Properly handle selects with None values for label-valued attrs in na…

…tive.existing_rule and native.existing_rules

Label-valued attributes have a default value of null in Java. The BuildType.Selector
constructor casts None values of select branches to that default - i.e. to null; but
we must transform that null back to None when reconstructing a SelectorValue in
native.existing_rule/s for use in Starlark - or we would miss a branch.

And take the opportunity to ensure we never attempt to construct an empty
SelectorValue or SelectorList - which is a fatal error.

PiperOrigin-RevId: 656122706
Change-Id: Id236af711ba7f7c515a1b238e4be9d7d63574173

src/main/java/com/google/devtools/build/lib/packages/StarlarkNativeModule.java

@@ -809,17 +809,30 @@ private static Object starlarkifyValue(Mutability mu, Object val, Package pkg) {
         selector.forEach(
             (rawKey, rawValue) -> {
               Object key = starlarkifyValue(mu, rawKey, pkg);
-              Object mapVal = starlarkifyValue(mu, rawValue, pkg);
+              // BuildType.Selector constructor transforms `None` values of selector branches into
+              // Java nulls if the selector original type's default value is null. We need to
+              // reverse this transformation.
+              Object mapVal =
+                  rawValue == null && selector.getOriginalType().getDefaultValue() == null
+                      ? Starlark.NONE
+                      : starlarkifyValue(mu, rawValue, pkg);
               if (key != null && mapVal != null) {
                 m.put(key, mapVal);
               }
             });
-        selectors.add(new SelectorValue(((Map<?, ?>) m.build(mu)), selector.getNoMatchError()));
+        Dict<?, ?> selectorDict = m.build(mu);
+        if (!selectorDict.isEmpty()) {
+          selectors.add(new SelectorValue(selectorDict, selector.getNoMatchError()));
+        }
       }
-      try {
-        return SelectorList.of(selectors);
-      } catch (EvalException e) {
+      if (selectors.isEmpty()) {
         return null;
+      } else {
+        try {
+          return SelectorList.of(selectors);
+        } catch (EvalException e) {
+          return null;
+        }
       }
     }
 

src/test/java/com/google/devtools/build/lib/packages/NativeExistingRulesTest.java

@@ -115,6 +115,48 @@ def rule_dict(name):
     assertThat(getConfiguredTarget("//test/getrule:x")).isNotNull();
   }
 
+  // Regression test for b/355432322
+  @Test
+  public void existingRule_handlesSelectWithNoneValues_forLabelValuedAttributes() throws Exception {
+    scratch.file("test/starlark/BUILD");
+    scratch.file(
+        "test/starlark/rulestr.bzl",
+        """
+        def save_dep(rule_name):
+            r = native.existing_rule(rule_name)
+            test.save("dep", r["dep"])
+
+        def _impl(ctx):
+            pass
+
+        my_rule = rule(
+            implementation = _impl,
+            attrs = {
+                "dep": attr.label(),
+            },
+        )
+        """);
+
+    scratch.file(
+        "test/getrule/BUILD",
+        """
+        load("//test/starlark:rulestr.bzl", "my_rule", "save_dep")
+
+        my_rule(
+            name = "x",
+            dep = select({"//conditions:default": None}),
+        )
+
+        save_dep("x")
+        """);
+
+    // Parse the BUILD file, to make sure select() makes it out of native.existing_rule().
+    assertThat(getConfiguredTarget("//test/getrule:x")).isNotNull();
+
+    // We have to compare by stringification because SelectorValue has reference equality semantics.
+    assertThat(getSaved("dep").toString()).isEqualTo("select({\"//conditions:default\": None})");
+  }
+
   @Test
   public void existingRule_returnsNone() throws Exception {
     scratch.file(