Move reachability checker generation into the ObjectReader object

Reachability checkers are retrieved from RevWalk and ObjectWalk objects:
* RevWalk.createReachabilityChecker()
* ObjectWalk.createObjectReachabilityChecker()

Since RevWalks and ObjectWalks are themselves directly instantiated
in hundreds of places (e.g. UploadPack...) overriding them in a
consistent way requires overloading 100s of methods, which isn't
feasible. Moving reachability checker generation to a more central
place solves that problem.

The ObjectReader object seems a good place from which to get
reachability checkers, because reachability checkers return
information about relationships between objects. ObjectDatabases
delegate many operations to ObjectReaders, and reachability bitmaps
are attached to ObjectReaders.

The Bitmapped and Pedestrian reachability checker objects were
package private in the org.eclipse.jgit.revwalk package. This change
makes them public and moves them to the
org.eclipse.jgit.internal.revwalk package. Corresponding tests are
also moved.

Motivation:
1) Reachability checking algorithms need to scale. One of the
   internal Android repositories has ~2.4 million refs/changes/*
   references, causing bad long tail performance in reachability
   checks.
2) Reachability check performance is impacted by repository
   topography: number of refs, number of objects, amounts of
   related vs. unrelated history.
3) Reachability check performance is also affected by per-branch
   access (Gerrit branch permissions) since different users can
   see different branches.
4) Reachability check performance isn't affected by any state in a
   RevWalk or ObjectWalk.

I don't yet know if a single algorithm will work for all cases in #2
and #3. We may need to evolve the ReachabilityChecker interfaces
over time to solve the Gerrit branch permissions case, or use
Gerrit-specific identity information to solve that in an efficient
way.

This change takes the existing public API and moves it to the
ObjectReader/whole repository level, which is where we can do
consistent customizations for #2 and #3. We intend to upstream the
best of whatever works, but anticipate the need for multiple rounds
of experimentation.

Change-Id: I9185feff43551fb387957c436112d5250486833d
Signed-off-by: Terry Parker <tparker@google.com>

1 files changed, 57 insertions, 0 deletions

diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/lib/ObjectReader.java b/org.eclipse.jgit/src/org/eclipse/jgit/lib/ObjectReader.java
index 6bb6ae590a..718ed89142 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/lib/ObjectReader.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/lib/ObjectReader.java
@@ -17,9 +17,18 @@ import java.util.Iterator;
 import java.util.List;
 import java.util.Set;
 
+import org.eclipse.jgit.annotations.NonNull;
 import org.eclipse.jgit.annotations.Nullable;
 import org.eclipse.jgit.errors.IncorrectObjectTypeException;
 import org.eclipse.jgit.errors.MissingObjectException;
+import org.eclipse.jgit.internal.revwalk.BitmappedObjectReachabilityChecker;
+import org.eclipse.jgit.internal.revwalk.BitmappedReachabilityChecker;
+import org.eclipse.jgit.internal.revwalk.PedestrianObjectReachabilityChecker;
+import org.eclipse.jgit.internal.revwalk.PedestrianReachabilityChecker;
+import org.eclipse.jgit.revwalk.ObjectReachabilityChecker;
+import org.eclipse.jgit.revwalk.ObjectWalk;
+import org.eclipse.jgit.revwalk.ReachabilityChecker;
+import org.eclipse.jgit.revwalk.RevWalk;
 
 /**
  * Reads an {@link org.eclipse.jgit.lib.ObjectDatabase} for a single thread.
@@ -408,6 +417,54 @@ public abstract class ObjectReader implements AutoCloseable {
 	}
 
 	/**
+	 * Create a reachability checker that will use bitmaps if possible.
+	 *
+	 * @param rw
+	 *            revwalk for use by the reachability checker
+	 * @return the most efficient reachability checker for this repository.
+	 * @throws IOException
+	 *             if it cannot open any of the underlying indices.
+	 *
+	 * @since 5.11
+	 */
+	@NonNull
+	public ReachabilityChecker createReachabilityChecker(RevWalk rw)
+			throws IOException {
+		if (getBitmapIndex() != null) {
+			return new BitmappedReachabilityChecker(rw);
+		}
+
+		return new PedestrianReachabilityChecker(true, rw);
+	}
+
+	/**
+	 * Create an object reachability checker that will use bitmaps if possible.
+	 *
+	 * This reachability checker accepts any object as target. For checks
+	 * exclusively between commits, use
+	 * {@link #createReachabilityChecker(RevWalk)}.
+	 *
+	 * @param ow
+	 *            objectwalk for use by the reachability checker
+	 * @return the most efficient object reachability checker for this
+	 *         repository.
+	 *
+	 * @throws IOException
+	 *             if it cannot open any of the underlying indices.
+	 *
+	 * @since 5.11
+	 */
+	@NonNull
+	public ObjectReachabilityChecker createObjectReachabilityChecker(
+			ObjectWalk ow) throws IOException {
+		if (getBitmapIndex() != null) {
+			return new BitmappedObjectReachabilityChecker(ow);
+		}
+
+		return new PedestrianObjectReachabilityChecker(ow);
+	}
+
+	/**
 	 * Get the {@link org.eclipse.jgit.lib.ObjectInserter} from which this
 	 * reader was created using {@code inserter.newReader()}
 	 *