[6.2.0] Construct TreeArtifactValues on multiple threads.

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

@@ -370,7 +370,10 @@ private TreeArtifactValue constructTreeArtifactValueFromFilesystem(SpecialArtifa
             throw new IOException(errorMessage, e);
           }
 
-          tree.putChild(child, metadata);
+          // visitTree() uses multiple threads and putChild() is not thread-safe
+          synchronized (tree) {
+            tree.putChild(child, metadata);
+          }
         });
 
     if (archivedTreeArtifactsEnabled) {

src/main/java/com/google/devtools/build/lib/skyframe/BUILD

@@ -2842,6 +2842,7 @@ java_library(
         "//src/main/java/com/google/devtools/build/lib/actions:artifacts",
         "//src/main/java/com/google/devtools/build/lib/actions:file_metadata",
         "//src/main/java/com/google/devtools/build/lib/actions:has_digest",
+        "//src/main/java/com/google/devtools/build/lib/concurrent",
         "//src/main/java/com/google/devtools/build/lib/skyframe/serialization/autocodec",
         "//src/main/java/com/google/devtools/build/lib/skyframe/serialization/autocodec:serialization-constant",
         "//src/main/java/com/google/devtools/build/lib/util",

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

@@ -421,7 +421,8 @@ private boolean treeArtifactIsDirty(Artifact artifact, TreeArtifactValue value)
 
     // This could be improved by short-circuiting as soon as we see a child that is not present in
     // the TreeArtifactValue, but it doesn't seem to be a major source of overhead.
-    Set<PathFragment> currentChildren = new HashSet<>();
+    // visitTree() is called from multiple threads in parallel so this need to be a hash set
+    Set<PathFragment> currentChildren = Sets.newConcurrentHashSet();
     try {
       TreeArtifactValue.visitTree(
           path,

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

@@ -35,10 +35,15 @@
 import com.google.devtools.build.lib.actions.FileStateType;
 import com.google.devtools.build.lib.actions.HasDigest;
 import com.google.devtools.build.lib.actions.cache.MetadataDigestUtils;
+import com.google.devtools.build.lib.concurrent.AbstractQueueVisitor;
+import com.google.devtools.build.lib.concurrent.ErrorClassifier;
+import com.google.devtools.build.lib.concurrent.NamedForkJoinPool;
+import com.google.devtools.build.lib.concurrent.ThreadSafety.ThreadSafe;
 import com.google.devtools.build.lib.skyframe.serialization.autocodec.AutoCodec;
 import com.google.devtools.build.lib.skyframe.serialization.autocodec.SerializationConstant;
 import com.google.devtools.build.lib.util.Fingerprint;
 import com.google.devtools.build.lib.vfs.Dirent;
+import com.google.devtools.build.lib.vfs.IORuntimeException;
 import com.google.devtools.build.lib.vfs.Path;
 import com.google.devtools.build.lib.vfs.PathFragment;
 import com.google.devtools.build.lib.vfs.Symlinks;
@@ -51,14 +56,17 @@
 import java.util.Map;
 import java.util.Objects;
 import java.util.Optional;
+import java.util.concurrent.ForkJoinPool;
 import javax.annotation.Nullable;
 
 /**
  * Value for TreeArtifacts, which contains a digest and the {@link FileArtifactValue}s of its child
  * {@link TreeFileArtifact}s.
  */
 public class TreeArtifactValue implements HasDigest, SkyValue {
-
+  private static final ForkJoinPool VISITOR_POOL =
+      NamedForkJoinPool.newNamedPool(
+          "tree-artifact-visitor", Runtime.getRuntime().availableProcessors());
   /**
    * Comparator based on exec path which works on {@link ActionInput} as opposed to {@link
    * com.google.devtools.build.lib.actions.Artifact}. This way, we can use an {@link ActionInput} to
@@ -487,10 +495,71 @@ public interface TreeArtifactVisitor {
      *
      * <p>If the implementation throws {@link IOException}, traversal is immediately halted and the
      * exception is propagated.
+     *
+     * <p>This method can be called from multiple threads in parallel during a single call of {@link
+     * TreeArtifactVisitor#visitTree(Path, TreeArtifactVisitor)}.
      */
+    @ThreadSafe
     void visit(PathFragment parentRelativePath, Dirent.Type type) throws IOException;
   }
 
