Emit Tree objects in topological order

remote-apis PR 230 added a way where producers of Tree messages can
indicate that the directories contained within are stored in topological
order. The advantage of using such an ordering is that it permits
instantiation of such objects onto a local file system in a streaming
fashion. The same holds for lookups of individual paths.

Even though Bazel currently does not gain from this, this change at
least modifies Bazel's REv2 client to emit topologically sorted trees.
This makes it possible for tools such as Buildbarn's bb-browser to
process them more efficiently.

More details:
- bazelbuild/remote-apis#229
- bazelbuild/remote-apis#230

src/main/java/com/google/devtools/build/lib/remote/UploadManifest.java

@@ -31,6 +31,7 @@
 import com.google.common.annotations.VisibleForTesting;
 import com.google.common.base.Preconditions;
 import com.google.common.collect.ImmutableList;
+import com.google.common.collect.Lists;
 import com.google.devtools.build.lib.actions.ActionExecutionMetadata;
 import com.google.devtools.build.lib.actions.ActionUploadFinishedEvent;
 import com.google.devtools.build.lib.actions.ActionUploadStartedEvent;
@@ -54,20 +55,24 @@
 import com.google.devtools.build.lib.vfs.PathFragment;
 import com.google.devtools.build.lib.vfs.Symlinks;
 import com.google.protobuf.ByteString;
+import com.google.protobuf.CodedOutputStream;
 import com.google.protobuf.Timestamp;
 import io.reactivex.rxjava3.core.Completable;
 import io.reactivex.rxjava3.core.Flowable;
 import io.reactivex.rxjava3.core.Single;
+import java.io.ByteArrayOutputStream;
 import java.io.IOException;
 import java.time.Duration;
 import java.time.Instant;
 import java.util.ArrayList;
 import java.util.Collection;
 import java.util.Comparator;
 import java.util.HashMap;
+import java.util.LinkedHashSet;
 import java.util.List;
 import java.util.Map;
 import java.util.Optional;
+import java.util.Set;
 import java.util.stream.Collectors;
 import javax.annotation.Nullable;
 
@@ -299,12 +304,27 @@ private void addFile(Digest digest, Path file) throws IOException {
     digestToFile.put(digest, file);
   }
 
+  // Field numbers of the 'root' and 'directory' fields in the Tree message.
+  private static final int TREE_ROOT_FIELD_NUMBER = 1;
+  private static final int TREE_CHILDREN_FIELD_NUMBER = 2;
+
   private void addDirectory(Path dir) throws ExecException, IOException {
-    Tree.Builder tree = Tree.newBuilder();
-    Directory root = computeDirectory(dir, tree);
-    tree.setRoot(root);
+    Set<ByteString> directories = new LinkedHashSet<>();
+    computeDirectory(dir, directories);
+
+    // Convert individual Directory messages to a Tree message. As we want the
+    // records to be topologically sorted (parents before children), we iterate
+    // over the directories in reverse insertion order.
+    ByteArrayOutputStream byteArrayOutputStream = new ByteArrayOutputStream();
+    CodedOutputStream codedOutputStream = CodedOutputStream.newInstance(byteArrayOutputStream);
+    int fieldNumber = TREE_ROOT_FIELD_NUMBER;
+    for (ByteString directory : Lists.reverse(new ArrayList<ByteString>(directories))) {
+      codedOutputStream.writeBytes(fieldNumber, directory);
+      fieldNumber = TREE_CHILDREN_FIELD_NUMBER;
+    }
+    codedOutputStream.flush();
 
-    ByteString data = tree.build().toByteString();
+    ByteString data = ByteString.copyFrom(byteArrayOutputStream.toByteArray());
     Digest digest = digestUtil.compute(data.toByteArray());
 
     if (result != null) {
@@ -317,7 +337,7 @@ private void addDirectory(Path dir) throws ExecException, IOException {
     digestToBlobs.put(digest, data);
   }
 
-  private Directory computeDirectory(Path path, Tree.Builder tree)
+  private ByteString computeDirectory(Path path, Set<ByteString> directories)
       throws ExecException, IOException {
     Directory.Builder b = Directory.newBuilder();
 
