A prototype implementation of top-level aspects.

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diff --git a/src/main/java/com/google/devtools/build/lib/skyframe/CompletionFunction.java b/src/main/java/com/google/devtools/build/lib/skyframe/CompletionFunction.java
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+// Copyright 2014 Google Inc. 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.eventbus.EventBus;
+import com.google.devtools.build.lib.actions.ActionExecutionException;
+import com.google.devtools.build.lib.actions.Artifact;
+import com.google.devtools.build.lib.actions.MissingInputFileException;
+import com.google.devtools.build.lib.analysis.AspectCompleteEvent;
+import com.google.devtools.build.lib.analysis.ConfiguredTarget;
+import com.google.devtools.build.lib.analysis.LabelAndConfiguration;
+import com.google.devtools.build.lib.analysis.TargetCompleteEvent;
+import com.google.devtools.build.lib.analysis.TopLevelArtifactContext;
+import com.google.devtools.build.lib.analysis.TopLevelArtifactHelper;
+import com.google.devtools.build.lib.analysis.TopLevelArtifactHelper.ArtifactsToBuild;
+import com.google.devtools.build.lib.collect.nestedset.NestedSet;
+import com.google.devtools.build.lib.collect.nestedset.NestedSetBuilder;
+import com.google.devtools.build.lib.events.Event;
+import com.google.devtools.build.lib.skyframe.AspectValue.AspectKey;
+import com.google.devtools.build.lib.syntax.Label;
+import com.google.devtools.build.skyframe.SkyFunction;
+import com.google.devtools.build.skyframe.SkyFunctionException;
+import com.google.devtools.build.skyframe.SkyKey;
+import com.google.devtools.build.skyframe.SkyValue;
+import com.google.devtools.build.skyframe.ValueOrException2;
+
+import java.util.Map;
+import java.util.concurrent.atomic.AtomicReference;
+
+import javax.annotation.Nullable;
+
+/**
+ * CompletionFunction builds the artifactsToBuild collection of a {@link ConfiguredTarget}.
+ */
+public final class CompletionFunction<TValue extends SkyValue, TResult extends SkyValue>
+    implements SkyFunction {
+
+  /**
+   * A strategy for completing the build.
+   */
+  public interface Completor<TValue, TResult extends SkyValue> {
+
+    /**
+     * Obtains an analysis result value from environment.
+     */
+    TValue getValueFromSkyKey(SkyKey skyKey, Environment env);
+
+    /**
+     * Returns all artefacts that need to be built to complete the {@code value}
+     */
+    ArtifactsToBuild getAllArtifactsToBuild(TValue value, TopLevelArtifactContext context);
+
+    /**
+     * Creates an event reporting an absent input artifact.
+     */
+    Event getRootCauseError(TValue value, Label rootCause);
+
+    /**
+     * Creates an error message reporting {@code missingCount} missing input files.
+     */
+    MissingInputFileException getMissingFilesException(TValue value, int missingCount);
+
+    /**
+     * Creates a successful completion value.
+     */
+    TResult createResult(TValue value);
+
+    /**
+     * Creates a failed completion value.
+     */
+    SkyValue createFailed(TValue value, NestedSet<Label> rootCauses);
+  }
+
+  private static class TargetCompletor
+      implements Completor<ConfiguredTargetValue, TargetCompletionValue> {
+    @Override
+    public ConfiguredTargetValue getValueFromSkyKey(SkyKey skyKey, Environment env) {
+      LabelAndConfiguration lac = (LabelAndConfiguration) skyKey.argument();
+      return (ConfiguredTargetValue)
+          env.getValue(ConfiguredTargetValue.key(lac.getLabel(), lac.getConfiguration()));
+    }
+
+    @Override
+    public ArtifactsToBuild getAllArtifactsToBuild(
+        ConfiguredTargetValue value, TopLevelArtifactContext topLevelContext) {
+      return TopLevelArtifactHelper.getAllArtifactsToBuild(
+          value.getConfiguredTarget(), topLevelContext);
+    }
+
+    @Override
+    public Event getRootCauseError(ConfiguredTargetValue ctValue, Label rootCause) {
+      return Event.error(
+          ctValue.getConfiguredTarget().getTarget().getLocation(),
+          String.format(
+              "%s: missing input file '%s'", ctValue.getConfiguredTarget().getLabel(), rootCause));
+    }
+
+    @Override
+    public MissingInputFileException getMissingFilesException(
+        ConfiguredTargetValue value, int missingCount) {
+      return new MissingInputFileException(
+          value.getConfiguredTarget().getTarget().getLocation()
+              + " "
+              + missingCount
+              + " input file(s) do not exist",
+          value.getConfiguredTarget().getTarget().getLocation());
+    }
+
+    @Override
+    public TargetCompletionValue createResult(ConfiguredTargetValue value) {
+      return new TargetCompletionValue(value.getConfiguredTarget());
+    }
+
+    @Override
+    public SkyValue createFailed(ConfiguredTargetValue value, NestedSet<Label> rootCauses) {
+      return TargetCompleteEvent.createFailed(value.getConfiguredTarget(), rootCauses);
+    }
+  }
+
+  private static class AspectCompletor implements Completor<AspectValue, AspectCompletionValue> {
+    @Override
+    public AspectValue getValueFromSkyKey(SkyKey skyKey, Environment env) {