+  /** An {@link AbstractQueueVisitor} that visits every file in the tree artifact. */
+  static class Visitor extends AbstractQueueVisitor {
+    private final Path parentDir;
+    private final TreeArtifactVisitor visitor;
+
+    Visitor(Path parentDir, TreeArtifactVisitor visitor) {
+      super(
+          VISITOR_POOL,
+          /* shutdownOnCompletion= */ false,
+          /* failFastOnException= */ true,
+          ErrorClassifier.DEFAULT);
+      this.parentDir = checkNotNull(parentDir);
+      this.visitor = checkNotNull(visitor);
+    }
+
+    void run() throws IOException, InterruptedException {
+      execute(() -> visitTree(PathFragment.EMPTY_FRAGMENT));
+      try {
+        awaitQuiescence(true);
+      } catch (IORuntimeException e) {
+        throw e.getCauseIOException();
+      }
+    }
+
+    // IOExceptions are wrapped in IORuntimeException so that it can be propagated to the main
+    // thread
+    private void visitTree(PathFragment subdir) {
+      try {
+        for (Dirent dirent : parentDir.getRelative(subdir).readdir(Symlinks.NOFOLLOW)) {
+          PathFragment parentRelativePath = subdir.getChild(dirent.getName());
+          Dirent.Type type = dirent.getType();
+
+          if (type == Dirent.Type.UNKNOWN) {
+            throw new IOException(
+                "Could not determine type of file for "
+                    + parentRelativePath
+                    + " under "
+                    + parentDir);
+          }
+
+          if (type == Dirent.Type.SYMLINK) {
+            checkSymlink(subdir, parentDir.getRelative(parentRelativePath));
+          }
+
+          visitor.visit(parentRelativePath, type);
+
+          if (type == Dirent.Type.DIRECTORY) {
+            execute(() -> visitTree(parentRelativePath));
+          }
+        }
+      } catch (IOException e) {
+        // We can't throw checked exceptions here since AQV expects Runnables
+        throw new IORuntimeException(e);
+      }
+    }
+  }
+
   /**
    * Recursively visits all descendants under a directory.
    *
@@ -502,33 +571,18 @@ public interface TreeArtifactVisitor {
    * symlink that traverses outside of the tree artifact and any entry of {@link
    * Dirent.Type#UNKNOWN}, such as a named pipe.
    *
+   * <p>The visitor will be called on multiple threads in parallel. Accordingly, it must be
+   * thread-safe.
+   *
    * @throws IOException if there is any problem reading or validating outputs under the given tree
    *     artifact directory, or if {@link TreeArtifactVisitor#visit} throws {@link IOException}
    */
-  public static void visitTree(Path parentDir, TreeArtifactVisitor visitor) throws IOException {
-    visitTree(parentDir, PathFragment.EMPTY_FRAGMENT, checkNotNull(visitor));
-  }
-
-  private static void visitTree(Path parentDir, PathFragment subdir, TreeArtifactVisitor visitor)
-      throws IOException {
-    for (Dirent dirent : parentDir.getRelative(subdir).readdir(Symlinks.NOFOLLOW)) {
-      PathFragment parentRelativePath = subdir.getChild(dirent.getName());
-      Dirent.Type type = dirent.getType();
-
-      if (type == Dirent.Type.UNKNOWN) {
-        throw new IOException(
-            "Could not determine type of file for " + parentRelativePath + " under " + parentDir);
-      }
-
-      if (type == Dirent.Type.SYMLINK) {
-        checkSymlink(subdir, parentDir.getRelative(parentRelativePath));
-      }
-
-      visitor.visit(parentRelativePath, type);
-
-      if (type == Dirent.Type.DIRECTORY) {
-        visitTree(parentDir, parentRelativePath, visitor);
-      }
+  public static void visitTree(Path parentDir, TreeArtifactVisitor treeArtifactVisitor) throws IOException {
+    try {
+      Visitor visitor = new Visitor(parentDir, treeArtifactVisitor);
+      visitor.run();
+    } catch (InterruptedException e) {
+      throw new IOException(e);
     }
   }
 

src/test/java/com/google/devtools/build/lib/actions/ActionCacheCheckerTest.java

@@ -1240,7 +1240,9 @@ public TreeArtifactValue getTreeArtifactValue(SpecialArtifact output) throws IOE
                   Artifact.TreeFileArtifact.createTreeOutput(output, parentRelativePath);
               FileArtifactValue metadata =
                   FileArtifactValue.createForTesting(treeDir.getRelative(parentRelativePath));
-              tree.putChild(child, metadata);
+              synchronized (tree) {
+                tree.putChild(child, metadata);
+              }
             });
       }
 

src/test/java/com/google/devtools/build/lib/skyframe/TreeArtifactValueTest.java

@@ -373,7 +373,13 @@ public void visitTree_visitsEachChild() throws Exception {
     scratch.resolve("tree/a/b/dangling_link").createSymbolicLink(PathFragment.create("?"));
     List<Pair<PathFragment, Dirent.Type>> children = new ArrayList<>();
 
-    TreeArtifactValue.visitTree(treeDir, (child, type) -> children.add(Pair.of(child, type)));
+    TreeArtifactValue.visitTree(
+        treeDir,
+        (child, type) -> {
+          synchronized (children) {
+            children.add(Pair.of(child, type));
+          }
+        });
 
     assertThat(children)
         .containsExactly(
@@ -435,7 +441,13 @@ public void visitTree_pemitsUpLevelSymlinkInsideTree() throws Exception {
     scratch.resolve("tree/a/up_link").createSymbolicLink(PathFragment.create("../file"));
     List<Pair<PathFragment, Dirent.Type>> children = new ArrayList<>();
 
-    TreeArtifactValue.visitTree(treeDir, (child, type) -> children.add(Pair.of(child, type)));
+    TreeArtifactValue.visitTree(
+        treeDir,
+        (child, type) -> {
+          synchronized (children) {
+            children.add(Pair.of(child, type));
+          }
+        });
 
     assertThat(children)
         .containsExactly(
@@ -450,7 +462,13 @@ public void visitTree_permitsAbsoluteSymlink() throws Exception {
     scratch.resolve("tree/absolute_link").createSymbolicLink(PathFragment.create("/tmp"));
     List<Pair<PathFragment, Dirent.Type>> children = new ArrayList<>();
 
-    TreeArtifactValue.visitTree(treeDir, (child, type) -> children.add(Pair.of(child, type)));
+    TreeArtifactValue.visitTree(
+        treeDir,
+        (child, type) -> {
+          synchronized (children) {
+            children.add(Pair.of(child, type));
+          }
+        });
 
     assertThat(children)
         .containsExactly(Pair.of(PathFragment.create("absolute_link"), Dirent.Type.SYMLINK));