@@ -328,9 +348,8 @@ private Directory computeDirectory(Path path, Tree.Builder tree)
       String name = dirent.getName();
       Path child = path.getRelative(name);
       if (dirent.getType() == Dirent.Type.DIRECTORY) {
-        Directory dir = computeDirectory(child, tree);
-        b.addDirectoriesBuilder().setName(name).setDigest(digestUtil.compute(dir));
-        tree.addChildren(dir);
+        ByteString dir = computeDirectory(child, directories);
+        b.addDirectoriesBuilder().setName(name).setDigest(digestUtil.compute(dir.toByteArray()));
       } else if (dirent.getType() == Dirent.Type.SYMLINK) {
         PathFragment target = child.readSymbolicLink();
         if (!followSymlinks && !target.isAbsolute()) {
@@ -349,9 +368,8 @@ private Directory computeDirectory(Path path, Tree.Builder tree)
           b.addFilesBuilder().setName(name).setDigest(digest).setIsExecutable(child.isExecutable());
           digestToFile.put(digest, child);
         } else if (statFollow.isDirectory()) {
-          Directory dir = computeDirectory(child, tree);
-          b.addDirectoriesBuilder().setName(name).setDigest(digestUtil.compute(dir));
-          tree.addChildren(dir);
+          ByteString dir = computeDirectory(child, directories);
+          b.addDirectoriesBuilder().setName(name).setDigest(digestUtil.compute(dir.toByteArray()));
         } else {
           illegalOutput(child);
         }
@@ -364,7 +382,9 @@ private Directory computeDirectory(Path path, Tree.Builder tree)
       }
     }
 
-    return b.build();
+    ByteString directory = b.build().toByteString();
+    directories.add(directory);
+    return directory;
   }
 
   private void illegalOutput(Path path) throws ExecException {

src/test/java/com/google/devtools/build/lib/remote/UploadManifestTest.java

@@ -41,6 +41,8 @@
 import com.google.devtools.build.lib.vfs.SyscallCache;
 import com.google.devtools.build.lib.vfs.inmemoryfs.InMemoryFileSystem;
 import java.io.IOException;
+import java.util.ArrayList;
+import java.util.List;
 import org.junit.Before;
 import org.junit.Test;
 import org.junit.runner.RunWith;
@@ -1053,6 +1055,73 @@ public void actionResult_followSymlinks_specialFileSymlinkInDirectoryError() thr
     assertThat(e).hasMessageThat().contains("dir/link");
   }
 
+  @Test
+  public void actionResult_topologicallySortedAndDeduplicatedTree()
+      throws Exception {
+    // Create 5^3 identical files named "dir/%d/%d/%d/file".
+    Path dir = execRoot.getRelative("dir");
+    dir.createDirectory();
+    byte fileContents[] = new byte[] {1, 2, 3, 4, 5};
+    final int childrenPerDirectory = 5;
+    for (int a = 0; a < childrenPerDirectory; a++) {
+      Path pathA = dir.getRelative(Integer.toString(a));
+      pathA.createDirectory();
+      for (int b = 0; b < childrenPerDirectory; b++) {
+        Path pathB = pathA.getRelative(Integer.toString(b));
+        pathB.createDirectory();
+        for (int c = 0; c < childrenPerDirectory; c++) {
+          Path pathC = pathB.getRelative(Integer.toString(c));
+          pathC.createDirectory();
+          Path file = pathC.getRelative("file");
+          FileSystemUtils.writeContent(file, fileContents);
+        }
+      }
+    }
+
+    ActionResult.Builder result = ActionResult.newBuilder();
+    UploadManifest um =
+        new UploadManifest(
+            digestUtil,
+            remotePathResolver,
+            result,
+            /*followSymlinks=*/ false,
+            /*allowDanglingSymlinks=*/ false,
+            /*allowAbsoluteSymlinks=*/ false);
+    um.addFiles(ImmutableList.of(dir));
+
+    // Even though we constructed 1 + 5 + 5^2 + 5^3 directories, the resulting
+    // Tree message should only contain four unique instances. The directories
+    // should also be topologically sorted.
+    List<Directory> children = new ArrayList<>();
+    Directory root = Directory.newBuilder()
+        .addFiles(
+            FileNode.newBuilder().
+               setName("file").setDigest(digestUtil.compute(fileContents)))
+         .build();
+    for (int depth = 0; depth < 3; depth++) {
+      Directory.Builder b = Directory.newBuilder();
+      Digest parentDigest = digestUtil.compute(root.toByteArray());
+      for (int i = 0; i < childrenPerDirectory; i++) {
+        b.addDirectories(
+            DirectoryNode.newBuilder()
+                .setName(Integer.toString(i))
+                .setDigest(parentDigest));
+      }
+      children.add(0, root);
+      root = b.build();
+    }
+    Tree tree =
+        Tree.newBuilder()
+            .setRoot(root)
+            .addAllChildren(children)
+            .build();
+    Digest treeDigest = digestUtil.compute(tree);
+
+    ActionResult.Builder expectedResult = ActionResult.newBuilder();
+    expectedResult.addOutputDirectoriesBuilder().setPath("dir").setTreeDigest(treeDigest);
+    assertThat(result.build()).isEqualTo(expectedResult.build());
+  }
+
   private Path createSpecialFile(String execPath) throws IOException {
     Path special = mock(Path.class);
     when(special.statIfFound(Symlinks.NOFOLLOW)).thenReturn(SPECIAL_FILE_STATUS);