MDEV-12061 Allow innodb_log_files_in_group=1

The InnoDB redo log consists of a list of files that logically form
a bigger file, as if the individual files were concatenated together.

The first file will always be written on redo log checkpoint, because
the two checkpoint pages are at the start of the single logical
redo log file.

There is no technical reason why InnoDB requires at least 2 files
to exist. Let us reduce the minimum number to 1. In that way,
restoring from backups will become easier, since InnoDB can directly
deal with a single backed-up redo log file.

Author: dr-m

mysql-test/suite/innodb/r/log_file.result

@@ -335,50 +335,12 @@ ibdata2
 undo001
 undo002
 undo003
-SELECT * FROM INFORMATION_SCHEMA.ENGINES
-WHERE engine = 'innodb'
-AND support IN ('YES', 'DEFAULT', 'ENABLED');
-ENGINE	SUPPORT	COMMENT	TRANSACTIONS	XA	SAVEPOINTS
-FOUND /Only one log file found/ in mysqld.1.err
-bak_ib_logfile0
-bak_ib_logfile1
-bak_ib_logfile2
-bak_ibdata1
-bak_ibdata2
-bak_undo001
-bak_undo002
-bak_undo003
-ib_buffer_pool
-ib_logfile0
-ib_logfile2
-ibdata1
-ibdata2
-undo001
-undo002
-undo003
-# 12. With ibdata*, without ib_logfile2
-bak_ib_logfile0
-bak_ib_logfile1
-bak_ib_logfile2
-bak_ibdata1
-bak_ibdata2
-bak_undo001
-bak_undo002
-bak_undo003
-ib_buffer_pool
-ib_logfile0
-ib_logfile1
-ibdata1
-ibdata2
-undo001
-undo002
-undo003
 SELECT COUNT(*) `1` FROM INFORMATION_SCHEMA.ENGINES
 WHERE engine='innodb'
 AND support IN ('YES', 'DEFAULT', 'ENABLED');
 1
 1
-FOUND /Resizing redo log from 2\*\d+ to 3\*\d+ pages, LSN=\d+/ in mysqld.1.err
+FOUND /Resizing redo log from 1\*\d+ to 3\*\d+ pages, LSN=\d+/ in mysqld.1.err
 # Cleanup
 bak_ib_logfile0
 bak_ib_logfile1

mysql-test/suite/innodb/r/log_file_size.result

@@ -53,8 +53,6 @@ ERROR 42000: Unknown storage engine 'InnoDB'
 SELECT * FROM t1;
 ERROR 42000: Unknown storage engine 'InnoDB'
 SELECT * FROM t1;
-ERROR 42000: Unknown storage engine 'InnoDB'
-SELECT * FROM t1;
 a
 42
 123

mysql-test/suite/innodb/t/log_file.test

@@ -26,7 +26,7 @@ let bugdir= $MYSQLTEST_VARDIR/tmp/log_file;
 --mkdir $bugdir
 
 let SEARCH_FILE= $MYSQLTEST_VARDIR/log/mysqld.1.err;
-let SEARCH_RANGE = -50000;
+let SEARCH_RANGE = -100000;
 let $check_no_innodb=SELECT * FROM INFORMATION_SCHEMA.ENGINES
 WHERE engine = 'innodb'
 AND support IN ('YES', 'DEFAULT', 'ENABLED');
@@ -217,21 +217,9 @@ eval $check_no_innodb;
 --remove_file $bugdir/ib_logfile1
 --list_files $bugdir
 --source include/start_mysqld.inc
-eval $check_no_innodb;
---source include/shutdown_mysqld.inc
-let SEARCH_PATTERN=Only one log file found;
---source include/search_pattern_in_file.inc
-
-# clean up & Restore
---source ../include/log_file_cleanup.inc
-
---echo # 12. With ibdata*, without ib_logfile2
---remove_file $bugdir/ib_logfile2
---list_files $bugdir
---source include/start_mysqld.inc
 eval $check_yes_innodb;
 --source include/shutdown_mysqld.inc
-let SEARCH_PATTERN=Resizing redo log from 2\*\d+ to 3\*\d+ pages, LSN=\d+;
+--let SEARCH_PATTERN=Resizing redo log from 1\*\d+ to 3\*\d+ pages, LSN=\d+
 --source include/search_pattern_in_file.inc
 
