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// 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.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}. 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 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.
*/
@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 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, boolean mandatory) {
Preconditions.checkArgument(sourceArtifact.isSourceArtifact());
this.artifact = Preconditions.checkNotNull(sourceArtifact);
this.isMandatory = mandatory;
}
/**
* 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.
}
@ThreadSafety.ThreadSafe
@AutoCodec.Instantiator
public static ArtifactSkyKey key(Artifact artifact, boolean isMandatory) {
return INTERNER.intern(
artifact.isSourceArtifact()
? new ArtifactSkyKey(artifact, isMandatory)
: new ArtifactSkyKey(artifact));
}
@ThreadSafety.ThreadSafe
public static Iterable<ArtifactSkyKey> mandatoryKeys(Iterable<Artifact> artifacts) {
return Iterables.transform(artifacts, TO_MANDATORY_KEY);
}
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
public SkyFunctionName functionName() {
return Artifact.ARTIFACT;
}
@Override
public int 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
public boolean equals(Object that) {
if (this == that) {
return true;
}
if (!(that instanceof ArtifactSkyKey)) {
return false;
}
ArtifactSkyKey thatArtifactSkyKey = ((ArtifactSkyKey) that);
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();
}
}
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