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

// Copyright 2017 The Bazel Authors. All rights reserved.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//    http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
package com.google.devtools.build.lib.packages;

import com.google.common.collect.ImmutableMap;
import com.google.devtools.build.lib.concurrent.ThreadSafety.Immutable;
import com.google.devtools.build.lib.events.Location;
import com.google.devtools.build.lib.syntax.EvalException;
import com.google.devtools.build.lib.syntax.FunctionSignature;
import com.google.devtools.build.lib.syntax.SkylarkType;
import com.google.devtools.build.lib.util.Pair;
import java.util.Map;
import javax.annotation.Nullable;

/**
 * Base class for declared providers {@see Provider} defined in native code.
 *
 * <p>Every non-abstract derived class of {@link NativeProvider} corresponds to a single declared
 * provider. This is enforced by final {@link #equals(Object)} and {@link #hashCode()}.
 *
 * <p>Typical implementation of a non-constructable from Skylark declared provider is as follows:
 *
 * <pre>
 *     public static final Provider PROVIDER =
 *       new NativeProvider("link_params") { };
 * </pre>
 *
 * To allow construction from Skylark and custom construction logic, override {@link
 * #createInstanceFromSkylark(Object[], Location)} (see {@link #STRUCT} for an example.
 */
@Immutable
public abstract class NativeProvider<V extends Info> extends Provider {
  private final NativeKey key;
  private final String errorMessageFormatForUnknownField;

  /** "struct" function. */
  public static final StructProvider STRUCT = new StructProvider();

  private final Class<V> valueClass;

  public Class<V> getValueClass() {
    return valueClass;
  }

  /**
   * Implement this to mark that a native provider should be exported with certain name to Skylark.
   * Broken: only works for rules, not for aspects. DO NOT USE FOR NEW CODE!
   *
   * <p>Use native declared providers mechanism exclusively to expose providers to both native and
   * Skylark code.
   */
  @Deprecated
  public interface WithLegacySkylarkName {
    String getSkylarkName();
  }

  /**
   * The provider for the built-in type {@code struct}.
   *
   * <p>Its singleton instance is {@link #STRUCT}.
   */
  public static final class StructProvider extends NativeProvider<Info> {
    private StructProvider() {
      super(Info.class, "struct");
    }

    @Override
    protected Info createInstanceFromSkylark(Object[] args, Location loc) {
      @SuppressWarnings("unchecked")
      Map<String, Object> kwargs = (Map<String, Object>) args[0];
      return SkylarkInfo.createSchemaless(this, kwargs, loc);
    }

    /**
     * Creates a struct with the he given field values and message format for unknown fields.
     *
     * <p>The custom message is useful for objects that have fields but aren't exactly used as
     * providers, such as the {@code native} object, and the struct fields of {@code ctx} like
     * {@code ctx.attr}.
     * */
    public SkylarkInfo create(
        Map<String, Object> values, String errorMessageFormatForUnknownField) {
      return SkylarkInfo.createSchemalessWithCustomMessage(
          this, values, errorMessageFormatForUnknownField);
    }

    /** Creates an empty struct with the given location. */
    public SkylarkInfo createEmpty(Location loc) {
      return SkylarkInfo.createSchemaless(this, ImmutableMap.of(), loc);
    }
  }

  private static final FunctionSignature.WithValues<Object, SkylarkType> SIGNATURE =
      FunctionSignature.WithValues.create(FunctionSignature.KWARGS);

  protected NativeProvider(Class<V> clazz, String name) {
    this(clazz, name, SIGNATURE);
  }

  protected NativeProvider(
      Class<V> valueClass,
      String name,
      FunctionSignature.WithValues<Object, SkylarkType> signature) {
    super(name, signature, Location.BUILTIN);
    key = new NativeKey(name, getClass());
    this.valueClass = valueClass;
    errorMessageFormatForUnknownField = String.format("'%s' object has no attribute '%%s'", name);
  }

  /**
   * equals() implements singleton class semantics.
   *
   * <p>Every non-abstract derived class of {@link NativeProvider} corresponds to a single declared
   * provider.
   */
  @Override
  public final boolean equals(@Nullable Object other) {
    return other != null && this.getClass().equals(other.getClass());
  }

  /**
   * hashCode() implements singleton class semantics.
   *
   * <p>Every non-abstract derived class of {@link NativeProvider} corresponds to a single declared
   * provider.
   */
  @Override
  public final int hashCode() {
    return getClass().hashCode();
  }

  @Override
  public String getPrintableName() {
    return getName();
  }

  @Override
  public String getErrorMessageFormatForUnknownField() {
    return errorMessageFormatForUnknownField;
  }

  @Override
  public boolean isExported() {
    return true;
  }

  @Override
  public NativeKey getKey() {
    return key;
  }

  @Override
  protected Info createInstanceFromSkylark(Object[] args, Location loc) throws EvalException {
    throw new EvalException(
        loc, String.format("'%s' cannot be constructed from Skylark", getPrintableName()));
  }

  public static Pair<String, String> getSerializedRepresentationForNativeKey(NativeKey key) {
    return Pair.of(key.name, key.aClass.getName());
  }

  public static NativeKey getNativeKeyFromSerializedRepresentation(Pair<String, String> serialized)
      throws ClassNotFoundException {
    Class<? extends NativeProvider> aClass =
        Class.forName(serialized.second).asSubclass(NativeProvider.class);
    return new NativeKey(serialized.first, aClass);
  }

  /**
   * A serializable representation of {@link NativeProvider}.
   *
   * <p>Just a wrapper around its class.
   */
  @Immutable
  public static final class NativeKey extends Key {
    private final String name;
    private final Class<? extends NativeProvider> aClass;

    private NativeKey(String name, Class<? extends NativeProvider> aClass) {
      this.name = name;
      this.aClass = aClass;
    }

    @Override
    public int hashCode() {
      return aClass.hashCode();
    }

    @Override
    public boolean equals(Object obj) {
      return obj instanceof NativeKey && aClass.equals(((NativeKey) obj).aClass);
    }

    @Override
    public String toString() {
      return name;
    }
  }
}