Expose CoverageOutputGenerator on a Fragment

This allows Starlark rules to define _lcov_merger as a
late-bound Starlark configuration_field in order not to pay the cost of
building it when coverage isn't enabled.

src/main/java/com/google/devtools/build/lib/analysis/BUILD

@@ -253,6 +253,7 @@ java_library(
         "test/AnalysisTestActionBuilder.java",
         "test/BaselineCoverageAction.java",
         "test/CoverageCommon.java",
+        "test/CoverageConfiguration.java",
         "test/InstrumentedFileManifestAction.java",
         "test/InstrumentedFilesCollector.java",
         "test/TestActionBuilder.java",
@@ -287,6 +288,7 @@ java_library(
         ":config/build_options",
         ":config/config_conditions",
         ":config/config_matching_provider",
+        ":config/core_option_converters",
         ":config/core_options",
         ":config/execution_transition_factory",
         ":config/fragment",
@@ -380,6 +382,7 @@ java_library(
         "//src/main/java/com/google/devtools/build/lib/actions:package_roots",
         "//src/main/java/com/google/devtools/build/lib/analysis/platform",
         "//src/main/java/com/google/devtools/build/lib/analysis/platform:utils",
+        "//src/main/java/com/google/devtools/build/lib/analysis/starlark/annotations",
         "//src/main/java/com/google/devtools/build/lib/analysis/stringtemplate",
         "//src/main/java/com/google/devtools/build/lib/bugreport",
         "//src/main/java/com/google/devtools/build/lib/buildeventstream",

src/main/java/com/google/devtools/build/lib/analysis/BaseRuleClasses.java

@@ -34,6 +34,7 @@
 import com.google.devtools.build.lib.analysis.config.RunUnder;
 import com.google.devtools.build.lib.analysis.constraints.ConstraintConstants;
 import com.google.devtools.build.lib.analysis.platform.ConstraintValueInfo;
+import com.google.devtools.build.lib.analysis.test.CoverageConfiguration;
 import com.google.devtools.build.lib.analysis.test.TestConfiguration;
 import com.google.devtools.build.lib.cmdline.Label;
 import com.google.devtools.build.lib.packages.Attribute;
@@ -139,9 +140,6 @@ public static LabelLateBoundDefault<TestConfiguration> coverageSupportAttribute(
   public static final String DEFAULT_COVERAGE_REPORT_GENERATOR_VALUE =
       "//tools/test:coverage_report_generator";
 
-  private static final String DEFAULT_COVERAGE_OUTPUT_GENERATOR_VALUE =
-      "@bazel_tools//tools/test:lcov_merger";
-
   @SerializationConstant @AutoCodec.VisibleForSerialization
   static final Resolver<TestConfiguration, Label> COVERAGE_REPORT_GENERATOR_CONFIGURATION_RESOLVER =
       (rule, attributes, configuration) -> configuration.getCoverageReportGenerator();
@@ -152,20 +150,14 @@ public static LabelLateBoundDefault<TestConfiguration> coverageReportGeneratorAt
         TestConfiguration.class, defaultValue, COVERAGE_REPORT_GENERATOR_CONFIGURATION_RESOLVER);
   }
 
-  public static LabelLateBoundDefault<BuildConfigurationValue> getCoverageOutputGeneratorLabel() {
+  public static LabelLateBoundDefault<CoverageConfiguration> getCoverageOutputGeneratorLabel() {
     return LabelLateBoundDefault.fromTargetConfiguration(
-        BuildConfigurationValue.class, null, COVERAGE_OUTPUT_GENERATOR_RESOLVER);
+        CoverageConfiguration.class, null, COVERAGE_OUTPUT_GENERATOR_RESOLVER);
   }
 
   @SerializationConstant @AutoCodec.VisibleForSerialization
-  static final Resolver<BuildConfigurationValue, Label> COVERAGE_OUTPUT_GENERATOR_RESOLVER =
-      (rule, attributes, configuration) -> {
-        if (configuration.isCodeCoverageEnabled()) {
-          return Label.parseAbsoluteUnchecked(DEFAULT_COVERAGE_OUTPUT_GENERATOR_VALUE);
-        } else {
-          return null;
-        }
-      };
+  static final Resolver<CoverageConfiguration, Label> COVERAGE_OUTPUT_GENERATOR_RESOLVER =
+      (rule, attributes, configuration) -> configuration.outputGenerator();
 
