Expose the list of pack files in the DfsBlockCache

Callers may want to inspect the contents of the cache, which this allows
them to do in a read-only fashion without any locking.

Change-Id: Ifd78e8ce34e26e5cc33e9dd61d70c593ce479ee0

org.eclipse.jgit/src/org/eclipse/jgit/storage/dfs/DfsBlockCache.java

 
 import java.io.IOException;
 import java.util.ArrayList;
-import java.util.HashMap;
+import java.util.Collection;
+import java.util.Collections;
 import java.util.List;
 import java.util.Map;
+import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.ThreadPoolExecutor;
 import java.util.concurrent.atomic.AtomicLong;
 import java.util.concurrent.atomic.AtomicReferenceArray;
 	/** Cache of pack files, indexed by description. */
 	private final Map<DfsPackDescription, DfsPackFile> packCache;
 
+	/** View of pack files in the pack cache. */
+	private final Collection<DfsPackFile> packFiles;
+
 	/** Number of times a block was found in the cache. */
 	private final AtomicLong statHit;
 
 		readAheadLimit = cfg.getReadAheadLimit();
 		readAheadService = cfg.getReadAheadService();
 
-		packCache = new HashMap<DfsPackDescription, DfsPackFile>();
+		packCache = new ConcurrentHashMap<DfsPackDescription, DfsPackFile>(
+				16, 0.75f, 1);
+		packFiles = Collections.unmodifiableCollection(packCache.values());
 
 		statHit = new AtomicLong();
 		statMiss = new AtomicLong();
 		return statEvict;
 	}
 
+	/**
+	 * Get the pack files stored in this cache.
+	 *
+	 * @return a collection of pack files, some of which may not actually be
+	 *             present; the caller should check the pack's cached size.
+	 */
+	public Collection<DfsPackFile> getPackFiles() {
+		return packFiles;
+	}
+
 	DfsPackFile getOrCreate(DfsPackDescription dsc, DfsPackKey key) {
 		// TODO This table grows without bound. It needs to clean up
 		// entries that aren't in cache anymore, and aren't being used