MDEV-26547 Restoring InnoDB buffer pool dump is single-threaded for no reason

buf_read_page_background(): Remove the parameter "bool sync"
and always actually initiate a page read in the background.

buf_load(): Always submit asynchronous reads. This allows
page checksums to be verified in concurrent threads as
soon as the reads are completed.

storage/innobase/btr/btr0cur.cc

@@ -3341,16 +3341,16 @@ static void btr_cur_prefetch_siblings(const buf_block_t *block,
   uint32_t prev= mach_read_from_4(my_assume_aligned<4>(page + FIL_PAGE_PREV));
   uint32_t next= mach_read_from_4(my_assume_aligned<4>(page + FIL_PAGE_NEXT));
 
+  fil_space_t *space= index->table->space;
+
   if (prev == FIL_NULL);
-  else if (index->table->space->acquire())
-    buf_read_page_background(index->table->space,
-                             page_id_t(block->page.id().space(), prev),
-                             block->zip_size(), false);
+  else if (space->acquire())
+    buf_read_page_background(space, page_id_t(space->id, prev),
+                             block->zip_size());
   if (next == FIL_NULL);
-  else if (index->table->space->acquire())
-    buf_read_page_background(index->table->space,
-                             page_id_t(block->page.id().space(), next),
-                             block->zip_size(), false);
+  else if (space->acquire())
+    buf_read_page_background(space, page_id_t(space->id, next),
+                             block->zip_size());
 }
 
 /*************************************************************//**

storage/innobase/buf/buf0dump.cc

 /*****************************************************************************
 
 Copyright (c) 2011, 2017, Oracle and/or its affiliates. All Rights Reserved.
-Copyright (c) 2017, 2020, MariaDB Corporation.
+Copyright (c) 2017, 2021, 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
@@ -678,7 +678,7 @@ buf_load()
 		}
 
 		space->reacquire();
-		buf_read_page_background(space, dump[i], zip_size, true);
+		buf_read_page_background(space, dump[i], zip_size);
 
 		if (buf_load_abort_flag) {
 			if (space) {

storage/innobase/buf/buf0rea.cc

@@ -501,14 +501,13 @@ an exclusive lock on the buffer frame. The flag is cleared and the x-lock
 released by the i/o-handler thread.
 @param[in,out]	space		tablespace
 @param[in]	page_id		page id
-@param[in]	zip_size	ROW_FORMAT=COMPRESSED page size, or 0
-@param[in]	sync		true if synchronous aio is desired */
+@param[in]	zip_size	ROW_FORMAT=COMPRESSED page size, or 0 */
 void buf_read_page_background(fil_space_t *space, const page_id_t page_id,
-			      ulint zip_size, bool sync)
+                              ulint zip_size)
 {
 	dberr_t		err;
 
-	if (buf_read_page_low(&err, space, sync, BUF_READ_ANY_PAGE,
+	if (buf_read_page_low(&err, space, false, BUF_READ_ANY_PAGE,
 			      page_id, zip_size, false)) {
 		srv_stats.buf_pool_reads.add(1);
 	}

storage/innobase/include/buf0rea.h

 /*****************************************************************************
 
 Copyright (c) 1995, 2015, Oracle and/or its affiliates. All Rights Reserved.
-Copyright (c) 2015, 2020, MariaDB Corporation.
+Copyright (c) 2015, 2021, 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
@@ -48,10 +48,9 @@ an exclusive lock on the buffer frame. The flag is cleared and the x-lock
 released by the i/o-handler thread.
 @param[in,out]	space		tablespace
 @param[in]	page_id		page id
-@param[in]	zip_size	ROW_FORMAT=COMPRESSED page size, or 0
-@param[in]	sync		true if synchronous aio is desired */
+@param[in]	zip_size	ROW_FORMAT=COMPRESSED page size, or 0 */
 void buf_read_page_background(fil_space_t *space, const page_id_t page_id,
-			      ulint zip_size, bool sync)
+                              ulint zip_size)
   MY_ATTRIBUTE((nonnull));
 
 /** Applies a random read-ahead in buf_pool if there are at least a threshold