Add REST endpoint to list caches

GET /config/server/caches/ lists the server caches. Caches defined by
plugins are included.

The implementation of the SSH show-caches was adapted so that it uses
the new REST endpoint to get the information about the caches. This
avoids code duplication.

For disk caches a new interface was introduced in gerrit-server. This
is done because the list caches REST endpoint needs extra information
for disk caches which can only be provided by the disk cache, aka
H2CacheImpl. H2CacheImpl is defined in gerrit-cache-h2, but
gerrit-server doesn't have a dependency on it and hence can't use
instanceof to check whether a cache is a disk cache. Adding this
dependency is not wanted and also not possible (because
gerrit-cache-h2 already depends on gerrit-server and we can't have
cycles in the dependencies).

Change-Id: I21bf6e114310f98051a0a8db01012511f58d4add
Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>

gerrit-cache-h2/src/main/java/com/google/gerrit/server/cache/h2/H2CacheImpl.java

@@ -11,6 +11,7 @@ import com.google.common.hash.BloomFilter;
 import com.google.common.hash.Funnel;
 import com.google.common.hash.Funnels;
 import com.google.common.hash.PrimitiveSink;
+import com.google.gerrit.server.cache.PersistentCache;
 import com.google.gerrit.server.util.TimeUtil;
 import com.google.inject.TypeLiteral;
 
@@ -63,7 +64,8 @@ import java.util.concurrent.atomic.AtomicLong;
  *
  * @see H2CacheFactory
  */
-public class H2CacheImpl<K, V> extends AbstractLoadingCache<K, V> {
+public class H2CacheImpl<K, V> extends AbstractLoadingCache<K, V> implements
+    PersistentCache {
   private static final Logger log = LoggerFactory.getLogger(H2CacheImpl.class);
 
   private final Executor executor;
@@ -156,6 +158,7 @@ public class H2CacheImpl<K, V> extends AbstractLoadingCache<K, V> {
     return mem.stats();
   }
 
+  @Override
   public DiskStats diskStats() {
     return store.diskStats();
   }
@@ -193,29 +196,6 @@ public class H2CacheImpl<K, V> extends AbstractLoadingCache<K, V> {
     }, delay, TimeUnit.MILLISECONDS);
   }
 
-  public static class DiskStats {
-    long size;
-    long space;
-    long hitCount;
-    long missCount;
-
-    public long size() {
-      return size;
-    }
-
-    public long space() {
-      return space;
-    }
-
-    public long hitCount() {
-      return hitCount;
-    }
-
-    public long requestCount() {
-      return hitCount + missCount;
-    }
-  }
-
   static class ValueHolder<V> {
     final V value;
     long created;
@@ -599,9 +579,8 @@ public class H2CacheImpl<K, V> extends AbstractLoadingCache<K, V> {
     }
 
     DiskStats diskStats() {
-      DiskStats d = new DiskStats();
-      d.hitCount = hitCount.get();
-      d.missCount = missCount.get();
+      long size = 0;
+      long space = 0;
       SqlHandle c = null;
       try {
         c = acquire();
@@ -613,8 +592,8 @@ public class H2CacheImpl<K, V> extends AbstractLoadingCache<K, V> {
               + " FROM data");
           try {
             if (r.next()) {
-              d.size = r.getLong(1);
-              d.space = r.getLong(2);
+              size = r.getLong(1);
+              space = r.getLong(2);
             }
           } finally {
             r.close();
@@ -628,7 +607,7 @@ public class H2CacheImpl<K, V> extends AbstractLoadingCache<K, V> {
       } finally {
         release(c);
       }
-      return d;
+      return new DiskStats(size, space, hitCount.get(), missCount.get());
     }
 
     private SqlHandle acquire() throws SQLException {