Add getCacheKey() to CurrentUser and its implementations

getCacheKey() can be used when grouping objects by user in caches. The
first user of this new method is a PerThreadCache added in a later commit.

Change-Id: I00a9c1528e2fdd2219f511ba749074efe9d967b6

java/com/google/gerrit/server/AnonymousUser.java

@@ -26,6 +26,12 @@ public class AnonymousUser extends CurrentUser {
     return new ListGroupMembership(Collections.singleton(SystemGroupBackend.ANONYMOUS_USERS));
   }
 
+  @Override
+  public Object getCacheKey() {
+    // Treat all anonymous users as a single user
+    return "anonymous";
+  }
+
   @Override
   public String toString() {
     return "ANONYMOUS";

java/com/google/gerrit/server/CurrentUser.java

@@ -90,6 +90,12 @@ public abstract class CurrentUser {
    */
   public abstract GroupMembership getEffectiveGroups();
 
+  /**
+   * Returns a unique identifier for this user that is intended to be used as a cache key. Returned
+   * object should to implement {@code equals()} and {@code hashCode()} for effective caching.
+   */
+  public abstract Object getCacheKey();
+
   /** Unique name of the user on this server, if one has been assigned. */
   public Optional<String> getUserName() {
     return Optional.empty();

java/com/google/gerrit/server/IdentifiedUser.java

@@ -382,6 +382,11 @@ public class IdentifiedUser extends CurrentUser {
     return effectiveGroups;
   }
 
+  @Override
+  public Object getCacheKey() {
+    return getAccountId();
+  }
+
   public PersonIdent newRefLogIdent() {
     return newRefLogIdent(new Date(), TimeZone.getDefault());
   }

java/com/google/gerrit/server/InternalUser.java

@@ -35,6 +35,11 @@ public class InternalUser extends CurrentUser {
     return GroupMembership.EMPTY;
   }
 
+  @Override
+  public String getCacheKey() {
+    return "internal";
+  }
+
   @Override
   public boolean isInternalUser() {
     return true;

java/com/google/gerrit/server/PeerDaemonUser.java

@@ -40,6 +40,11 @@ public class PeerDaemonUser extends CurrentUser {
     return GroupMembership.EMPTY;
   }
 
+  @Override
+  public Object getCacheKey() {
+    return getRemoteAddress();
+  }
+
   public SocketAddress getRemoteAddress() {
     return peer;
   }

java/com/google/gerrit/server/query/change/SingleGroupUser.java

@@ -36,4 +36,9 @@ public final class SingleGroupUser extends CurrentUser {
   public GroupMembership getEffectiveGroups() {
     return groups;
   }
+
+  @Override
+  public Object getCacheKey() {
+    return groups.getKnownGroups();
+  }
 }

javatests/com/google/gerrit/server/extensions/webui/UiActionsTest.java

@@ -50,6 +50,11 @@ public class UiActionsTest {
           throw new UnsupportedOperationException("not implemented");
         }
 
+        @Override
+        public Object getCacheKey() {
+          return new Object();
+        }
+
         @Override
         public boolean isIdentifiedUser() {
           return true;

javatests/com/google/gerrit/server/permissions/RefControlTest.java

@@ -1012,6 +1012,11 @@ public class RefControlTest {
       return groups;
     }
 
+    @Override
+    public Object getCacheKey() {
+      return new Object();
+    }
+
     @Override
     public Optional<String> getUserName() {
       return Optional.ofNullable(username);