BUG#21798355 SLOW STARTUP OF 5.7.X SLAVE WITH RELAY_LOG_RECOVERY = ON

             AND MANY RELAY LOGS

Problem
-------

The slave initialization was scanning relay log files to update the
Retrieved_Gtid_Set and the transaction parser state even when the
relay_log_recovery option was set.

Analysis
--------

During the slave initialization, it opens the relay log to compute the
Retrieved_Gtid_Set and to set the transaction parser in the correct
state in order to keep track of already replicated GTID sets.

The initialization procedure was performing the above actions even when
the relay_log_recovery option was set.

When the relay_log_recovery option is set, the slave I/O thread should
not use the current relay log files and will base it initial state on
the SQL thread state. If the I/O thread will not rely on relay log
files, it is a waste of time to compute the Retrieved_Gtid_Set and the
transaction parser state when the relay_log_recovery option is set.

Fix
---

The Retrieved_Gtid_Set and the transaction parser states are now
initialized only if the relay_log_recovery option is not set.

Author: Joao Gramacho

mysql-test/suite/rpl/r/rpl_trx_boundary_parser_relay_log_recovery.result

@@ -0,0 +1,20 @@
+include/master-slave.inc
+Warnings:
+Note	####	Sending passwords in plain text without SSL/TLS is extremely insecure.
+Note	####	Storing MySQL user name or password information in the master info repository is not secure and is therefore not recommended. Please consider using the USER and PASSWORD connection options for START SLAVE; see the 'START SLAVE Syntax' in the MySQL Manual for more information.
+[connection master]
+CALL mtr.add_suppression('Recovery from master pos');
+CREATE TABLE t1 (c1 INT);
+INSERT INTO t1 VALUES (1);
+FLUSH LOCAL BINARY LOGS;
+INSERT INTO t1 VALUES (1);
+DROP TABLE t1;
+[connection slave]
+include/start_slave_io.inc
+[connection master]
+include/sync_slave_io_with_master.inc
+include/rpl_restart_server.inc [server_number=2 parameters: --relay_log_recovery=ON]
+include/assert_grep.inc [Found no lines about reading events on the relay log.]
+[connection slave]
+include/start_slave.inc
+include/rpl_end.inc

mysql-test/suite/rpl/t/rpl_trx_boundary_parser_relay_log_recovery.test

@@ -0,0 +1,52 @@
+# ==== Purpose ====
+#
+# This test will verify if a slave will skip the transaction boundary parser
+# and the Retrieved_Gtid_Set initialization when relay_log_recovery is set.
+#
+# ==== Related Bugs and Worklogs ====
+#
+# BUG#21798355 SLOW STARTUP OF 5.7.X SLAVE WITH RELAY_LOG_RECOVERY = ON
+#              AND MANY RELAY LOGS
+
+# This test case is binary log format agnostic
+--source include/have_binlog_format_row.inc
+--let $rpl_skip_start_slave=1
+--source include/master-slave.inc
+# Suppress recovery warning messages
+CALL mtr.add_suppression('Recovery from master pos');
+
+# Insert some content on the master
+CREATE TABLE t1 (c1 INT);
+INSERT INTO t1 VALUES (1);
+
+# Rotate the binary log to have events on more than one relay log file
+FLUSH LOCAL BINARY LOGS;
+INSERT INTO t1 VALUES (1);
+DROP TABLE t1;
+
+# Start and sync the I/O thread before restarting the slave
+--source include/rpl_connection_slave.inc
+--source include/start_slave_io.inc
+--source include/rpl_connection_master.inc
+--source include/sync_slave_io_with_master.inc
+
+# Restart the slave enabling relay_log_recovery
+--let $rpl_server_number= 2
+--let $rpl_server_parameters= --relay_log_recovery=ON
+--source include/rpl_restart_server.inc
+
+# File to GREP
+--let $assert_file=$MYSQLTEST_VARDIR/log/mysqld.2.err
+# Show entries only after the last occurrence of the following pattern
+--let $assert_only_after=.* \[Note\] Shutting down slave threads
+# Assert that there is no expected note log line about reading relay log events
+--let $assert_count= 0
+--let $assert_select=.* \[Note\] .* events read in relaylog file .*
+--let $assert_text= Found no lines about reading events on the relay log.
+--source include/assert_grep.inc
+
+# Cleanup
+--source include/rpl_connection_slave.inc
+--source include/start_slave.inc
+
+--source include/rpl_end.inc

sql/rpl_rli.cc

@@ -2096,8 +2096,13 @@ a file name for --relay-log-index option.", opt_relaylog_index_name);
       be useful to ensure the Retrieved_Gtid_Set behavior when auto
       positioning is disabled (we could have transactions spanning multiple
       relay log files in this case).
+      We will skip this initialization if relay_log_recovery is set in order
+      to save time, as neither the GTIDs nor the transaction_parser state
+      would be useful when the relay log will be cleaned up later when calling
+      init_recovery.
     */
-    if (!gtid_retrieved_initialized &&
+    if (!is_relay_log_recovery &&
+        !gtid_retrieved_initialized &&
         relay_log.init_gtid_sets(&gtid_set, NULL,
                                  opt_slave_sql_verify_checksum,
                                  true/*true=need lock*/,