   // TODO(b/65746853): provide a way to do this without passing the entire configuration
   /** Implementation for the :run_under attribute. */

src/main/java/com/google/devtools/build/lib/analysis/test/CoverageConfiguration.java

@@ -0,0 +1,66 @@
+package com.google.devtools.build.lib.analysis.test;
+
+import com.google.devtools.build.lib.analysis.config.BuildOptions;
+import com.google.devtools.build.lib.analysis.config.CoreOptionConverters.LabelConverter;
+import com.google.devtools.build.lib.analysis.config.CoreOptions;
+import com.google.devtools.build.lib.analysis.config.Fragment;
+import com.google.devtools.build.lib.analysis.config.FragmentOptions;
+import com.google.devtools.build.lib.analysis.config.RequiresOptions;
+import com.google.devtools.build.lib.analysis.starlark.annotations.StarlarkConfigurationField;
+import com.google.devtools.build.lib.cmdline.Label;
+import com.google.devtools.build.lib.concurrent.ThreadSafety.Immutable;
+import com.google.devtools.build.lib.starlarkbuildapi.test.CoverageConfigurationApi;
+import com.google.devtools.common.options.Option;
+import com.google.devtools.common.options.OptionDocumentationCategory;
+import com.google.devtools.common.options.OptionEffectTag;
+import javax.annotation.Nullable;
+
+@Immutable
+@RequiresOptions(options = {CoreOptions.class, CoverageConfiguration.CoverageOptions.class})
+public class CoverageConfiguration extends Fragment implements CoverageConfigurationApi {
+
+  /**
+   * Command-line options.
+   */
+  public static class CoverageOptions extends FragmentOptions {
+
+    @Option(
+        name = "coverage_output_generator",
+        converter = LabelConverter.class,
+        defaultValue = "@bazel_tools//tools/test:lcov_merger",
+        documentationCategory = OptionDocumentationCategory.TOOLCHAIN,
+        effectTags = {
+            OptionEffectTag.CHANGES_INPUTS,
+            OptionEffectTag.AFFECTS_OUTPUTS,
+            OptionEffectTag.LOADING_AND_ANALYSIS
+        },
+        help =
+            "Location of the binary that is used to postprocess raw coverage reports. This must "
+                + "currently be a filegroup that contains a single file, the binary. Defaults to "
+                + "'//tools/test:lcov_merger'."
+    )
+    public Label coverageOutputGenerator;
+  }
+
+  private final CoverageOptions coverageOptions;
+
+  public CoverageConfiguration(BuildOptions buildOptions) {
+    if (!buildOptions.get(CoreOptions.class).collectCodeCoverage) {
+      this.coverageOptions = null;
+      return;
+    }
+    this.coverageOptions = buildOptions.get(CoverageOptions.class);
+  }
+
+  @Override
+  @StarlarkConfigurationField(
+      name = "output_generator",
+      doc = "Label for the coverage output generator.")
+  @Nullable
+  public Label outputGenerator() {
+    if (coverageOptions == null) {
+      return null;
+    }
+    return coverageOptions.coverageOutputGenerator;
+  }
+}

src/main/java/com/google/devtools/build/lib/rules/core/CoreRules.java

@@ -17,6 +17,7 @@
 import com.google.devtools.build.lib.analysis.BaseRuleClasses;
 import com.google.devtools.build.lib.analysis.ConfiguredRuleClassProvider;
 import com.google.devtools.build.lib.analysis.ConfiguredRuleClassProvider.RuleSet;
+import com.google.devtools.build.lib.analysis.test.CoverageConfiguration;
 import com.google.devtools.build.lib.analysis.test.TestConfiguration;
 import com.google.devtools.build.lib.analysis.test.TestTrimmingTransitionFactory;
 
@@ -33,6 +34,7 @@ public void init(ConfiguredRuleClassProvider.Builder builder) {
     builder.setShouldInvalidateCacheForOptionDiff(
         TestConfiguration.SHOULD_INVALIDATE_FOR_OPTION_DIFF);
     builder.addConfigurationFragment(TestConfiguration.class);
+    builder.addConfigurationFragment(CoverageConfiguration.class);
     builder.addTrimmingTransitionFactory(new TestTrimmingTransitionFactory());
     builder.addRuleDefinition(new BaseRuleClasses.NativeBuildRule());
     builder.addRuleDefinition(new BaseRuleClasses.NativeActionCreatingRule());

src/main/java/com/google/devtools/build/lib/starlarkbuildapi/test/CoverageConfigurationApi.java