+      AspectKey aspectKey = (AspectKey) skyKey.argument();
+      return (AspectValue) env.getValue(AspectValue.key(aspectKey));
+    }
+
+    @Override
+    public ArtifactsToBuild getAllArtifactsToBuild(
+        AspectValue value, TopLevelArtifactContext topLevelArtifactContext) {
+      return TopLevelArtifactHelper.getAllArtifactsToBuild(value, topLevelArtifactContext);
+    }
+
+    @Override
+    public Event getRootCauseError(AspectValue value, Label rootCause) {
+      return Event.error(
+          value.getLocation(),
+          String.format(
+              "%s, aspect %s: missing input file '%s'",
+              value.getLabel(),
+              value.getAspect().getName(),
+              rootCause));
+    }
+
+    @Override
+    public MissingInputFileException getMissingFilesException(AspectValue value, int missingCount) {
+      return new MissingInputFileException(
+          value.getLabel()
+              + ", aspect "
+              + value.getAspect().getName()
+              + missingCount
+              + " input file(s) do not exist",
+          value.getLocation());
+    }
+
+    @Override
+    public AspectCompletionValue createResult(AspectValue value) {
+      return new AspectCompletionValue(value);
+    }
+
+    @Override
+    public SkyValue createFailed(AspectValue value, NestedSet<Label> rootCauses) {
+      return AspectCompleteEvent.createFailed(value, rootCauses);
+    }
+  }
+
+  public static SkyFunction targetCompletionFunction(AtomicReference<EventBus> eventBusRef) {
+    return new CompletionFunction<>(eventBusRef, new TargetCompletor());
+  }
+
+  public static SkyFunction aspectCompletionFunction(AtomicReference<EventBus> eventBusRef) {
+    return new CompletionFunction<>(eventBusRef, new AspectCompletor());
+  }
+
+  private final AtomicReference<EventBus> eventBusRef;
+  private final Completor<TValue, TResult> completor;
+
+  private CompletionFunction(
+      AtomicReference<EventBus> eventBusRef, Completor<TValue, TResult> completor) {
+    this.eventBusRef = eventBusRef;
+    this.completor = completor;
+  }
+
+  @Nullable
+  @Override
+  public SkyValue compute(SkyKey skyKey, Environment env) throws CompletionFunctionException {
+    TValue value = completor.getValueFromSkyKey(skyKey, env);
+    TopLevelArtifactContext topLevelContext = PrecomputedValue.TOP_LEVEL_CONTEXT.get(env);
+    if (env.valuesMissing()) {
+      return null;
+    }
+
+    Map<SkyKey, ValueOrException2<MissingInputFileException, ActionExecutionException>> inputDeps =
+        env.getValuesOrThrow(
+            ArtifactValue.mandatoryKeys(
+                completor.getAllArtifactsToBuild(value, topLevelContext).getAllArtifacts()),
+            MissingInputFileException.class,
+            ActionExecutionException.class);
+
+    int missingCount = 0;
+    ActionExecutionException firstActionExecutionException = null;
+    MissingInputFileException missingInputException = null;
+    NestedSetBuilder<Label> rootCausesBuilder = NestedSetBuilder.stableOrder();
+    for (Map.Entry<SkyKey, ValueOrException2<MissingInputFileException, ActionExecutionException>>
+        depsEntry : inputDeps.entrySet()) {
+      Artifact input = ArtifactValue.artifact(depsEntry.getKey());
+      try {
+        depsEntry.getValue().get();
+      } catch (MissingInputFileException e) {
+        missingCount++;
+        final Label inputOwner = input.getOwner();
+        if (inputOwner != null) {
+          rootCausesBuilder.add(inputOwner);
+          env.getListener().handle(completor.getRootCauseError(value, inputOwner));
+        }
+      } catch (ActionExecutionException e) {
+        rootCausesBuilder.addTransitive(e.getRootCauses());
+        if (firstActionExecutionException == null) {
+          firstActionExecutionException = e;
+        }
+      }
+    }
+
+    if (missingCount > 0) {
+      missingInputException = completor.getMissingFilesException(value, missingCount);
+    }
+
+    NestedSet<Label> rootCauses = rootCausesBuilder.build();
+    if (!rootCauses.isEmpty()) {
+      eventBusRef.get().post(completor.createFailed(value, rootCauses));
+      if (firstActionExecutionException != null) {
+        throw new CompletionFunctionException(firstActionExecutionException);
+      } else {
+        throw new CompletionFunctionException(missingInputException);
+      }
+    }
+
+    return env.valuesMissing() ? null : completor.createResult(value);
+  }
+
+  @Override
+  public String extractTag(SkyKey skyKey) {
+    return Label.print(((LabelAndConfiguration) skyKey.argument()).getLabel());
+  }
+
+  private static final class CompletionFunctionException extends SkyFunctionException {
+
+    private final ActionExecutionException actionException;
+
+    public CompletionFunctionException(ActionExecutionException e) {
+      super(e, Transience.PERSISTENT);
+      this.actionException = e;
+    }
+
+    public CompletionFunctionException(MissingInputFileException e) {
+      super(e, Transience.TRANSIENT);
+      this.actionException = null;
+    }
+
+    @Override
+    public boolean isCatastrophic() {
+      return actionException != null && actionException.isCatastrophe();
+    }
+  }
+}