Gate external-repo source dir package boundary checks behind a flag

src/main/java/com/google/devtools/build/lib/packages/semantics/BuildLanguageOptions.java

@@ -707,6 +707,19 @@ public abstract class BuildLanguageOptions extends OptionsBase {
               + " attributes of symbolic macros or attribute default values.")
   public abstract boolean getIncompatibleSimplifyUnconditionalSelectsInRuleAttrs();
 
+  @Option(
+      name = "incompatible_check_external_repo_source_dir_package_boundary",
+      defaultValue = "false",
+      documentationCategory = OptionDocumentationCategory.STARLARK_SEMANTICS,
+      effectTags = {OptionEffectTag.LOADING_AND_ANALYSIS},
+      metadataTags = {OptionMetadataTag.INCOMPATIBLE_CHANGE},
+      help =
+          "If true, a source directory in an external repository fails the build if it crosses a"
+              + " package boundary into a sub-package, matching the behavior that already applies"
+              + " to source directories in the main repository. If false, such package boundary"
+              + " crossings inside external repositories are not detected.")
+  public abstract boolean getIncompatibleCheckExternalRepoSourceDirPackageBoundary();
+
   @Option(
       name = "experimental_enable_starlark_set",
       defaultValue = "true",
@@ -859,6 +872,9 @@ EXPERIMENTAL_SIBLING_REPOSITORY_LAYOUT, getExperimentalSiblingRepositoryLayout()
             .setBool(
                 INCOMPATIBLE_ALWAYS_CHECK_DEPSET_ELEMENTS,
                 getIncompatibleAlwaysCheckDepsetElements())
+            .setBool(
+                INCOMPATIBLE_CHECK_EXTERNAL_REPO_SOURCE_DIR_PACKAGE_BOUNDARY,
+                getIncompatibleCheckExternalRepoSourceDirPackageBoundary())
             .setBool(INCOMPATIBLE_DISALLOW_EMPTY_GLOB, getIncompatibleDisallowEmptyGlob())
             .setBool(
                 INCOMPATIBLE_PACKAGE_GROUP_HAS_PUBLIC_SYNTAX,
@@ -1050,6 +1066,8 @@ public FlagConsumer setBool(String key, boolean ignored) {
       "-experimental_sibling_repository_layout";
   public static final String INCOMPATIBLE_ALWAYS_CHECK_DEPSET_ELEMENTS =
       "+incompatible_always_check_depset_elements";
+  public static final String INCOMPATIBLE_CHECK_EXTERNAL_REPO_SOURCE_DIR_PACKAGE_BOUNDARY =
+      "-incompatible_check_external_repo_source_dir_package_boundary";
 
   // Note that INCOMPATIBLE_DISALLOW_EMPTY_GLOB differs in Google and in OSS Bazel.
   public static final String INCOMPATIBLE_DISALLOW_EMPTY_GLOB = "+incompatible_disallow_empty_glob";

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

@@ -2292,6 +2292,7 @@ java_library(
         ":dirents",
         ":filesystem_keys",
         ":package_lookup_value",
+        ":precomputed_value",
         ":sky_functions",
         ":tree_artifact_value",
         "//src/main/java/com/google/devtools/build/lib/actions",
@@ -2304,6 +2305,7 @@ java_library(
         "//src/main/java/com/google/devtools/build/lib/io:file_symlink_infinite_expansion_uniqueness_function",
         "//src/main/java/com/google/devtools/build/lib/io:inconsistent_filesystem_exception",
         "//src/main/java/com/google/devtools/build/lib/packages",
+        "//src/main/java/com/google/devtools/build/lib/packages/semantics",
         "//src/main/java/com/google/devtools/build/lib/profiler",
         "//src/main/java/com/google/devtools/build/lib/skyframe/serialization:visible-for-serialization",
         "//src/main/java/com/google/devtools/build/lib/skyframe/serialization/autocodec",
@@ -2313,6 +2315,7 @@ java_library(
         "//src/main/java/com/google/devtools/build/lib/vfs:pathfragment",
         "//src/main/java/com/google/devtools/build/skyframe",
         "//src/main/java/com/google/devtools/build/skyframe:skyframe-objects",
+        "//src/main/java/net/starlark/java/eval",
         "//src/main/protobuf:failure_details_java_proto",
         "//third_party:auto_value",
         "//third_party:error_prone_annotations",

src/main/java/com/google/devtools/build/lib/skyframe/RecursiveFilesystemTraversalFunction.java

@@ -39,6 +39,7 @@
 import com.google.devtools.build.lib.io.FileSymlinkInfiniteExpansionUniquenessFunction;
 import com.google.devtools.build.lib.io.InconsistentFilesystemException;
 import com.google.devtools.build.lib.packages.BuildFileNotFoundException;
+import com.google.devtools.build.lib.packages.semantics.BuildLanguageOptions;
 import com.google.devtools.build.lib.profiler.Profiler;
 import com.google.devtools.build.lib.profiler.ProfilerTask;
 import com.google.devtools.build.lib.profiler.SilentCloseable;
@@ -72,6 +73,7 @@
 import java.util.List;
 import java.util.Map;
 import javax.annotation.Nullable;
+import net.starlark.java.eval.StarlarkSemantics;
 
