Expose late-bound-attributes to Skylark.

RELNOTES: Late-bound attributes are exposed to skylark. This is a new API (`configuration_field()`) to depend on certain configuration-defined targets from skylark rules.
PiperOrigin-RevId: 174534104

4 files changed, 33 insertions, 2 deletions

diff --git a/src/main/java/com/google/devtools/build/lib/packages/AbstractAttributeMapper.java b/src/main/java/com/google/devtools/build/lib/packages/AbstractAttributeMapper.java
index 8b99d8182b..ec03962931 100644
--- a/src/main/java/com/google/devtools/build/lib/packages/AbstractAttributeMapper.java
+++ b/src/main/java/com/google/devtools/build/lib/packages/AbstractAttributeMapper.java
@@ -58,6 +58,8 @@ public abstract class AbstractAttributeMapper implements AttributeMap {
     Object value = attributes.getAttributeValue(index);
     if (value instanceof Attribute.ComputedDefault) {
       value = ((Attribute.ComputedDefault) value).getDefault(this);
+    } else if (value instanceof Attribute.LateBoundDefault) {
+      value = ((Attribute.LateBoundDefault) value).getDefault();
     }
     try {
       return type.cast(value);
@@ -87,6 +89,25 @@ public abstract class AbstractAttributeMapper implements AttributeMap {
     }
   }
 
+  /**
+   * Returns the given attribute if it's a {@link Attribute.LateBoundDefault}, null otherwise.
+   *
+   * @throws IllegalArgumentException if the given attribute doesn't exist with the specified
+   *         type. This happens whether or not it's a late bound default.
+   */
+  @Nullable
+  @SuppressWarnings("unchecked")
+  public <T> Attribute.LateBoundDefault<?, T> getLateBoundDefault(
+      String attributeName, Type<T> type) {
+    int index = getIndexWithTypeCheck(attributeName, type);
+    Object value = attributes.getAttributeValue(index);
+    if (value instanceof Attribute.LateBoundDefault) {
+      return (Attribute.LateBoundDefault<?, T>) value;
+    } else {
+      return null;
+    }
+  }
+
   @Override
   public Iterable<String> getAttributeNames() {
     ImmutableList.Builder<String> names = ImmutableList.builder();
diff --git a/src/main/java/com/google/devtools/build/lib/packages/Attribute.java b/src/main/java/com/google/devtools/build/lib/packages/Attribute.java
index 54f6255226..9b158e6a58 100644
--- a/src/main/java/com/google/devtools/build/lib/packages/Attribute.java
+++ b/src/main/java/com/google/devtools/build/lib/packages/Attribute.java
@@ -680,7 +680,6 @@ public final class Attribute implements Comparable<Attribute> {
      */
     public Builder<TYPE> value(LateBoundDefault<?, ? extends TYPE> defaultValue) {
       Preconditions.checkState(!valueSet, "the default value is already set");
-      Preconditions.checkState(name.isEmpty() || isLateBound(name));
       value = defaultValue;
       valueSource = AttributeValueSource.LATE_BOUND;
       valueSet = true;
@@ -1082,6 +1081,9 @@ public final class Attribute implements Comparable<Attribute> {
      */
     public Attribute build(String name) {
       Preconditions.checkState(!name.isEmpty(), "name has not been set");
+      if (valueSource == AttributeValueSource.LATE_BOUND) {
+        Preconditions.checkState(isLateBound(name));
+      }
       // TODO(bazel-team): Set the default to be no file type, then remove this check, and also
       // remove all allowedFileTypes() calls without parameters.
 
@@ -1715,7 +1717,7 @@ public final class Attribute implements Comparable<Attribute> {
     private static final LateBoundDefault<Void, Void> ALWAYS_NULL =
         new SimpleLateBoundDefault<>(false, Void.class, null, (rule, attributes, unused) -> null);
 
-    private LateBoundDefault(
+    protected LateBoundDefault(
         boolean useHostConfiguration,
         Class<FragmentT> fragmentClass,
         ValueT defaultValue) {
diff --git a/src/main/java/com/google/devtools/build/lib/packages/RuleClass.java b/src/main/java/com/google/devtools/build/lib/packages/RuleClass.java
index 7dd8acba2d..bf99b43ade 100644
--- a/src/main/java/com/google/devtools/build/lib/packages/RuleClass.java
+++ b/src/main/java/com/google/devtools/build/lib/packages/RuleClass.java
@@ -1601,6 +1601,8 @@ public class RuleClass {
         // that depends on non-computed default attribute values, and that condition predicate is
         // evaluated by the call to Attribute#getDefaultValue.
         attrsWithComputedDefaults.add(attr);
+      } else if (attr.isLateBound()) {
+        rule.setAttributeValue(attr, attr.getLateBoundDefault(), /*explicit=*/ false);
       } else {
         Object defaultValue = getAttributeNoncomputedDefaultValue(attr, pkgBuilder);
         rule.setAttributeValue(attr, defaultValue, /*explicit=*/ false);
diff --git a/src/main/java/com/google/devtools/build/lib/packages/RuleClassProvider.java b/src/main/java/com/google/devtools/build/lib/packages/RuleClassProvider.java
index 5235523d53..302b05bde2 100644
--- a/src/main/java/com/google/devtools/build/lib/packages/RuleClassProvider.java
+++ b/src/main/java/com/google/devtools/build/lib/packages/RuleClassProvider.java
@@ -93,4 +93,10 @@ public interface RuleClassProvider {
    * Retrieves an aspect from the aspect factory map using the key provided
    */
   NativeAspectClass getNativeAspectClass(String key);
+
+  /**
+   * Retrieves a {@link Map} from skylark configuration fragment name to configuration fragment
+   * class.
+   */
+  Map<String, Class<?>> getConfigurationFragmentMap();
 }