Changed background_rollback to return true/false depending on if

background rollback did happen.
Background rollback should be skipped if the aborting happens due to KILL command issued by user.
Some KILL signals, like KILL CONECTION, wake up the victim too early so that background rollback
could happen in parallel with the victim waking up and continuing execution.

Author: sjaakola

dbsim/db_server_service.cpp

@@ -86,8 +86,9 @@ int db::server_service::start_sst(
     return 0;
 }
 
-void db::server_service::background_rollback(wsrep::client_state&)
+bool db::server_service::background_rollback(wsrep::client_state&)
 {
+    return false;
 }
 
 void db::server_service::bootstrap()

dbsim/db_server_service.hpp

@@ -41,7 +41,7 @@ namespace db
         bool sst_before_init() const override;
         int start_sst(const std::string&, const wsrep::gtid&, bool) override;
         std::string sst_request() override;
-        void background_rollback(wsrep::client_state&) override;
+        bool background_rollback(wsrep::client_state&) override;
         void bootstrap() override;
         void log_message(enum wsrep::log::level, const char* message) override;
         void log_dummy_write_set(wsrep::client_state&, const wsrep::ws_meta&)

include/wsrep/server_service.hpp

@@ -86,7 +86,7 @@ namespace wsrep
         /**
          * Perform a background rollback for a transaction.
          */
-        virtual void background_rollback(wsrep::client_state&) = 0;
+        virtual bool background_rollback(wsrep::client_state&) = 0;
 
         /**
          * Bootstrap a DBMS state for a new cluster.

src/transaction.cpp

@@ -1074,7 +1074,9 @@ bool wsrep::transaction::bf_abort(
             }
 
             lock.unlock();
-            server_service_.background_rollback(client_state_);
+            /* if background rollback is skipped, reset rollbacker activity */
+            if (server_service_.background_rollback(client_state_))
+	        client_state_.set_rollbacker_active(false);
             lock.lock();
         }
     }

test/mock_server_state.hpp

@@ -183,11 +183,12 @@ namespace wsrep
             return start_sst_action();
         }
 
-        void
+        bool
         background_rollback(wsrep::client_state& client_state) WSREP_OVERRIDE
         {
             client_state.before_rollback();
             client_state.after_rollback();
+            return false;
         }
 
         int wait_committing_transactions(int) WSREP_OVERRIDE { return 0; }