Avoid downloading the empty Tree message

Valid empty `Tree` messages, which correspond to empty output directories, serialize to two bytes since they always contain an empty but present `Tree` message in the `root` field. Since such directories are very common (for example, as undeclared test output directories), it pays off to avoid downloading them.

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

@@ -1158,6 +1158,15 @@ private static DirectoryMetadata parseDirectory(
     return new DirectoryMetadata(filesBuilder.build(), symlinksBuilder.build());
   }
 
+  // The Tree message representing an empty directory.
+  private static final Tree EMPTY_DIRECTORY =
+      Tree.newBuilder().setRoot(Directory.getDefaultInstance()).build();
+
+  static {
+    // See logic in parseActionResultMetadata below.
+    Preconditions.checkState(EMPTY_DIRECTORY.toByteString().size() == 2);
+  }
+
   static ActionResultMetadata parseActionResultMetadata(
       CombinedCache combinedCache,
       DigestUtil digestUtil,
@@ -1171,13 +1180,25 @@ static ActionResultMetadata parseActionResultMetadata(
         Maps.newHashMapWithExpectedSize(result.getOutputDirectoriesCount());
     for (OutputDirectory dir : result.getOutputDirectoriesList()) {
       var outputPath = dir.getPath();
-      dirMetadataDownloads.put(
-          remotePathResolver.outputPathToLocalPath(unicodeToInternal(outputPath)),
-          Futures.transformAsync(
-              combinedCache.downloadBlob(context, outputPath, dir.getTreeDigest()),
-              (treeBytes) ->
-                  immediateFuture(Tree.parseFrom(treeBytes, ExtensionRegistry.getEmptyRegistry())),
-              directExecutor()));
+      var localPath = remotePathResolver.outputPathToLocalPath(unicodeToInternal(outputPath));
+      if (dir.getTreeDigest().getSizeBytes() == 2) {
+        // A valid Tree message contains at least a non-empty root field. The only way for a Tree
+        // message to have a size of 2 bytes is if the root field is the only non-empty field and
+        // the Directory message in the root field is empty, which corresponds to one byte for the
+        // LEN tag and field number and one byte for the zero-length varint. Since empty tree
+        // artifacts are relatively common (e.g., as the undeclared test output directory), we avoid
+        // downloading these messages here.
+        dirMetadataDownloads.put(localPath, immediateFuture(EMPTY_DIRECTORY));
+      } else {
+        dirMetadataDownloads.put(
+            localPath,
+            Futures.transformAsync(
+                combinedCache.downloadBlob(context, outputPath, dir.getTreeDigest()),
+                (treeBytes) ->
+                    immediateFuture(
+                        Tree.parseFrom(treeBytes, ExtensionRegistry.getEmptyRegistry())),
+                directExecutor()));
+      }
     }
 
     waitForBulkTransfer(dirMetadataDownloads.values(), /* cancelRemainingOnInterrupt= */ true);

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

@@ -589,8 +589,9 @@ public void downloadOutputs_emptyOutputDirectories_works() throws Exception {
 
     // arrange
     Tree barTreeMessage = Tree.newBuilder().setRoot(Directory.getDefaultInstance()).build();
-    Digest barTreeDigest =
-        cache.addContents(remoteActionExecutionContext, barTreeMessage.toByteArray());
+    // Don't add barTreeMessage to the cache, the Tree proto for an empty output directory is
+    // recognized by its digest.
+    Digest barTreeDigest = digestUtil.compute(barTreeMessage);
     ActionResult.Builder builder = ActionResult.newBuilder();
     builder.addOutputDirectoriesBuilder().setPath("outputs/a/bar").setTreeDigest(barTreeDigest);
     RemoteActionResult result =