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

PROBLEM
--------
1. ibuf_contarct_in background() function is trying to merge
   all pages related to a certain table (which contains no record).
   while in background heavy DML operations are happening.
   When contracting the pages in ibuf_merge_space() we call
   ibuf_get_merge_pages() to get the pages to be merged.This function
   call returns the volume ( in bytes) of pages to be merged.
   In our case , the returned volume is zero since there are no pages
   to be merged ,but we increment the volume returned by 1 .
2. This volume is returned to ibuf_contract_in_background() which
   assumes that some pages have been merged since the volume
   returned is greater than zero and waits it reach the required
   number of pages to be merged and gets stuck in the loop.

FIX

1. The reason we increment the volume by 1 ,is becasue for delete
   marked records the volume returned is zero and the logic in
   ibuf_merge_space() is such that acutual merging happens only
   if volume returned is non zero.
2. To fix this we have changed the logic to merge only if the
   number of pages returned is greater than zero.We return zero
   to the ibuf_contarct_in background() when there are no pages
   to merge which enables it break the loop.

[ Reviewed by Jimmy #rb12209 ]

Author: Aditya A

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

@@ -0,0 +1,36 @@
+#
+#Bug #21133329   HANGING "SYSTEM LOCK" WHEN EXECUTING "FLUSH TABLE ... FOR EXPORT"
+#
+CREATE TABLE t1 (
+c1 BIGINT NOT NULL AUTO_INCREMENT PRIMARY KEY,
+c2 BIGINT,
+c3 VARCHAR(2048),
+c4 VARCHAR(2048),
+INDEX idx1(c2),
+INDEX idx2(c3(512)),
+INDEX idx3(c4(512))) Engine=InnoDB;
+CREATE TABLE t2 ( f1 int PRIMARY KEY) engine=innodb;
+SET GLOBAL INNODB_PURGE_STOP_NOW=ON;
+SET GLOBAL innodb_disable_background_merge=ON;
+SET GLOBAL innodb_stats_persistent=OFF;
+show variables like '%innodb_stats_persistent%';
+Variable_name	Value
+innodb_stats_persistent	OFF
+innodb_stats_persistent_sample_pages	20
+INSERT INTO t1(c2, c3, c4) VALUES
+(1, REPEAT('a', 2048), REPEAT('a', 2048)),
+(2, REPEAT('b', 2048), REPEAT('b', 2048)),
+(3, REPEAT('c', 2048), REPEAT('c', 2048)),
+(4, REPEAT('d', 2048), REPEAT('d', 2048));
+INSERT INTO t1(c2, c3, c4) SELECT c2, c3, c4 FROM t1;
+INSERT INTO t1(c2, c3, c4) SELECT c2, c3, c4 FROM t1;
+INSERT INTO t1(c2, c3, c4) SELECT c2, c3, c4 FROM t1;
+INSERT INTO t1(c2, c3, c4) SELECT c2, c3, c4 FROM t1;
+INSERT INTO t1(c2, c3, c4) SELECT c2, c3, c4 FROM t1;
+INSERT INTO t1(c2, c3, c4) SELECT c2, c3, c4 FROM t1;
+FLUSH TABLES t2 FOR EXPORT;
+UNLOCK TABLES;
+SET GLOBAL innodb_disable_background_merge=OFF;
+SET GLOBAL INNODB_PURGE_RUN_NOW=ON;
+SET GLOBAL innodb_stats_persistent=ON;
+DROP TABLE t1,t2;

mysql-test/suite/innodb/t/innodb-flush-hang.test

@@ -0,0 +1,50 @@
+
+--echo #
+--echo #Bug #21133329   HANGING "SYSTEM LOCK" WHEN EXECUTING "FLUSH TABLE ... FOR EXPORT"
+--echo #
+
+--source include/not_embedded.inc
+--source include/have_debug.inc
+--source include/have_innodb.inc
+
+CREATE TABLE t1 (
+	c1 BIGINT NOT NULL AUTO_INCREMENT PRIMARY KEY,
+	c2 BIGINT,
+	c3 VARCHAR(2048),
+	c4 VARCHAR(2048),
+	INDEX idx1(c2),
+	INDEX idx2(c3(512)),
+	INDEX idx3(c4(512))) Engine=InnoDB;
+
+CREATE TABLE t2 ( f1 int PRIMARY KEY) engine=innodb;
+
+# Stop purge so that it doesn't remove the delete marked entries.
+SET GLOBAL INNODB_PURGE_STOP_NOW=ON;
+
+# Disable change buffer merge from the master thread, additionally
+# enable aggressive flushing so that more changes are buffered.
+SET GLOBAL innodb_disable_background_merge=ON;
+SET GLOBAL innodb_stats_persistent=OFF;
+show variables like '%innodb_stats_persistent%';
+
+INSERT INTO t1(c2, c3, c4) VALUES
+	(1, REPEAT('a', 2048), REPEAT('a', 2048)),
+	(2, REPEAT('b', 2048), REPEAT('b', 2048)),
+	(3, REPEAT('c', 2048), REPEAT('c', 2048)),
+	(4, REPEAT('d', 2048), REPEAT('d', 2048));
+
+INSERT INTO t1(c2, c3, c4) SELECT c2, c3, c4 FROM t1;
+INSERT INTO t1(c2, c3, c4) SELECT c2, c3, c4 FROM t1;
+INSERT INTO t1(c2, c3, c4) SELECT c2, c3, c4 FROM t1;
+INSERT INTO t1(c2, c3, c4) SELECT c2, c3, c4 FROM t1;
+INSERT INTO t1(c2, c3, c4) SELECT c2, c3, c4 FROM t1;
+INSERT INTO t1(c2, c3, c4) SELECT c2, c3, c4 FROM t1;
+
+FLUSH TABLES t2 FOR EXPORT;
+UNLOCK TABLES;
+
+SET GLOBAL innodb_disable_background_merge=OFF;
+SET GLOBAL INNODB_PURGE_RUN_NOW=ON;
+SET GLOBAL innodb_stats_persistent=ON;
+
+DROP TABLE t1,t2;

storage/innobase/ibuf/ibuf0ibuf.cc

@@ -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 */
+empty or there are no pages to merge*/
 static
 ulint
 ibuf_merge_pages(
@@ -2617,10 +2617,15 @@ 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 + 1);
+	return(sum_sizes);
 }
 
 /*********************************************************************//**
@@ -2646,7 +2651,8 @@ 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 */
+empty or if there are no pages to merge */
+
 static
 ulint
 ibuf_merge_space(
@@ -2693,16 +2699,13 @@ ibuf_merge_space(
 			&pages[0], &spaces[0], &versions[0], n_pages,
 			&mtr);
 
-		++sum_sizes;
 	}
 
 	ibuf_mtr_commit(&mtr);
 
 	btr_pcur_close(&pcur);
 
-	if (sum_sizes > 0) {
-
-		ut_a(*n_pages > 0 || sum_sizes == 1);
+	if (*n_pages > 0) {
 
 #ifdef UNIV_DEBUG
 		ut_ad(*n_pages <= UT_ARR_SIZE(pages));