Allow to use an external ExecutorService for background auto-gc

If set use the external executor, otherwise use JGit's own simple
WorkQueue. Move WorkQueue to an internal package so we can reuse it
without exposing it in the public API.

Change-Id: I060d62ffd6692362a88b4bf13ee07b0dc857abe9
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

org.eclipse.jgit/META-INF/MANIFEST.MF

@@ -85,6 +85,7 @@ Export-Package: org.eclipse.jgit.annotations;version="4.8.0",
    org.eclipse.jgit.treewalk,
    org.eclipse.jgit.transport,
    org.eclipse.jgit.submodule",
+ org.eclipse.jgit.lib.internal;version="4.8.0";x-internal:=true,
  org.eclipse.jgit.merge;version="4.8.0";
   uses:="org.eclipse.jgit.lib,
    org.eclipse.jgit.treewalk,

org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/GC.java

@@ -78,7 +78,6 @@ import java.util.TreeMap;
 import java.util.concurrent.Callable;
 import java.util.concurrent.ExecutionException;
 import java.util.concurrent.ExecutorService;
-import java.util.concurrent.Executors;
 import java.util.concurrent.Future;
 import java.util.regex.Pattern;
 import java.util.stream.Collectors;
@@ -107,6 +106,7 @@ import org.eclipse.jgit.lib.ObjectReader;
 import org.eclipse.jgit.lib.ProgressMonitor;
 import org.eclipse.jgit.lib.Ref;
 import org.eclipse.jgit.lib.Ref.Storage;
+import org.eclipse.jgit.lib.internal.WorkQueue;
 import org.eclipse.jgit.lib.RefDatabase;
 import org.eclipse.jgit.lib.ReflogEntry;
 import org.eclipse.jgit.lib.ReflogReader;
@@ -151,7 +151,19 @@ public class GC {
 
 	private static final int DEFAULT_AUTOLIMIT = 6700;
 
-	private static ExecutorService executor = Executors.newFixedThreadPool(1);
+	private static volatile ExecutorService executor;
+
+	/**
+	 * Set the executor for running auto-gc in the background. If no executor is
+	 * set JGit's own WorkQueue will be used.
+	 *
+	 * @param e
+	 *            the executor to be used for running auto-gc
+	 * @since 4.8
+	 */
+	public static void setExecutor(ExecutorService e) {
+		executor = e;
+	}
 
 	private final FileRepository repo;
 
@@ -271,7 +283,7 @@ public class GC {
 			}
 			return Collections.emptyList();
 		};
-		Future<Collection<PackFile>> result = executor.submit(gcTask);
+		Future<Collection<PackFile>> result = executor().submit(gcTask);
 		if (background) {
 			// TODO(ms): in 5.0 change signature and return the Future
 			return Collections.emptyList();
@@ -283,6 +295,10 @@ public class GC {
 		}
 	}
 
+	private ExecutorService executor() {
+		return (executor != null) ? executor : WorkQueue.getExecutor();
+	}
+
 	private Collection<PackFile> doGc() throws IOException, ParseException {
 		if (automatic && !needGc()) {
 			return Collections.emptyList();

org.eclipse.jgit/src/org/eclipse/jgit/lib/BatchingProgressMonitor.java

@@ -46,6 +46,8 @@ package org.eclipse.jgit.lib;
 import java.util.concurrent.Future;
 import java.util.concurrent.TimeUnit;
 
+import org.eclipse.jgit.lib.internal.WorkQueue;
+
 /** ProgressMonitor that batches update events. */
 public abstract class BatchingProgressMonitor implements ProgressMonitor {
 	private long delayStartTime;

org.eclipse.jgit/src/org/eclipse/jgit/lib/RepositoryCache.java

@@ -55,6 +55,7 @@ import java.util.concurrent.TimeUnit;
 import org.eclipse.jgit.annotations.NonNull;
 import org.eclipse.jgit.errors.RepositoryNotFoundException;
 import org.eclipse.jgit.internal.storage.file.FileRepository;
+import org.eclipse.jgit.lib.internal.WorkQueue;
 import org.eclipse.jgit.util.FS;
 import org.eclipse.jgit.util.IO;
 import org.eclipse.jgit.util.RawParseUtils;

org.eclipse.jgit/src/org/eclipse/jgit/lib/internal/WorkQueue.java

@@ -41,7 +41,7 @@
  * ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
-package org.eclipse.jgit.lib;
+package org.eclipse.jgit.lib.internal;
 
 import java.util.concurrent.Executors;
 import java.util.concurrent.ScheduledThreadPoolExecutor;
@@ -50,7 +50,7 @@ import java.util.concurrent.ThreadFactory;
 /**
  * Simple work queue to run tasks in the background
  */
-class WorkQueue {
+public class WorkQueue {
 	private static final ScheduledThreadPoolExecutor executor;
 
 	static final Object executorKiller;
@@ -94,7 +94,10 @@ class WorkQueue {
 		};
 	}
 
-	static ScheduledThreadPoolExecutor getExecutor() {
+	/**
+	 * @return the WorkQueue's executor
+	 */
+	public static ScheduledThreadPoolExecutor getExecutor() {
 		return executor;
 	}
 }