super(BazelPythonSemantics.GET_INIT_PY_FILES);
+  }
+}

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

@@ -58,6 +58,12 @@
 public abstract class PyBuiltins implements StarlarkValue {
   public static final String NAME = "py_builtins";
 
+  private final Runfiles.EmptyFilesSupplier emptyFilesSupplier;
+
+  protected PyBuiltins(Runfiles.EmptyFilesSupplier emptyFilesSupplier) {
+    this.emptyFilesSupplier = emptyFilesSupplier;
+  }
+
   @StarlarkMethod(
       name = "is_singleton_depset",
       doc = "Efficiently checks if the depset is a singleton.",
@@ -259,6 +265,36 @@ public Object newEmptyRunfilesWithMiddleman(
         .build();
   }
 
+  @StarlarkMethod(
+      name = "merge_runfiles_with_generated_inits_empty_files_supplier",
+      doc =
+          "Create a runfiles that generates missing __init__.py files using Java and "
+              + "the internal EmptyFilesProvider interface",
+      parameters = {
+        @Param(name = "ctx", positional = false, named = true, defaultValue = "unbound"),
+        @Param(
+            name = "runfiles",
+            positional = false,
+            named = true,
+            defaultValue = "unbound",
+            doc = "Runfiles to merge into the result; must be non-empty"),
+      })
+  public Object mergeRunfilesWithGeneratedInitsEmptyFilesSupplier(
+      StarlarkRuleContext starlarkCtx, Runfiles runfiles) throws EvalException {
+    if (runfiles.isEmpty()) {
+      // The Runfiles merge functions have an optimization to detect an empty runfiles and return a
+      // singleton. Unfortunately, this optimization considers an empty runfiles with
+      // a emptyFilesSupplier as empty, so then drops it when returning the singleton. To work
+      // around this, require that there is *something* in the runfiles.
+      throw Starlark.errorf("input runfiles cannot be empty");
+    }
+    return new Runfiles.Builder(
+            starlarkCtx.getWorkspaceName(), starlarkCtx.getConfiguration().legacyExternalRunfiles())
+        .setEmptyFilesSupplier(emptyFilesSupplier)
+        .merge(runfiles)
+        .build();
+  }
+
   @StarlarkMethod(
       name = "declare_constant_metadata_file",
       doc = "Declare a file that always reports it is unchanged.",