Clean up ib_sequence::m_max_value

Correctly document the usage of m_max_value. Remove the const
qualifier, so that the implicit assignment operator can be used.
Make all members of ib_sequence private, and add an accessor
member function max_value().

storage/innobase/handler/handler0alter.cc

@@ -2721,7 +2721,7 @@ prepare_inplace_alter_table_dict(
 		(ha_alter_info->handler_ctx);
 
 	DBUG_ASSERT((ctx->add_autoinc != ULINT_UNDEFINED)
-		    == (ctx->sequence.m_max_value > 0));
+		    == (ctx->sequence.max_value() > 0));
 	DBUG_ASSERT(!ctx->num_to_drop_index == !ctx->drop_index);
 	DBUG_ASSERT(!ctx->num_to_drop_fk == !ctx->drop_fk);
 	DBUG_ASSERT(!add_fts_doc_id || add_fts_doc_id_idx);

storage/innobase/include/handler0alter.h

 /*****************************************************************************
 
 Copyright (c) 2005, 2016, Oracle and/or its affiliates. All Rights Reserved.
+Copyright (c) 2019, MariaDB Corporation.
 
 This program is free software; you can redistribute it and/or modify it under
 the terms of the GNU General Public License as published by the Free Software
@@ -96,9 +97,13 @@ struct ib_sequence_t {
 		return(m_next_value);
 	}
 
-	/** Maximum calumn value if adding an AUTOINC column else 0. Once
-	we reach the end of the sequence it will be set to ~0. */
-	const ulonglong	m_max_value;
+	/** @return maximum column value
+	@retval	0	if not adding AUTO_INCREMENT column */
+	ulonglong max_value() const { return m_max_value; }
+
+private:
+	/** Maximum value if adding an AUTO_INCREMENT column, else 0 */
+	ulonglong	m_max_value;
 
 	/** Value of auto_increment_increment */
 	ulong		m_increment;

storage/xtradb/handler/handler0alter.cc

@@ -2727,7 +2727,7 @@ prepare_inplace_alter_table_dict(
 		(ha_alter_info->handler_ctx);
 
 	DBUG_ASSERT((ctx->add_autoinc != ULINT_UNDEFINED)
-		    == (ctx->sequence.m_max_value > 0));
+		    == (ctx->sequence.max_value() > 0));
 	DBUG_ASSERT(!ctx->num_to_drop_index == !ctx->drop_index);
 	DBUG_ASSERT(!ctx->num_to_drop_fk == !ctx->drop_fk);
 	DBUG_ASSERT(!add_fts_doc_id || add_fts_doc_id_idx);

storage/xtradb/include/handler0alter.h

 /*****************************************************************************
 
 Copyright (c) 2005, 2016, Oracle and/or its affiliates. All Rights Reserved.
+Copyright (c) 2019, MariaDB Corporation.
 
 This program is free software; you can redistribute it and/or modify it under
 the terms of the GNU General Public License as published by the Free Software
@@ -96,9 +97,13 @@ struct ib_sequence_t {
 		return(m_next_value);
 	}
 
-	/** Maximum calumn value if adding an AUTOINC column else 0. Once
-	we reach the end of the sequence it will be set to ~0. */
-	const ulonglong	m_max_value;
+	/** @return maximum column value
+	@retval	0	if not adding AUTO_INCREMENT column */
+	ulonglong max_value() const { return m_max_value; }
+
+private:
+	/** Maximum value if adding an AUTO_INCREMENT column, else 0 */
+	ulonglong	m_max_value;
 
 	/** Value of auto_increment_increment */
 	ulong		m_increment;