MXS-2834: Make retrying of trx deadlocks configurable

The behavior is not desirable in all cases and should be enabled only if
needed.

Documentation/Routers/ReadWriteSplit.md

@@ -537,6 +537,19 @@ controls how many server and network failures a single transaction replay
 tolerates. If a transaction is replayed successfully, the counter for failed
 attempts is reset.
 
+### `transaction_replay_retry_on_deadlock`
+
+Enable automatic retrying of transactions that end up in a deadlock. This
+parameter was added in MaxScale 2.4.6 and the feature is disabled by
+default. MaxScale versions from 2.4.0 to 2.4.5 always tried to replay deadlocked
+transactions.
+
+If this feature is enabled and a transaction returns a deadlock error
+(e.g. `SQLSTATE 40001: Deadlock found when trying to get lock; try restarting transaction`),
+the transaction is automatically retried. If the retrying of the transaction
+results in another deadlock error, it is retried until it either succeeds or a
+transaction checksum error is encountered.
+
 ### `optimistic_trx`
 
 Enable optimistic transaction execution. This parameter controls whether normal

server/modules/routing/readwritesplit/readwritesplit.cc

@@ -520,6 +520,7 @@ extern "C" MXS_MODULE* MXS_CREATE_MODULE()
             {"transaction_replay",         MXS_MODULE_PARAM_BOOL,      "false"},
             {"transaction_replay_max_size",MXS_MODULE_PARAM_SIZE,      "1Mi"},
             {"transaction_replay_attempts",MXS_MODULE_PARAM_COUNT,     "5"},
+            {"transaction_replay_retry_on_deadlock",MXS_MODULE_PARAM_BOOL,      "false"},
             {"optimistic_trx",             MXS_MODULE_PARAM_BOOL,      "false"},
             {"lazy_connect",               MXS_MODULE_PARAM_BOOL,      "false"},
             {MXS_END_MODULE_PARAMS}

server/modules/routing/readwritesplit/readwritesplit.hh

@@ -161,6 +161,7 @@ struct Config
         , transaction_replay(params->get_bool("transaction_replay"))
         , trx_max_size(params->get_size("transaction_replay_max_size"))
         , trx_max_attempts(params->get_integer("transaction_replay_attempts"))
+        , trx_retry_on_deadlock(params->get_bool("transaction_replay_retry_on_deadlock"))
         , optimistic_trx(params->get_bool("optimistic_trx"))
         , lazy_connect(params->get_bool("lazy_connect"))
     {
@@ -225,6 +226,7 @@ struct Config
     bool        transaction_replay;     /**< Replay failed transactions */
     size_t      trx_max_size;           /**< Max transaction size for replaying */
     int64_t     trx_max_attempts;       /**< Maximum number of transaction replay attempts */
+    bool        trx_retry_on_deadlock;  /**< Replay the transaction if it ends up in a deadlock */
     bool        optimistic_trx;         /**< Enable optimistic transactions */
     bool        lazy_connect;           /**< Create connections only when needed */
 };

server/modules/routing/readwritesplit/rwsplitsession.cc

@@ -709,7 +709,8 @@ void RWSplitSession::clientReply(GWBUF* writebuf, DCB* backend_dcb)
         }
     }
 
-    if ((error.is_rollback() || error.is_wsrep_error()) && handle_ignorable_error(backend))
+    if (((m_config.trx_retry_on_deadlock && error.is_rollback()) || error.is_wsrep_error())
+        && handle_ignorable_error(backend))
     {
         // We can ignore this error and treat it as if the connection to the server was broken.
         gwbuf_free(writebuf);