@@ -0,0 +1,34 @@
+package com.google.devtools.build.lib.starlarkbuildapi.test;
+
+import com.google.devtools.build.docgen.annot.DocCategory;
+import com.google.devtools.build.lib.cmdline.Label;
+import javax.annotation.Nullable;
+import net.starlark.java.annot.StarlarkBuiltin;
+import net.starlark.java.annot.StarlarkMethod;
+import net.starlark.java.eval.StarlarkValue;
+
+@StarlarkBuiltin(
+    name = "coverage",
+    category = DocCategory.CONFIGURATION_FRAGMENT,
+    doc = "A configuration fragment representing the coverage configuration.")
+public interface CoverageConfigurationApi extends StarlarkValue {
+
+  @StarlarkMethod(
+      name = "output_generator",
+      allowReturnNones = true,
+      structField = true,
+      doc =
+          "Returns label pointed to by "
+              + "<a href=\"../../user-manual.html#flag--coverage_output_generator\">"
+              + "<code>--coverage_output_generator</code></a> option. Can be accessed with"
+              + " <a href=\"globals.html#configuration_field\"><code>configuration_field"
+              + "</code></a>:<br/>"
+              + "<pre>attr.label(<br/>"
+              + "    default = configuration_field(<br/>"
+              + "        fragment = \"coverage\",<br/>"
+              + "        name = \"output_generator\"<br/>"
+              + "    )<br/>"
+              + ")</pre>")
+  @Nullable
+  Label outputGenerator();
+}

src/test/shell/bazel/bazel_coverage_starlark_test.sh

@@ -107,4 +107,90 @@ EOF
         || fail "Coverage report did not contain evidence of custom lcov_merger."
 }
 
+function test_starlark_rule_with_configuration_field_lcov_merger_coverage_enabled() {
+
+    cat <<EOF > lcov_merger.sh
+for var in "\$@"
+do
+    if [[ "\$var" == "--output_file="* ]]; then
+        path="\${var##--output_file=}"
+        mkdir -p "\$(dirname \$path)"
+        echo lcov_merger_called >> \$path
+        exit 0
+    fi
+done
+EOF
+chmod +x lcov_merger.sh
+
+    cat <<EOF > rules.bzl
+def _impl(ctx):
+    output = ctx.actions.declare_file(ctx.attr.name)
+    ctx.actions.write(output, "", is_executable = True)
+    return [DefaultInfo(executable=output)]
+
+custom_test = rule(
+    implementation = _impl,
+    test = True,
+    attrs = {
+        "_lcov_merger": attr.label(
+            default = configuration_field(fragment = "coverage", name = "output_generator"),
+            cfg = "exec"
+        ),
+    },
+    fragments = ["coverage"],
+)
+EOF
+
+    cat <<EOF > BUILD
+load(":rules.bzl", "custom_test")
+
+sh_binary(
+    name = "lcov_merger",
+    srcs = ["lcov_merger.sh"],
+)
+custom_test(name = "foo_test")
+EOF
+
+    bazel coverage --test_output=all //:foo_test --combined_report=lcov --coverage_output_generator=//:lcov_merger > $TEST_log \
+        || fail "Coverage run failed but should have succeeded."
+
+    local coverage_file_path="$( get_coverage_file_path_from_test_log )"
+    cat $coverage_file_path
+    grep "lcov_merger_called"  "$coverage_file_path" \
+        || fail "Coverage report did not contain evidence of custom lcov_merger."
+}
+
+function test_starlark_rule_with_configuration_field_lcov_merger_coverage_disabled() {
+
+    cat <<EOF > rules.bzl
+def _impl(ctx):
+    if ctx.attr._lcov_merger:
+        fail("Expected _lcov_merger to be None if coverage is not collected")
+    output = ctx.actions.declare_file(ctx.attr.name)
+    ctx.actions.write(output, "", is_executable = True)
+    return [DefaultInfo(executable=output)]
+
+custom_test = rule(
+    implementation = _impl,
+    test = True,
+    attrs = {
+        "_lcov_merger": attr.label(
+            default = configuration_field(fragment = "coverage", name = "output_generator"),
+            cfg = "exec"
+        ),
+    },
+    fragments = ["coverage"],
+)
+EOF
+
+    cat <<EOF > BUILD
+load(":rules.bzl", "custom_test")
+
+custom_test(name = "foo_test")
+EOF
+
+    bazel test --test_output=all //:foo_test > $TEST_log \
+        || fail "Test run failed but should have succeeded."
+}
+
 run_suite "test tests"