MDEV-20813 Assertion failure in buf_flush_init_for_writing() for innodb_immediate_scrub_data_uncompressed=ON

The assertion that was added in
commit c0c003beb4
to augment the fix of MDEV-20805 turns out to be invalid when
innodb_immediate_scrub_data_uncompressed is enabled.
In this mode, fsp_init_file_page() will be invoked on data pages
that have been freed, causing writes of almost-all-zero pages.

btr_page_free(): Adjust the comment.

buf_flush_init_for_writing(): Disable the assertion with a note
that it should be re-enabled in MDEV-15528.

storage/innobase/btr/btr0btr.cc

@@ -761,9 +761,11 @@ void btr_page_free(dict_index_t* index, buf_block_t* block, mtr_t* mtr,
 	ut_ad(mtr_is_block_fix(mtr, block, MTR_MEMO_PAGE_X_FIX, index->table));
 
 	if (srv_immediate_scrub_data_uncompressed) {
-		/* In MDEV-15528 this call must be removed, and we should
-		zero out the page after the redo log for this mini-transaction
-		has been durably written. */
+		/* In MDEV-15528 this code must be removed and the
+		check in buf_flush_init_for_writing() re-enabled.  We
+		should zero out the page after the redo log for this
+		mini-transaction has been durably written. The log
+		would include the 10.4 MLOG_INIT_FREE_PAGE record. */
 		fsp_init_file_page(fil_space_get(index->space), block, mtr);
 	}
 }

storage/innobase/buf/buf0flu.cc

@@ -856,7 +856,14 @@ buf_flush_init_for_writing(
 	      || &block->page.zip == page_zip_);
 	ut_ad(!srv_safe_truncate || !block || newest_lsn);
 	ut_ad(page);
+#if 0 /* MDEV-15528 TODO: reinstate this check */
+	/* innodb_immediate_scrub_data_uncompressed=ON would cause
+	fsp_init_file_page() to be called on freed pages, and thus
+	cause them to be written as almost-all-zeroed.
+	In MDEV-15528 we should change that implement an option to
+	make freed pages appear all-zero, bypassing this code. */
 	ut_ad(!srv_safe_truncate || !newest_lsn || fil_page_get_type(page));
+#endif
 
 	if (page_zip_) {
 		page_zip_des_t*	page_zip;