Persist the ChangeKindCache

What kind a patch set is relative to the previous patch set is
basically immutable, given the project config and merge strategy name.
(It could conceivably change if the underlying JGit merge algorithm
changes, but hopefully we would notice and bump the
serialVersionUID of the Key and recommend people flush their caches.)

Change-Id: Ib95cf23e39a3b43dbb38901b52ef7bd71da87f41

gerrit-server/src/main/java/com/google/gerrit/server/change/ChangeKindCache.java

@@ -15,10 +15,13 @@
 package com.google.gerrit.server.change;
 
 import static com.google.common.base.Preconditions.checkNotNull;
+import static org.eclipse.jgit.lib.ObjectIdSerialization.readNotNull;
+import static org.eclipse.jgit.lib.ObjectIdSerialization.writeNotNull;
 
 import com.google.common.base.Objects;
 import com.google.common.cache.CacheLoader;
 import com.google.common.cache.LoadingCache;
+import com.google.common.cache.Weigher;
 import com.google.gerrit.server.cache.CacheModule;
 import com.google.gerrit.server.config.GerritServerConfig;
 import com.google.gerrit.server.git.MergeUtil;
@@ -38,6 +41,8 @@ import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
 import java.io.IOException;
+import java.io.ObjectInputStream;
+import java.io.ObjectOutputStream;
 import java.io.Serializable;
 import java.util.concurrent.ExecutionException;
 
@@ -57,11 +62,10 @@ public class ChangeKindCache {
     return new CacheModule() {
       @Override
       protected void configure() {
-        cache(ID_CACHE,
-            Key.class,
-            ChangeKind.class)
-          .maximumWeight(0)
-          .loader(Loader.class);
+        persist(ID_CACHE, Key.class, ChangeKind.class)
+            .maximumWeight(2 << 20)
+            .weigher(ChangeKindWeigher.class)
+            .loader(Loader.class);
       }
     };
   }
@@ -69,10 +73,11 @@ public class ChangeKindCache {
   public static class Key implements Serializable {
     private static final long serialVersionUID = 1L;
 
-    private final ObjectId prior;
-    private final ObjectId next;
-    private final String strategyName;
-    private transient Repository repo;
+    private transient ObjectId prior;
+    private transient ObjectId next;
+    private transient String strategyName;
+
+    private transient Repository repo; // Passed through to loader on miss.
 
     private Key(ObjectId prior, ObjectId next, String strategyName,
         Repository repo) {
@@ -109,6 +114,18 @@ public class ChangeKindCache {
     public int hashCode() {
       return Objects.hashCode(prior, next, strategyName);
     }
+
+    private void writeObject(ObjectOutputStream out) throws IOException {
+      writeNotNull(out, prior);
+      writeNotNull(out, next);
+      out.writeUTF(strategyName);
+    }
+
+    private void readObject(ObjectInputStream in) throws IOException {
+      prior = readNotNull(in);
+      next = readNotNull(in);
+      strategyName = in.readUTF();
+    }
   }
 
   @Singleton
@@ -168,6 +185,14 @@ public class ChangeKindCache {
     }
   }
 
+  public static class ChangeKindWeigher implements Weigher<Key, ChangeKind> {
+    @Override
+    public int weigh(Key key, ChangeKind changeKind) {
+      return 16 + 2*36 + 2*key.strategyName.length() // Size of Key, 64 bit JVM
+          + 2*changeKind.name().length(); // Size of ChangeKind, 64 bit JVM
+    }
+  }
+
   private final LoadingCache<Key, ChangeKind> cache;
   private final boolean useRecursiveMerge;