Revert "Bug #21133329 HANGING "SYSTEM LOCK" WHEN EXECUTING "FLUSH TABLE ... FOR EXPORT""

This reverts commit 66b7e4af49af98f8d38caf1f52f6ab0dc1cc99ab.

Author: Aditya A

mysql-test/suite/innodb/r/innodb-flush-hang.result

@@ -2562,7 +2562,7 @@ ibuf_get_merge_pages(
 Contracts insert buffer trees by reading pages to the buffer pool.
 @return a lower limit for the combined size in bytes of entries which
 will be merged from ibuf trees to the pages read, 0 if ibuf is
-empty or there are no pages to merge*/
+empty */
 static
 ulint
 ibuf_merge_pages(
@@ -2617,15 +2617,10 @@ ibuf_merge_pages(
 	ibuf_mtr_commit(&mtr);
 	btr_pcur_close(&pcur);
 
-        if (*n_pages == 0) {
-              ut_ad(sum_sizes == 0);
-              return(0);
-        }
-
 	buf_read_ibuf_merge_pages(
 		sync, space_ids, space_versions, page_nos, *n_pages);
 
-	return(sum_sizes);
+	return(sum_sizes + 1);
 }
 
 /*********************************************************************//**
@@ -2651,8 +2646,7 @@ ibuf_get_table(
 Contracts insert buffer trees by reading pages to the buffer pool.
 @return a lower limit for the combined size in bytes of entries which
 will be merged from ibuf trees to the pages read, 0 if ibuf is
-empty or if there are no pages to merge */
-
+empty */
 static
 ulint
 ibuf_merge_space(
@@ -2699,13 +2693,16 @@ ibuf_merge_space(
 			&pages[0], &spaces[0], &versions[0], n_pages,
 			&mtr);
 
+		++sum_sizes;
 	}
 
 	ibuf_mtr_commit(&mtr);
 
 	btr_pcur_close(&pcur);
 
-	if (*n_pages > 0) {
+	if (sum_sizes > 0) {
+
+		ut_a(*n_pages > 0 || sum_sizes == 1);
 
 #ifdef UNIV_DEBUG
 		ut_ad(*n_pages <= UT_ARR_SIZE(pages));