Fixes according to review comments

Author: sjaakola

include/wsrep/server_service.hpp

@@ -85,6 +85,7 @@ namespace wsrep
 
         /**
          * Perform a background rollback for a transaction.
+         * @return true if rollbacker was not started, false otherwise
          */
         virtual bool background_rollback(wsrep::client_state&) = 0;
 

src/transaction.cpp

@@ -1076,8 +1076,17 @@ bool wsrep::transaction::bf_abort(
             lock.unlock();
             /* if background rollback is skipped, reset rollbacker activity */
             if (server_service_.background_rollback(client_state_))
-	        client_state_.set_rollbacker_active(false);
-            lock.lock();
+            {
+                lock.lock();
+                client_state_.set_rollbacker_active(false);
+
+                /* release the victim from waiting, if it has advanced to
+                   wait_rollback_complete_and_acquire_ownership stage */
+                if (client_state_.state() == wsrep::client_state::s_idle)
+                  client_state_.cond_.notify_all();
+            }
+            else
+                lock.lock();
         }
     }
     return ret;