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>

7 files changed, 17 insertions, 6 deletions

diff --git a/org.eclipse.jgit.test/META-INF/MANIFEST.MF b/org.eclipse.jgit.test/META-INF/MANIFEST.MF
index a1239468db..0540cfb045 100644
--- a/org.eclipse.jgit.test/META-INF/MANIFEST.MF
+++ b/org.eclipse.jgit.test/META-INF/MANIFEST.MF
@@ -34,6 +34,7 @@ Import-Package: com.googlecode.javaewah;version="[1.1.6,2.0.0)",
  org.eclipse.jgit.ignore.internal;version="[5.11.0,5.12.0)",
  org.eclipse.jgit.internal;version="[5.11.0,5.12.0)",
  org.eclipse.jgit.internal.fsck;version="[5.11.0,5.12.0)",
+ org.eclipse.jgit.internal.revwalk;version="[5.11.0,5.12.0)",
  org.eclipse.jgit.internal.storage.dfs;version="[5.11.0,5.12.0)",
  org.eclipse.jgit.internal.storage.file;version="[5.11.0,5.12.0)",
  org.eclipse.jgit.internal.storage.io;version="[5.11.0,5.12.0)",
diff --git a/org.eclipse.jgit.test/tst/org/eclipse/jgit/revwalk/BitmappedObjectReachabilityTest.java b/org.eclipse.jgit.test/tst/org/eclipse/jgit/internal/revwalk/BitmappedObjectReachabilityTest.java
index d2b6e89168..f2f74050b9 100644
--- a/org.eclipse.jgit.test/tst/org/eclipse/jgit/revwalk/BitmappedObjectReachabilityTest.java
+++ b/org.eclipse.jgit.test/tst/org/eclipse/jgit/internal/revwalk/BitmappedObjectReachabilityTest.java
@@ -7,11 +7,12 @@
  *
  * SPDX-License-Identifier: BSD-3-Clause
  */
-package org.eclipse.jgit.revwalk;
+package org.eclipse.jgit.internal.revwalk;
 
 import org.eclipse.jgit.internal.storage.file.FileRepository;
 import org.eclipse.jgit.internal.storage.file.GC;
 import org.eclipse.jgit.junit.TestRepository;
+import org.eclipse.jgit.revwalk.ObjectReachabilityChecker;
 
 public class BitmappedObjectReachabilityTest
 	extends ObjectReachabilityTestCase {
diff --git a/org.eclipse.jgit.test/tst/org/eclipse/jgit/revwalk/BitmappedReachabilityCheckerTest.java b/org.eclipse.jgit.test/tst/org/eclipse/jgit/internal/revwalk/BitmappedReachabilityCheckerTest.java
index 2cb88b979e..5833c7a81b 100644
--- a/org.eclipse.jgit.test/tst/org/eclipse/jgit/revwalk/BitmappedReachabilityCheckerTest.java
+++ b/org.eclipse.jgit.test/tst/org/eclipse/jgit/internal/revwalk/BitmappedReachabilityCheckerTest.java
@@ -7,13 +7,14 @@
  *
  * SPDX-License-Identifier: BSD-3-Clause
  */
-package org.eclipse.jgit.revwalk;
+package org.eclipse.jgit.internal.revwalk;
 
 import static org.junit.Assert.assertNotNull;
 
 import org.eclipse.jgit.internal.storage.file.FileRepository;
 import org.eclipse.jgit.internal.storage.file.GC;
 import org.eclipse.jgit.junit.TestRepository;
+import org.eclipse.jgit.revwalk.ReachabilityChecker;
 
 public class BitmappedReachabilityCheckerTest
 		extends ReachabilityCheckerTestCase {
diff --git a/org.eclipse.jgit.test/tst/org/eclipse/jgit/revwalk/ObjectReachabilityTestCase.java b/org.eclipse.jgit.test/tst/org/eclipse/jgit/internal/revwalk/ObjectReachabilityTestCase.java
index 267b163f43..37ff40bdf7 100644
--- a/org.eclipse.jgit.test/tst/org/eclipse/jgit/revwalk/ObjectReachabilityTestCase.java
+++ b/org.eclipse.jgit.test/tst/org/eclipse/jgit/internal/revwalk/ObjectReachabilityTestCase.java
@@ -7,7 +7,7 @@
  *
  * SPDX-License-Identifier: BSD-3-Clause
  */
-package org.eclipse.jgit.revwalk;
+package org.eclipse.jgit.internal.revwalk;
 
