MDEV-20805 follow-up: Catch writes of bogus pages

buf_flush_init_for_writing(): Assert that FIL_PAGE_TYPE is set
except when creating a new data file with a dummy first page.

buf_dblwr_create(): Ensure that FIL_PAGE_TYPE on all pages
will be initialized. Reset buf_dblwr_being_created at the end.

storage/innobase/buf/buf0dblwr.cc

 /*****************************************************************************
 
 Copyright (c) 1995, 2017, Oracle and/or its affiliates. All Rights Reserved.
-Copyright (c) 2013, 2018, MariaDB Corporation.
+Copyright (c) 2013, 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
@@ -291,6 +291,13 @@ buf_dblwr_create()
 
 		ut_ad(rw_lock_get_x_lock_count(&new_block->lock) == 1);
 		page_no = buf_block_get_page_no(new_block);
+		/* We only do this in the debug build, to ensure that
+		both the check in buf_flush_init_for_writing() and
+		recv_parse_or_apply_log_rec_body() will see a valid
+		page type. The flushes of new_block are actually
+		unnecessary here.  */
+		ut_d(mlog_write_ulint(FIL_PAGE_TYPE + new_block->frame,
+				      FIL_PAGE_TYPE_SYS, MLOG_2BYTES, &mtr));
 
 		if (i == FSP_EXTENT_SIZE / 2) {
 			ut_a(page_no == FSP_EXTENT_SIZE);
@@ -353,6 +360,7 @@ buf_dblwr_create()
 
 	/* Flush the modified pages to disk and make a checkpoint */
 	log_make_checkpoint_at(LSN_MAX, TRUE);
+	buf_dblwr_being_created = FALSE;
 
 	/* Remove doublewrite pages from LRU */
 	buf_pool_invalidate();

storage/innobase/buf/buf0flu.cc

 /*****************************************************************************
 
 Copyright (c) 1995, 2017, Oracle and/or its affiliates. All Rights Reserved.
-Copyright (c) 2013, 2017, MariaDB Corporation.
+Copyright (c) 2013, 2019, MariaDB Corporation.
 Copyright (c) 2013, 2014, Fusion-io
 
 This program is free software; you can redistribute it and/or modify it under
@@ -745,6 +745,7 @@ buf_flush_init_for_writing(
 	ib_uint32_t	checksum = 0 /* silence bogus gcc warning */;
 
 	ut_ad(page);
+	ut_ad(!newest_lsn || fil_page_get_type(page));
 
 	if (page_zip_) {
 		page_zip_des_t*	page_zip;

storage/xtradb/buf/buf0dblwr.cc

 /*****************************************************************************
 
 Copyright (c) 1995, 2017, Oracle and/or its affiliates. All Rights Reserved.
-Copyright (c) 2013, 2018, MariaDB Corporation.
+Copyright (c) 2013, 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
@@ -291,6 +291,13 @@ buf_dblwr_create()
 
 		ut_ad(rw_lock_get_x_lock_count(&new_block->lock) == 1);
 		page_no = buf_block_get_page_no(new_block);
+		/* We only do this in the debug build, to ensure that
+		both the check in buf_flush_init_for_writing() and
+		recv_parse_or_apply_log_rec_body() will see a valid
+		page type. The flushes of new_block are actually
+		unnecessary here.  */
+		ut_d(mlog_write_ulint(FIL_PAGE_TYPE + new_block->frame,
+				      FIL_PAGE_TYPE_SYS, MLOG_2BYTES, &mtr));
 
 		if (i == FSP_EXTENT_SIZE / 2) {
 			ut_a(page_no == FSP_EXTENT_SIZE);
@@ -353,6 +360,7 @@ buf_dblwr_create()
 
 	/* Flush the modified pages to disk and make a checkpoint */
 	log_make_checkpoint_at(LSN_MAX, TRUE);
+	buf_dblwr_being_created = FALSE;
 
 	/* Remove doublewrite pages from LRU */
 	buf_pool_invalidate();

storage/xtradb/buf/buf0flu.cc

 /*****************************************************************************
 
 Copyright (c) 1995, 2017, Oracle and/or its affiliates. All Rights Reserved.
-Copyright (c) 2013, 2017, MariaDB Corporation.
+Copyright (c) 2013, 2019, MariaDB Corporation.
 Copyright (c) 2013, 2014, Fusion-io
 
 This program is free software; you can redistribute it and/or modify it under
@@ -787,6 +787,7 @@ buf_flush_init_for_writing(
 	ib_uint32_t	checksum = 0 /* silence bogus gcc warning */;
 
 	ut_ad(page);
+	ut_ad(!newest_lsn || fil_page_get_type(page));
 
 	if (page_zip_) {
 		page_zip_des_t*	page_zip;