 /** A {@link SkyFunction} to build {@link RecursiveFilesystemTraversalValue}s. */
 public final class RecursiveFilesystemTraversalFunction implements SkyFunction {
@@ -557,6 +559,14 @@ private PkgLookupResult checkIfPackage(
     // those are symlinks to other locations (e.g. in the case of a local_repository), so all other
     // roots are part of the main repository.
     if (rootPath.startsWith(externalDirectory)) {
+      StarlarkSemantics semantics = PrecomputedValue.STARLARK_SEMANTICS.get(env);
+      if (semantics == null) {
+        return null;
+      }
+      if (!semantics.getBool(
+          BuildLanguageOptions.INCOMPATIBLE_CHECK_EXTERNAL_REPO_SOURCE_DIR_PACKAGE_BOUNDARY)) {
+        return PkgLookupResult.directory(traversal, rootInfo);
+      }
       checkState(
           rootPath.relativeTo(externalDirectory).isSingleSegment(),
           "%s is expected to be directly under %s",

src/test/java/com/google/devtools/build/lib/packages/semantics/ConsistencyTest.java

@@ -140,6 +140,7 @@ private static BuildLanguageOptions buildRandomOptions(Random rand) throws Excep
         "--experimental_dormant_deps=" + rand.nextBoolean(),
         "--force_starlark_stack_trace=" + rand.nextBoolean(),
         "--incompatible_always_check_depset_elements=" + rand.nextBoolean(),
+        "--incompatible_check_external_repo_source_dir_package_boundary=" + rand.nextBoolean(),
         "--incompatible_disallow_empty_glob=" + rand.nextBoolean(),
         "--incompatible_do_not_split_linking_cmdline=" + rand.nextBoolean(),
         "--incompatible_enable_deprecated_label_apis=" + rand.nextBoolean(),
@@ -185,6 +186,9 @@ private static StarlarkSemantics buildRandomSemantics(Random rand) {
         .setBool(BuildLanguageOptions.EXPERIMENTAL_DORMANT_DEPS, rand.nextBoolean())
         .setBool(StarlarkSemantics.FORCE_STARLARK_STACK_TRACE, rand.nextBoolean())
         .setBool(BuildLanguageOptions.INCOMPATIBLE_ALWAYS_CHECK_DEPSET_ELEMENTS, rand.nextBoolean())
+        .setBool(
+            BuildLanguageOptions.INCOMPATIBLE_CHECK_EXTERNAL_REPO_SOURCE_DIR_PACKAGE_BOUNDARY,
+            rand.nextBoolean())
         .setBool(BuildLanguageOptions.INCOMPATIBLE_DISALLOW_EMPTY_GLOB, rand.nextBoolean())
         .setBool(BuildLanguageOptions.INCOMPATIBLE_DO_NOT_SPLIT_LINKING_CMDLINE, rand.nextBoolean())
         .setBool(BuildLanguageOptions.INCOMPATIBLE_ENABLE_DEPRECATED_LABEL_APIS, rand.nextBoolean())

src/test/java/com/google/devtools/build/lib/skyframe/SourceDirectoryIntegrationTest.java

@@ -299,6 +299,74 @@ public void crossingRepoPackageCollision_doesNotFail() throws Exception {
     buildTarget("//:consume");
   }
 
+  /**
+   * With --incompatible_check_external_repo_source_dir_package_boundary, a source directory in an
+   * external repository that crosses a package boundary into a sub-package of that same repository
+   * fails the build, just like in the main repository.
+   */
+  @Test
+  public void crossingExternalRepoPackageBoundary_withFlag_fails() throws Exception {
+    if (!AnalysisMock.get().isThisBazel()) {
+      return;
+    }
+    setUpCrossingExternalRepoPackageBoundary();
+    addOptions("--incompatible_check_external_repo_source_dir_package_boundary");
+
+    assertThrows(BuildFailedException.class, () -> buildTarget("//:consume"));
+    assertContainsEvent("crosses package boundary into package rooted at data/sub");
+  }
+
+  /**
+   * Without --incompatible_check_external_repo_source_dir_package_boundary (the default), the same
+   * package boundary crossing inside an external repository is not detected and the build succeeds.
+   */
+  @Test
+  public void crossingExternalRepoPackageBoundary_withoutFlag_doesNotFail() throws Exception {
+    if (!AnalysisMock.get().isThisBazel()) {
+      return;
+    }
+    setUpCrossingExternalRepoPackageBoundary();
+
+    buildTarget("//:consume");
+  }
+
+  private void setUpCrossingExternalRepoPackageBoundary() throws Exception {
+    write(
+        "MODULE.bazel",
+        """
+        bazel_dep(name = "ext")
+        local_path_override(
+            module_name = "ext",
+            path = "ext",
+        )
+        """);
+    write("ext/MODULE.bazel", "module(name = 'ext')");
+    write(
+        "ext/BUILD",
+        """
+        filegroup(
+            name = "dir",
+            srcs = ["data"],
+            visibility = ["//visibility:public"],
+        )
+        """);
+    // The presence of this BUILD file makes ext's `data/sub` a package, so the `data` source directory
+    // crosses a package boundary.
+    write("ext/data/sub/BUILD");
+    writeIsoLatin1(getWorkspace().getRelative("ext/data/top"), "content");
+
+    write(
+        "BUILD",
+        """
+        genrule(
+            name = "consume",
+            srcs = ["@ext//:dir"],
+            outs = ["consume.out"],
+            cmd = "touch $@",
+        )
+        """);
+  }
+
   private static final String GENRULE_EVENT = "Executing genrule //foo:foo";
 
   private void assertInvalidatedByBuild() throws Exception {