MDEV-19514: Correct a few outdated comments

There is no background change buffer merge any more.
Change buffer merge will only take place during a slow shutdown
(a shutdown initiated after SET GLOBAL innodb_fast_shutdown=0).

storage/innobase/dict/dict0stats.cc

@@ -3332,16 +3332,18 @@ dict_stats_update(
 	return(DB_SUCCESS);
 }
 
-/*********************************************************************//**
-Removes the information for a particular index's stats from the persistent
+/** Remove the information for a particular index's stats from the persistent
 storage if it exists and if there is data stored for this index.
 This function creates its own trx and commits it.
-A note from Marko why we cannot edit user and sys_* tables in one trx:
-marko: The problem is that ibuf merges should be disabled while we are
-rolling back dict transactions.
-marko: If ibuf merges are not disabled, we need to scan the *.ibd files.
-But we shouldn't open *.ibd files before we have rolled back dict
-transactions and opened the SYS_* records for the *.ibd files.
+
+We must modify system tables in a separate transaction in order to
+adhere to the InnoDB design constraint that dict_sys.latch prevents
+lock waits on system tables. If we modified system and user tables in
+the same transaction, we should exclusively hold dict_sys.latch until
+the transaction is committed, and effectively block other transactions
+that will attempt to open any InnoDB tables. Because we have no
+guarantee that user transactions will be committed fast, we cannot
+afford to keep the system tables locked in a user transaction.
 @return DB_SUCCESS or error code */
 dberr_t
 dict_stats_drop_index(

storage/innobase/fil/fil0fil.cc

@@ -2200,8 +2200,8 @@ fil_close_tablespace(
 
 	/* Invalidate in the buffer pool all pages belonging to the
 	tablespace. Since we have set space->stop_new_ops = true, readahead
-	or ibuf merge can no longer read more pages of this tablespace to the
-	buffer pool. Thus we can clean the tablespace out of the buffer pool
+	can no longer read more pages of this tablespace to buf_pool.
+	Thus we can clean the tablespace out of buf_pool
 	completely and permanently. The flag stop_new_ops also prevents
 	fil_flush() from being applied to this tablespace. */
 	buf_LRU_flush_or_remove_pages(id, true);
@@ -2279,7 +2279,7 @@ dberr_t fil_delete_tablespace(ulint id, bool if_exists)
 	ut_a(path != 0);
 
 	/* IMPORTANT: Because we have set space::stop_new_ops there
-	can't be any new ibuf merges, reads or flushes. We are here
+	can't be any new reads or flushes. We are here
 	because node::n_pending was zero above. However, it is still
 	possible to have pending read and write requests:
 

storage/innobase/include/dict0stats.h

 /*****************************************************************************
 
 Copyright (c) 2009, 2018, Oracle and/or its affiliates. All Rights Reserved.
-Copyright (c) 2017, 2018, MariaDB Corporation.
+Copyright (c) 2017, 2020, 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
@@ -140,10 +140,18 @@ dict_stats_update(
 					the stats or to fetch them from
 					the persistent storage */
 
-/*********************************************************************//**
-Removes the information for a particular index's stats from the persistent
+/** Remove the information for a particular index's stats from the persistent
 storage if it exists and if there is data stored for this index.
 This function creates its own trx and commits it.
+
+We must modify system tables in a separate transaction in order to
+adhere to the InnoDB design constraint that dict_sys.latch prevents
+lock waits on system tables. If we modified system and user tables in
+the same transaction, we should exclusively hold dict_sys.latch until
+the transaction is committed, and effectively block other transactions
+that will attempt to open any InnoDB tables. Because we have no
+guarantee that user transactions will be committed fast, we cannot
+afford to keep the system tables locked in a user transaction.
 @return DB_SUCCESS or error code */
 dberr_t
 dict_stats_drop_index(

storage/innobase/include/fil0fil.h

@@ -100,9 +100,8 @@ struct fil_space_t
 				tablespace will be in named_spaces. */
 	/** set when an .ibd file is about to be deleted,
 	or an undo tablespace is about to be truncated.
-	When this is set following new ops are not allowed:
+	When this is set, the following new ops are not allowed:
 	* read IO request
-	* ibuf merge
 	* file flush
 	Note that we can still possibly have new write operations
 	because we don't check this flag when doing flush batches. */
@@ -134,7 +133,7 @@ struct fil_space_t
 	/** Number of pending buffer pool operations accessing the tablespace
 	without holding a table lock or dict_sys.latch S-latch
 	that would prevent the table (and tablespace) from being
-	dropped. An example is change buffer merge.
+	dropped. An example is fil_crypt_thread.
 	The tablespace cannot be dropped while this is nonzero,
 	or while fil_node_t::n_pending is nonzero.
 	Protected by fil_system.mutex and std::atomic. */

storage/innobase/include/srv0srv.h

@@ -417,13 +417,11 @@ extern ulong	srv_flushing_avg_loops;
 
 extern ulong	srv_force_recovery;
 