 --let $restart_parameters=

mysql-test/suite/innodb/t/log_file_size.test

@@ -163,25 +163,15 @@ let SEARCH_PATTERN= InnoDB: Setting log file .*ib_logfile[0-9]+ size to;
 
 # We should have perfectly synced files here.
 # Rename the log files, and trigger an error in recovery.
---move_file $MYSQLD_DATADIR/ib_logfile101 $MYSQLD_DATADIR/ib_logfile0
 --move_file $MYSQLD_DATADIR/ib_logfile1 $MYSQLD_DATADIR/ib_logfile1_hidden
 
---let $restart_parameters=
---source include/start_mysqld.inc
---error ER_UNKNOWN_STORAGE_ENGINE
-SELECT * FROM t1;
-
-let SEARCH_PATTERN= InnoDB: Only one log file found;
---source include/search_pattern_in_file.inc
---move_file $MYSQLD_DATADIR/ib_logfile0 $MYSQLD_DATADIR/ib_logfile101
-
 perl;
 die unless open(FILE, ">$ENV{MYSQLD_DATADIR}/ib_logfile0");
 print FILE "garbage";
 close(FILE);
 EOF
 
---source include/restart_mysqld.inc
+--source include/start_mysqld.inc
 --error ER_UNKNOWN_STORAGE_ENGINE
 SELECT * FROM t1;
 let SEARCH_PATTERN= InnoDB: Log file .*ib_logfile0 size 7 is not a multiple of innodb_page_size;

mysql-test/suite/sys_vars/r/sysvars_innodb.result

@@ -1470,7 +1470,7 @@ DEFAULT_VALUE	2
 VARIABLE_SCOPE	GLOBAL
 VARIABLE_TYPE	BIGINT UNSIGNED
 VARIABLE_COMMENT	Number of log files in the log group. InnoDB writes to the files in a circular fashion.
-NUMERIC_MIN_VALUE	2
+NUMERIC_MIN_VALUE	1
 NUMERIC_MAX_VALUE	100
 NUMERIC_BLOCK_SIZE	0
 ENUM_VALUE_LIST	NULL

storage/innobase/handler/ha_innodb.cc

@@ -21265,7 +21265,7 @@ static MYSQL_SYSVAR_LONGLONG(log_file_size, innobase_log_file_size,
 static MYSQL_SYSVAR_ULONG(log_files_in_group, srv_n_log_files,
   PLUGIN_VAR_RQCMDARG | PLUGIN_VAR_READONLY,
   "Number of log files in the log group. InnoDB writes to the files in a circular fashion.",
-  NULL, NULL, 2, 2, SRV_N_LOG_FILES_MAX, 0);
+  NULL, NULL, 2, 1, SRV_N_LOG_FILES_MAX, 0);
 
 static MYSQL_SYSVAR_ULONG(log_write_ahead_size, srv_log_write_ahead_size,
   PLUGIN_VAR_RQCMDARG,

storage/innobase/srv/srv0srv.cc

@@ -205,9 +205,9 @@ extern bool		trx_commit_disallowed;
 #endif /* UNIV_DEBUG */
 
 /*------------------------- LOG FILES ------------------------ */
-char*	srv_log_group_home_dir	= NULL;
+char*	srv_log_group_home_dir;
 
-ulong	srv_n_log_files		= SRV_N_LOG_FILES_MAX;
+ulong	srv_n_log_files;
 /** At startup, this is the current redo log file size.
 During startup, if this is different from srv_log_file_size_requested
 (innodb_log_file_size), the redo log will be rebuilt and this size

storage/innobase/srv/srv0start.cc

@@ -1977,11 +1977,6 @@ innobase_start_or_create_for_mysql(void)
 					crash recovery. */
 					flushed_lsn = log_get_lsn();
 					goto files_checked;
-				} else if (i < 2) {
-					/* must have at least 2 log files */
-					ib::error() << "Only one log file"
-						" found.";
-					return(srv_init_abort(err));
 				}
 
 				/* opened all files */