1 files changed, 105 insertions, 22 deletions
diff --git a/src/main/java/com/google/devtools/build/lib/actions/ArtifactSkyKey.java b/src/main/java/com/google/devtools/build/lib/actions/ArtifactSkyKey.java
index 701aacaec4..277b26933b 100644
--- a/src/main/java/com/google/devtools/build/lib/actions/ArtifactSkyKey.java
+++ b/src/main/java/com/google/devtools/build/lib/actions/ArtifactSkyKey.java
@@ -13,52 +13,99 @@
 // limitations under the License.
 package com.google.devtools.build.lib.actions;
 
+import com.google.common.base.Function;
 import com.google.common.base.Preconditions;
+import com.google.common.collect.Collections2;
 import com.google.common.collect.Interner;
+import com.google.common.collect.Iterables;
 import com.google.devtools.build.lib.concurrent.BlazeInterners;
 import com.google.devtools.build.lib.concurrent.ThreadSafety;
 import com.google.devtools.build.lib.skyframe.serialization.autocodec.AutoCodec;
 import com.google.devtools.build.skyframe.SkyFunctionName;
 import com.google.devtools.build.skyframe.SkyKey;
+import java.util.Collection;
 
 /**
- * A {@link SkyKey} coming from an {@link Artifact} that is not mandatory: absence of the Artifact
- * does not imply any error.
+ * A {@link SkyKey} coming from an {@link Artifact}. Source artifacts are checked for existence,
+ * while output artifacts imply creation of the output file.
  *
- * <p>Since {@link Artifact} is already a {@link SkyKey}, this wrapper is only needed when the
- * {@link Artifact} is not mandatory (discovered during include scanning).
+ * <p>There are effectively three kinds of output artifact values corresponding to these keys. 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.
+ *
+ * <p>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 just comparing Artifacts, we 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 these keys, since outside of Skyframe it is quite crucial that
+ * Artifacts with different owners be able to compare equal.
  */
-// TODO(janakr): pull mandatory/non-mandatory handling up to consumers and get rid of this wrapper.
 @AutoCodec
 public class ArtifactSkyKey implements SkyKey {
   private static final Interner<ArtifactSkyKey> INTERNER = BlazeInterners.newWeakInterner();
+  private static final Function<Artifact, ArtifactSkyKey> TO_MANDATORY_KEY =
+      artifact -> key(artifact, true);
+  private static final Function<ArtifactSkyKey, Artifact> TO_ARTIFACT = ArtifactSkyKey::getArtifact;
 
-  private final Artifact.SourceArtifact artifact;
+  private final Artifact artifact;
+  // Always true for derived artifacts.
+  private final boolean isMandatory;
+  // TODO(janakr): we may want to remove this field in the future. The expensive hash computation
+  // is already cached one level down (in the Artifact), so the CPU overhead here may not be
+  // worth the memory. However, when running with +CompressedOops, this field is free, so we leave
+  // it. When running with -CompressedOops, we might be able to save memory by using polymorphism
+  // for isMandatory and dropping this field.
+  private int hashCode = 0;
 
-  private ArtifactSkyKey(Artifact.SourceArtifact sourceArtifact) {
+  private ArtifactSkyKey(Artifact sourceArtifact, boolean mandatory) {
+    Preconditions.checkArgument(sourceArtifact.isSourceArtifact());
     this.artifact = Preconditions.checkNotNull(sourceArtifact);
+    this.isMandatory = mandatory;
   }
 
-  @ThreadSafety.ThreadSafe
-  public static SkyKey key(Artifact artifact, boolean isMandatory) {
-    if (isMandatory || !artifact.isSourceArtifact()) {
-      return artifact;
-    }
-    return create(((Artifact.SourceArtifact) artifact));
+  /**
+   * Constructs an ArtifactSkyKey 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 ArtifactSkyKey(Artifact derivedArtifact) {
+    this.artifact = Preconditions.checkNotNull(derivedArtifact);
+    Preconditions.checkArgument(!derivedArtifact.isSourceArtifact(), derivedArtifact);
+    this.isMandatory = true; // Unused.
   }
 
-  @AutoCodec.VisibleForSerialization
+  @ThreadSafety.ThreadSafe
   @AutoCodec.Instantiator
-  static ArtifactSkyKey create(Artifact.SourceArtifact artifact) {
-    return INTERNER.intern(new ArtifactSkyKey(artifact));
+  public static ArtifactSkyKey key(Artifact artifact, boolean isMandatory) {
+    return INTERNER.intern(
+        artifact.isSourceArtifact()
+            ? new ArtifactSkyKey(artifact, isMandatory)
+            : new ArtifactSkyKey(artifact));
   }
 
-  public static Artifact artifact(SkyKey key) {
-    return key instanceof Artifact ? (Artifact) key : ((ArtifactSkyKey) key).artifact;
+  @ThreadSafety.ThreadSafe
+  public static Iterable<ArtifactSkyKey> mandatoryKeys(Iterable<Artifact> artifacts) {
+    return Iterables.transform(artifacts, TO_MANDATORY_KEY);
   }
 
-  public static boolean isMandatory(SkyKey key) {
-    return key instanceof Artifact;
+  public static Collection<Artifact> artifacts(Collection<? extends ArtifactSkyKey> keys) {
+    return Collections2.transform(keys, TO_ARTIFACT);
+  }
+
+  public static Artifact artifact(SkyKey key) {
+    return TO_ARTIFACT.apply((ArtifactSkyKey) key.argument());
   }
 
   @Override
@@ -68,7 +115,32 @@ public class ArtifactSkyKey implements SkyKey {
 
   @Override
   public int hashCode() {
-    return artifact.hashCode();
+    // We use the hash code caching strategy employed by java.lang.String. There are three subtle
+    // things going on here:
+    //
+    // (1) We use a value of 0 to indicate that the hash code hasn't been computed and cached yet.
+    // Yes, this means that if the hash code is really 0 then we will "recompute" it each time.
+    // But this isn't a problem in practice since a hash code of 0 should be rare.
+    //
+    // (2) Since we have no synchronization, multiple threads can race here thinking there are the
+    // first one to compute and cache the hash code.
+    //
+    // (3) Moreover, since 'hashCode' is non-volatile, the cached hash code value written from one
+    // thread may not be visible by another. Note that we probably don't need to worry about
+    // multiple inefficient reads of 'hashCode' on the same thread since it's non-volatile.
+    //
+    // All three of these issues are benign from a correctness perspective; in the end we have no
+    // overhead from synchronization, at the cost of potentially computing the hash code more than
+    // once.
+    if (hashCode == 0) {
+      hashCode = computeHashCode();
+    }
+    return hashCode;
+  }
+
+  private int computeHashCode() {
+    int initialHash = artifact.hashCode() + artifact.getArtifactOwner().hashCode();
+    return isMandatory ? initialHash : 47 * initialHash + 1;
   }
 
   @Override
@@ -80,13 +152,24 @@ public class ArtifactSkyKey implements SkyKey {
       return false;
     }
     ArtifactSkyKey thatArtifactSkyKey = ((ArtifactSkyKey) that);
-    return artifact.equals(thatArtifactSkyKey.artifact);
+    return Artifact.equalWithOwner(artifact, thatArtifactSkyKey.artifact)
+        && isMandatory == thatArtifactSkyKey.isMandatory;
   }
 
   public 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();