Trim input build settings after transition

By convention, BuildOptions should not contain Starlark build settings
that are set to their default values. However, if a Starlark transition
has a build setting as an input, the value of that setting has to be
added to the input BuildOptions temporarily so that it can be read by
the transition.

Before this commit, if a build setting was an input but not an output of
a transition, its default value remained in the BuildOptions and could
cause action conflicts. With this commit, all build settings rather than
only output build settings are trimmed post-transition if they are set
to their default value.

Fixes #12888.

src/main/java/com/google/devtools/build/lib/analysis/starlark/StarlarkTransition.java

@@ -267,13 +267,15 @@ public static Map<String, BuildOptions> validate(
       Map<PackageValue.Key, PackageValue> buildSettingPackages,
       Map<String, BuildOptions> toOptions)
       throws TransitionException {
-    // collect settings changed during this transition and their types
+    // Collect settings changed during this transition and their types. This includes settings that
+    // were only used as inputs as to the transition and thus had their default values added to the
+    // fromOptions, which in case of a no-op transition directly end up in toOptions.
     Map<Label, Rule> changedSettingToRule = Maps.newHashMap();
     root.visit(
         (StarlarkTransitionVisitor)
             transition -> {
-              ImmutableSet<Label> changedSettings =
-                  getRelevantStarlarkSettingsFromTransition(transition, Settings.OUTPUTS);
+              ImmutableSet<Label> changedSettings = getRelevantStarlarkSettingsFromTransition(
+                  transition, Settings.INPUTS_AND_OUTPUTS);
               for (Label setting : changedSettings) {
                 changedSettingToRule.put(
                     setting, getActual(buildSettingPackages, setting).getAssociatedRule());

src/test/java/com/google/devtools/build/lib/analysis/StarlarkAttrTransitionProviderTest.java

@@ -2249,4 +2249,86 @@ public void testNoPlatformChange() throws Exception {
     assertThat(getConfiguration(dep).getOptions().get(PlatformOptions.class).platforms)
         .containsExactly(Label.parseAbsoluteUnchecked("//platforms:my_platform"));
   }
+
+  @Test
+  public void testTransitionDoesNotSetDefaultBuildSettingsExplicitly() throws Exception {
+    writeAllowlistFile();
+    scratch.file(
+        "test/starlark/rules.bzl",
+        "def _string_impl(ctx):",
+        "    return []",
+        "string_flag = rule(",
+        "    implementation = _string_impl,",
+        "    build_setting = config.string(flag = True),",
+        ")",
+        "def _no_op_transition_impl(settings, attr):",
+        "    return {",
+        "        '//test/starlark:input_and_output': settings['//test/starlark:input_and_output'],",
+        "        '//test/starlark:output_only': 'output_only_default',",
+        "    }",
+        "_no_op_transition = transition(",
+        "    implementation = _no_op_transition_impl,",
+        "    inputs = [",
+        "        '//test/starlark:input_only',",
+        "        '//test/starlark:input_and_output',",
+        "    ],",
+        "    outputs = [",
+        "        '//test/starlark:input_and_output',",
+        "        '//test/starlark:output_only',",
+        "    ],",
+        ")",
+        "def _apply_transition_impl(ctx):",
+        "    ctx.actions.symlink(",
+        "        output = ctx.outputs.out,",
+        "        target_file = ctx.file.target,",
+        "    )",
+        "    return [DefaultInfo(executable = ctx.outputs.out)]",
+        "apply_transition = rule(",
+        "    implementation = _apply_transition_impl,",
+        "    attrs = {",
+        "        'target': attr.label(",
+        "            cfg = _no_op_transition,",
+        "            allow_single_file = True,",
+        "        ),",
+        "        'out': attr.output(),",
+        "        '_allowlist_function_transition': attr.label(",
+        "            default = '//tools/allowlists/function_transition_allowlist',",
+        "        ),",
+        "    },",
+        "    executable = False,",
+        ")");
+    scratch.file(
+        "test/starlark/BUILD",
+        "load('//test/starlark:rules.bzl', 'apply_transition', 'string_flag')",
+        "",
+        "string_flag(",
+        "    name = 'input_only',",
+        "    build_setting_default = 'input_only_default',",
+        ")",
+        "string_flag(",
+        "    name = 'input_and_output',",
+        "    build_setting_default = 'input_and_output_default',",
+        ")",
+        "string_flag(",
+        "    name = 'output_only',",
+        "    build_setting_default = 'output_only_default',",
+        ")",
+        "cc_binary(",
+        "    name = 'main',",
+        "    srcs = ['main.cc'],",
+        ")",
+        "apply_transition(",
+        "    name = 'transitioned_main',",
+        "    target = ':main',",
+        "    out = 'main_out',",
+        ")");
+
+    update(
+        ImmutableList.of("//test/starlark:main", "//test/starlark:transitioned_main"),
+        /*keepGoing=*/ false,
+        LOADING_PHASE_THREADS,
+        /*doAnalysis=*/ true,
+        new EventBus());
+    assertNoEvents();
+  }
 }