Bug#21628087 innodb_log_checkpoint_now not fully compatible with WL#7142

In debug builds of MySQL, there is an configuration variable
that allows an InnoDB log checkpoint to be initiated:

SET GLOBAL innodb_log_checkpoint_now=ON;

Setting this variable while a table-rebuilding ALTER TABLE is executing
may result in an infinite loop.

checkpoint_now_set(): Account for log_sys->append_on_checkpoint->size().
Note that this function contains race conditions, because it is accessing
fields of log_sys without holding log_sys->mutex. We think that this is
acceptable, because this variable only exists for debugging purposes, in
debug builds of MySQL.

RB: 9947
Reviewed-by: Sunny Bains <sunny.bains@oracle.com>

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

+CREATE TABLE t1 (x INT NOT NULL UNIQUE KEY) ENGINE=InnoDB;
+INSERT INTO t1 VALUES(5);
+SET GLOBAL innodb_log_checkpoint_now=TRUE;
+SET DEBUG_SYNC='commit_cache_rebuild SIGNAL ready WAIT_FOR finish';
+ALTER TABLE t1 ADD PRIMARY KEY(x);
+SET DEBUG_SYNC='now WAIT_FOR ready';
+SET GLOBAL innodb_log_checkpoint_now=TRUE;
+SET DEBUG_SYNC='now SIGNAL finish';
+SHOW CREATE TABLE t1;
+Table	Create Table
+t1	CREATE TABLE `t1` (
+  `x` int(11) NOT NULL,
+  PRIMARY KEY (`x`),
+  UNIQUE KEY `x` (`x`)
+) ENGINE=InnoDB DEFAULT CHARSET=latin1
+DROP TABLE t1;
+SET DEBUG_SYNC='RESET';

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

+--source include/have_debug.inc
+--source include/have_debug_sync.inc
+--source include/have_innodb.inc
+--source include/count_sessions.inc
+
+CREATE TABLE t1 (x INT NOT NULL UNIQUE KEY) ENGINE=InnoDB;
+INSERT INTO t1 VALUES(5);
+
+SET GLOBAL innodb_log_checkpoint_now=TRUE;
+
+# Start an ALTER TABLE and stop it before renaming the files
+SET DEBUG_SYNC='commit_cache_rebuild SIGNAL ready WAIT_FOR finish';
+
+--send ALTER TABLE t1 ADD PRIMARY KEY(x)
+
+connect (con1,localhost,root,,);
+
+SET DEBUG_SYNC='now WAIT_FOR ready';
+
+SET GLOBAL innodb_log_checkpoint_now=TRUE;
+
+SET DEBUG_SYNC='now SIGNAL finish';
+
+disconnect con1;
+connection default;
+reap;
+SHOW CREATE TABLE t1;
+DROP TABLE t1;
+SET DEBUG_SYNC='RESET';
+
+--source include/wait_until_count_sessions.inc