nix-gerrit/gerrit/jgit-parallel-object-counting.patch

diff --git a/modules/jgit/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/WindowCursor.java b/modules/jgit/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/WindowCursor.java
index 01f514b93..c9888657a 100644
--- a/modules/jgit/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/WindowCursor.java
+++ b/modules/jgit/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/WindowCursor.java
@@ -18,6 +18,7 @@
 import java.util.List;
 import java.util.Optional;
 import java.util.Set;
+import java.util.concurrent.atomic.AtomicInteger;
 import java.util.zip.DataFormatException;
 import java.util.zip.Inflater;
 
@@ -166,10 +167,49 @@ public LocalObjectToPack newObjectToPack(AnyObjectId objectId, int type) {
 		return new LocalObjectToPack(objectId, type);
 	}
 
+	/* Above this number of objects, perform this counting in parallel. */
+	private static final int OBJECTS_TO_MULTITHREAD = 10_000;
+
 	@Override
 	public void selectObjectRepresentation(PackWriter packer,
 			ProgressMonitor monitor, Iterable<ObjectToPack> objects)
 			throws IOException, MissingObjectException {
+		if (objects instanceof Collection<ObjectToPack>) {
+			// If it isn't a collection, we can't tell how many objects there are.
+			Collection<ObjectToPack> sizedObjects = (Collection<ObjectToPack>)objects;
+			if (sizedObjects.size() >= OBJECTS_TO_MULTITHREAD) {
+				// Where we're going, we don't need single-threaded.
+				AtomicInteger counter = new AtomicInteger();
+				try {
+					sizedObjects.parallelStream()
+						.forEach(otp -> {
+							try {
+								// TODO: maybe creating a new WindowCursor each time is expensive?
+								db.selectObjectRepresentation(packer, otp, new WindowCursor(db));
+							} catch (IOException e) {
+								throw new RuntimeException(e);
+							}
+							if (counter.incrementAndGet() >= OBJECTS_TO_MULTITHREAD) {
+								int rem = counter.getAndSet(0);
+								if (rem >= OBJECTS_TO_MULTITHREAD) {
+									synchronized (monitor) {
+										monitor.update(rem);
+									}
+								} else {
+									counter.addAndGet(rem);
+								}
+							}
+						});
+					monitor.update(counter.get());
+				} catch (RuntimeException e) {
+					if (e.getCause() != null && e.getCause() instanceof IOException) throw (IOException)e.getCause();
+					throw e;
+				}
+				return;
+			}
+		}
+
+		// Otherwise, single-threaded mode.
 		for (ObjectToPack otp : objects) {
 			db.selectObjectRepresentation(packer, otp, this);
 			monitor.update(1);