Add GC_REST PackSource to better order DFS packs

Force reads to use a search ordering of:

  INSERT / RECEIVE
  COMPACT
  GC (heads)
  GC_REST (non-heads)
  GC_TXN (refs/txn)
  UNREACHABLE_GARBAGE

This has provided decent performance for object lookups.  Starting
from an arbitrary reference may find the content in a newer pack
created by DfsObjectInserter or a ReceivePack server.  Compaction of
recent packs also contains newer content, and then most interesting
data is in the "main" GC pack.  As the GC pack is self-contained (has
no edges leading outside) readers typically do not need to go further.

Adding a new GC_REST PackSource allows the DfsGarbageCollector to
identify to the pack ordering code which pack is which, so the
non-heads are scanned second during reads.  This removes a hack that
was unique to Google's implementation that enforced this behavior by
fixing up the lastModified timestamp.

Renumber the PackSource's categories to reflect this search ordering.

Change-Id: I19fdaab8a8d6687cbe8c88488e6daa0630bf189a

org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/dfs/DfsGarbageCollector.java

@@ -44,6 +44,7 @@
 package org.eclipse.jgit.internal.storage.dfs;
 
 import static org.eclipse.jgit.internal.storage.dfs.DfsObjDatabase.PackSource.GC;
+import static org.eclipse.jgit.internal.storage.dfs.DfsObjDatabase.PackSource.GC_REST;
 import static org.eclipse.jgit.internal.storage.dfs.DfsObjDatabase.PackSource.GC_TXN;
 import static org.eclipse.jgit.internal.storage.dfs.DfsObjDatabase.PackSource.UNREACHABLE_GARBAGE;
 import static org.eclipse.jgit.internal.storage.pack.PackExt.BITMAP_INDEX;
@@ -299,6 +300,7 @@ public class DfsGarbageCollector {
 				writePack(GC, pw, pm);
 		}
 	}
+
 	private void packRest(ProgressMonitor pm) throws IOException {
 		if (nonHeads.isEmpty())
 			return;
@@ -308,7 +310,7 @@ public class DfsGarbageCollector {
 				pw.excludeObjects(packedObjs);
 			pw.preparePack(pm, nonHeads, allHeads);
 			if (0 < pw.getObjectCount())
-				writePack(GC, pw, pm);
+				writePack(GC_REST, pw, pm);
 		}
 	}
 

org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/dfs/DfsObjDatabase.java

@@ -79,6 +79,17 @@ public abstract class DfsObjDatabase extends ObjectDatabase {
 		 */
 		RECEIVE(0),
 
+		/**
+		 * The pack was created by compacting multiple packs together.
+		 * <p>
+		 * Packs created by compacting multiple packs together aren't nearly as
+		 * efficient as a fully garbage collected repository, but may save disk
+		 * space by reducing redundant copies of base objects.
+		 *
+		 * @see DfsPackCompactor
+		 */
+		COMPACT(1),
+
 		/**
 		 * Pack was created by Git garbage collection by this implementation.
 		 * <p>
@@ -88,25 +99,17 @@ public abstract class DfsObjDatabase extends ObjectDatabase {
 		 *
 		 * @see DfsGarbageCollector
 		 */
-		GC(1),
+		GC(2),
+
+		/** Created from non-heads by {@link DfsGarbageCollector}. */
+		GC_REST(3),
 
 		/**
 		 * RefTreeGraph pack was created by Git garbage collection.
 		 *
 		 * @see DfsGarbageCollector
 		 */
-		GC_TXN(1),
+		GC_TXN(4),
-
-		/**
-		 * The pack was created by compacting multiple packs together.
-		 * <p>
-		 * Packs created by compacting multiple packs together aren't nearly as
-		 * efficient as a fully garbage collected repository, but may save disk
-		 * space by reducing redundant copies of base objects.
-		 *
-		 * @see DfsPackCompactor
-		 */
-		COMPACT(1),
 
 		/**
 		 * Pack was created by Git garbage collection.
@@ -115,7 +118,7 @@ public abstract class DfsObjDatabase extends ObjectDatabase {
 		 * last GC pass. It is retained in a new pack until it is safe to prune
 		 * these objects from the repository.
 		 */
-		UNREACHABLE_GARBAGE(2);
+		UNREACHABLE_GARBAGE(5);
 
 		final int category;