-extern uint	srv_fast_shutdown;	/*!< If this is 1, do not do a
-					purge and index buffer merge.
-					If this 2, do not even flush the
-					buffer pool to data files at the
-					shutdown: we effectively 'crash'
-					InnoDB (but lose no committed
-					transactions). */
+/** innodb_fast_shutdown=1 skips purge and change buffer merge.
+innodb_fast_shutdown=2 effectively crashes the server (no log checkpoint).
+innodb_fast_shutdown=3 is a clean shutdown that skips the rollback
+of active transaction (to be done on restart). */
+extern uint	srv_fast_shutdown;
 
 extern ibool	srv_innodb_status;
 
@@ -776,9 +774,8 @@ void srv_master_callback(void*);
 
 
 /**
-Perform shutdown tasks such as background drop,
-and optionally ibuf merge.
-*/
+Complete the shutdown tasks such as background DROP TABLE,
+and optionally change buffer merge (on innodb_fast_shutdown=0). */
 void srv_shutdown(bool ibuf_merge);
 
 

storage/innobase/row/row0mysql.cc

@@ -54,7 +54,6 @@ Created 9/17/2000 Heikki Tuuri
 #include "rem0cmp.h"
 #include "row0import.h"
 #include "row0ins.h"
-#include "row0merge.h"
 #include "row0row.h"
 #include "row0sel.h"
 #include "row0upd.h"

storage/innobase/row/row0upd.cc

@@ -503,11 +503,6 @@ row_upd_changes_field_size_or_external(
 		ut_ad(new_len != UNIV_SQL_DEFAULT);
 
 		if (dfield_is_null(new_val) && !rec_offs_comp(offsets)) {
-			/* A bug fixed on Dec 31st, 2004: we looked at the
-			SQL NULL size from the wrong field! We may backport
-			this fix also to 4.0. The merge to 5.0 will be made
-			manually immediately after we commit this to 4.1. */
-
 			new_len = dict_col_get_sql_null_size(
 				dict_index_get_nth_col(index,
 						       upd_field->field_no),

storage/innobase/srv/srv0srv.cc

@@ -306,10 +306,10 @@ my_bool	srv_print_all_deadlocks;
 INFORMATION_SCHEMA.innodb_cmp_per_index */
 my_bool	srv_cmp_per_index_enabled;
 
-/** innodb_fast_shutdown; if 1 then we do not run purge and insert buffer
-merge to completion before shutdown. If it is set to 2, do not even flush the
-buffer pool to data files at the shutdown: we effectively 'crash'
-InnoDB (but lose no committed transactions). */
+/** innodb_fast_shutdown=1 skips purge and change buffer merge.
+innodb_fast_shutdown=2 effectively crashes the server (no log checkpoint).
+innodb_fast_shutdown=3 is a clean shutdown that skips the rollback
+of active transaction (to be done on restart). */
 uint	srv_fast_shutdown;
 
 /** copy of innodb_status_file; generate a innodb_status.<pid> file */
@@ -1949,10 +1949,10 @@ srv_master_do_idle_tasks(void)
 				       counter_time);
 }
 
-/** Perform shutdown tasks.
-@param[in]	ibuf_merge	whether to complete the change buffer merge */
-void
-srv_shutdown(bool ibuf_merge)
+/**
+Complete the shutdown tasks such as background DROP TABLE,
+and optionally change buffer merge (on innodb_fast_shutdown=0). */
+void srv_shutdown(bool ibuf_merge)
 {
 	ulint		n_bytes_merged	= 0;
 	ulint		n_tables_to_drop;

storage/innobase/srv/srv0start.cc

@@ -1993,11 +1993,9 @@ dberr_t srv_start(bool create_new_db)
 	}
 
 	if (srv_force_recovery == 0) {
-		/* In the insert buffer we may have even bigger tablespace
+		/* In the change buffer we may have even bigger tablespace
 		id's, because we may have dropped those tablespaces, but
-		insert buffer merge has not had time to clean the records from
-		the ibuf tree. */
-
+		the buffered records have not been cleaned yet. */
 		ibuf_update_max_tablespace_id();
 	}