 import static org.junit.Assert.assertFalse;
 import static org.junit.Assert.assertTrue;
@@ -20,6 +20,10 @@ import org.eclipse.jgit.internal.storage.file.FileRepository;
 import org.eclipse.jgit.junit.LocalDiskRepositoryTestCase;
 import org.eclipse.jgit.junit.TestRepository;
 import org.eclipse.jgit.junit.TestRepository.CommitBuilder;
+import org.eclipse.jgit.revwalk.ObjectReachabilityChecker;
+import org.eclipse.jgit.revwalk.RevBlob;
+import org.eclipse.jgit.revwalk.RevCommit;
+import org.eclipse.jgit.revwalk.RevObject;
 import org.junit.Before;
 import org.junit.Test;
 
diff --git a/org.eclipse.jgit.test/tst/org/eclipse/jgit/revwalk/PedestrianObjectReachabilityTest.java b/org.eclipse.jgit.test/tst/org/eclipse/jgit/internal/revwalk/PedestrianObjectReachabilityTest.java
index b1c9556df8..f06a768340 100644
--- a/org.eclipse.jgit.test/tst/org/eclipse/jgit/revwalk/PedestrianObjectReachabilityTest.java
+++ b/org.eclipse.jgit.test/tst/org/eclipse/jgit/internal/revwalk/PedestrianObjectReachabilityTest.java
@@ -7,10 +7,11 @@
  *
  * SPDX-License-Identifier: BSD-3-Clause
  */
-package org.eclipse.jgit.revwalk;
+package org.eclipse.jgit.internal.revwalk;
 
 import org.eclipse.jgit.internal.storage.file.FileRepository;
 import org.eclipse.jgit.junit.TestRepository;
+import org.eclipse.jgit.revwalk.ObjectReachabilityChecker;
 
 public class PedestrianObjectReachabilityTest
 		extends ObjectReachabilityTestCase {
diff --git a/org.eclipse.jgit.test/tst/org/eclipse/jgit/revwalk/PedestrianReachabilityCheckerTest.java b/org.eclipse.jgit.test/tst/org/eclipse/jgit/internal/revwalk/PedestrianReachabilityCheckerTest.java
index 3029e056eb..f9d4e182a4 100644
--- a/org.eclipse.jgit.test/tst/org/eclipse/jgit/revwalk/PedestrianReachabilityCheckerTest.java
+++ b/org.eclipse.jgit.test/tst/org/eclipse/jgit/internal/revwalk/PedestrianReachabilityCheckerTest.java
@@ -7,10 +7,11 @@
  *
  * SPDX-License-Identifier: BSD-3-Clause
  */
-package org.eclipse.jgit.revwalk;
+package org.eclipse.jgit.internal.revwalk;
 
 import org.eclipse.jgit.internal.storage.file.FileRepository;
 import org.eclipse.jgit.junit.TestRepository;
+import org.eclipse.jgit.revwalk.ReachabilityChecker;
 
 public class PedestrianReachabilityCheckerTest
 		extends ReachabilityCheckerTestCase {
diff --git a/org.eclipse.jgit.test/tst/org/eclipse/jgit/revwalk/ReachabilityCheckerTestCase.java b/org.eclipse.jgit.test/tst/org/eclipse/jgit/internal/revwalk/ReachabilityCheckerTestCase.java
index 4695eaa695..1ff6e7defb 100644
--- a/org.eclipse.jgit.test/tst/org/eclipse/jgit/revwalk/ReachabilityCheckerTestCase.java
+++ b/org.eclipse.jgit.test/tst/org/eclipse/jgit/internal/revwalk/ReachabilityCheckerTestCase.java
@@ -7,7 +7,7 @@
  *
  * SPDX-License-Identifier: BSD-3-Clause
  */
-package org.eclipse.jgit.revwalk;
+package org.eclipse.jgit.internal.revwalk;
 
 import static org.junit.Assert.assertFalse;
 import static org.junit.Assert.assertTrue;
@@ -19,6 +19,8 @@ import java.util.stream.Stream;
 import org.eclipse.jgit.internal.storage.file.FileRepository;
 import org.eclipse.jgit.junit.LocalDiskRepositoryTestCase;
 import org.eclipse.jgit.junit.TestRepository;
+import org.eclipse.jgit.revwalk.ReachabilityChecker;
+import org.eclipse.jgit.revwalk.RevCommit;
 import org.junit.Before;
 import org.junit.Test;