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

// Copyright 2014 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.skyframe;

import com.google.common.base.Function;
import com.google.common.collect.Collections2;
import com.google.common.collect.Iterables;
import com.google.devtools.build.lib.actions.Artifact;
import com.google.devtools.build.lib.concurrent.ThreadSafety.Immutable;
import com.google.devtools.build.lib.concurrent.ThreadSafety.ThreadSafe;
import com.google.devtools.build.lib.util.Preconditions;
import com.google.devtools.build.skyframe.SkyKey;
import com.google.devtools.build.skyframe.SkyValue;

import java.util.Collection;

/**
 * A value representing an artifact. Source artifacts are checked for existence, while output
 * artifacts imply creation of the output file.
 *
 * <p>There are effectively three kinds of output artifact values. The first corresponds to an
 * ordinary artifact {@link FileArtifactValue}. It stores the relevant data for the artifact --
 * digest/mtime and size. The second corresponds to either an "aggregating" artifact -- the output
 * of an aggregating middleman action -- or a TreeArtifact. It stores the relevant data of all its
 * inputs, as well as a combined digest for itself.
 */
@Immutable
@ThreadSafe
public abstract class ArtifactValue implements SkyValue {

  @ThreadSafe
  public static SkyKey key(Artifact artifact, boolean isMandatory) {
    return SkyKey.create(
        SkyFunctions.ARTIFACT,
        artifact.isSourceArtifact()
            ? new OwnedArtifact(artifact, isMandatory)
            : new OwnedArtifact(artifact));
  }

  private static final Function<Artifact, SkyKey> TO_MANDATORY_KEY =
      new Function<Artifact, SkyKey>() {
        @Override
        public SkyKey apply(Artifact artifact) {
          return key(artifact, true);
        }
      };

  @ThreadSafe
  public static Iterable<SkyKey> mandatoryKeys(Iterable<Artifact> artifacts) {
    return Iterables.transform(artifacts, TO_MANDATORY_KEY);
  }

  private static final Function<OwnedArtifact, Artifact> TO_ARTIFACT =
      new Function<OwnedArtifact, Artifact>() {
    @Override
    public Artifact apply(OwnedArtifact key) {
      return key.getArtifact();
    }
  };

  public static Collection<Artifact> artifacts(Collection<? extends OwnedArtifact> keys) {
    return Collections2.transform(keys, TO_ARTIFACT);
  }

  public static Artifact artifact(SkyKey key) {
    return TO_ARTIFACT.apply((OwnedArtifact) key.argument());
  }

  public static boolean equalWithOwner(Artifact first, Artifact second) {
    return first.equals(second)
        && first.getArtifactOwner().equals(second.getArtifactOwner());
  }

  /**
   * Artifacts are compared using just their paths, but in Skyframe, the configured target that owns
   * an artifact must also be part of the comparison. For example, suppose we build //foo:foo in
   * configurationA, yielding artifact foo.out. If we change the configuration to configurationB in
   * such a way that the path to the artifact does not change, requesting foo.out from the graph
   * will result in the value entry for foo.out under configurationA being returned. This would
   * prevent caching the graph in different configurations, and also causes big problems with change
   * pruning, which assumes the invariant that a value's first dependency will always be the same.
   * In this case, the value entry's old dependency on //foo:foo in configurationA would cause it to
   * request (//foo:foo, configurationA) from the graph, causing an undesired re-analysis of
   * (//foo:foo, configurationA).
   *
   * <p>In order to prevent that, instead of using Artifacts as keys in the graph, we use
   * OwnedArtifacts, which compare for equality using both the Artifact, and the owner. The effect
   * is functionally that of making Artifact.equals() check the owner, but only within Skyframe,
   * since outside of Skyframe it is quite crucial that Artifacts with different owners be able to
   * compare equal.
   */
  static class OwnedArtifact {
    private final Artifact artifact;
    // Always true for derived artifacts.
    private final boolean isMandatory;

    /** Constructs an OwnedArtifact wrapper for a source artifact. */
    private OwnedArtifact(Artifact sourceArtifact, boolean mandatory) {
      Preconditions.checkArgument(sourceArtifact.isSourceArtifact());
      this.artifact = Preconditions.checkNotNull(sourceArtifact);
      this.isMandatory = mandatory;
    }

    /**
     * Constructs an OwnedArtifact wrapper for a derived artifact. The mandatory attribute is
     * not needed because a derived artifact must be a mandatory input for some action in order to
     * ensure that it is built in the first place. If it fails to build, then that fact is cached
     * in the node, so any action that has it as a non-mandatory input can retrieve that
     * information from the node.
     */
    private OwnedArtifact(Artifact derivedArtifact) {
      this.artifact = Preconditions.checkNotNull(derivedArtifact);
      Preconditions.checkArgument(!derivedArtifact.isSourceArtifact(), derivedArtifact);
      this.isMandatory = true; // Unused.
    }

    @Override
    public int hashCode() {
      int initialHash = artifact.hashCode() +  artifact.getArtifactOwner().hashCode();
      return isMandatory ? initialHash : 47 * initialHash + 1;
    }

    @Override
    public boolean equals(Object that) {
      if (this == that) {
        return true;
      }
      if (!(that instanceof OwnedArtifact)) {
        return false;
      }
      OwnedArtifact thatOwnedArtifact = ((OwnedArtifact) that);
      Artifact thatArtifact = thatOwnedArtifact.artifact;
      return equalWithOwner(artifact, thatArtifact) && isMandatory == thatOwnedArtifact.isMandatory;
    }

    Artifact getArtifact() {
      return artifact;
    }

    /**
     * Returns whether the artifact is a mandatory input of its requesting action. May only be
     * called for source artifacts, since a derived artifact must be a mandatory input of some
     * action in order to have been built in the first place.
     */
    public boolean isMandatory() {
      Preconditions.checkState(artifact.isSourceArtifact(), artifact);
      return isMandatory;
    }

    @Override
    public String toString() {
      return artifact.prettyPrint() + " " + artifact.getArtifactOwner();
    }
  }
}