Implement multiple-signal debug_sync

The patch is inspired from MySQL. Instead of using a single String to
hold the current active debug_sync signal, use a Hash_set to store
LEX_STRINGS. This patch ensures that a signal can not be lost, by being
overwritten by another thread via set DEBUG_SYNC = '... SIGNAL ...';

All signals are kepts "alive" until they are consumed by a wait event.

This requires updating test cases that assume the GLOBAL signal is never
consumed.

Follow-up work needed:
Port the additional syntax that allows one to set multiple signals
and also conditionally deactivate signals when waiting.

mysql-test/main/debug_sync.result

@@ -2,7 +2,7 @@ SET DEBUG_SYNC= 'RESET';
 DROP TABLE IF EXISTS t1;
 SHOW VARIABLES LIKE 'DEBUG_SYNC';
 Variable_name	Value
-debug_sync	ON - current signal: ''
+debug_sync	ON - current signals: ''
 SET DEBUG_SYNC='p0 SIGNAL s1 WAIT_FOR s2 TIMEOUT 6 EXECUTE 2 HIT_LIMIT 3';
 SET DEBUG_SYNC='p0 SIGNAL s1 WAIT_FOR s2 TIMEOUT 6 EXECUTE 2';
 SET DEBUG_SYNC='p0 SIGNAL s1 WAIT_FOR s2 TIMEOUT 6           HIT_LIMIT 3';
@@ -150,34 +150,34 @@ SET @myvar= 'now SIGNAL from_myvar';
 SET DEBUG_SYNC= @myvar;
 SHOW VARIABLES LIKE 'DEBUG_SYNC';
 Variable_name	Value
-debug_sync	ON - current signal: 'from_myvar'
+debug_sync	ON - current signals: 'from_myvar'
 SET DEBUG_SYNC= LEFT('now SIGNAL from_function_cut_here', 24);
 SHOW VARIABLES LIKE 'DEBUG_SYNC';
 Variable_name	Value
-debug_sync	ON - current signal: 'from_function'
+debug_sync	ON - current signals: 'from_myvar,from_function'
 SET DEBUG_SYNC= 'now SIGNAL something';
 SHOW VARIABLES LIKE 'DEBUG_SYNC';
 Variable_name	Value
-debug_sync	ON - current signal: 'something'
+debug_sync	ON - current signals: 'something,from_function,from_myvar'
 SET DEBUG_SYNC= 'now WAIT_FOR nothing TIMEOUT 0';
 Warnings:
 Warning	####	debug sync point wait timed out
 SET DEBUG_SYNC= 'now SIGNAL nothing';
 SHOW VARIABLES LIKE 'DEBUG_SYNC';
 Variable_name	Value
-debug_sync	ON - current signal: 'nothing'
+debug_sync	ON - current signals: 'something,from_function,nothing,from_myvar'
 SET DEBUG_SYNC= 'now WAIT_FOR nothing TIMEOUT 0';
 SET DEBUG_SYNC= 'now SIGNAL something EXECUTE 0';
 SHOW VARIABLES LIKE 'DEBUG_SYNC';
 Variable_name	Value
-debug_sync	ON - current signal: 'nothing'
+debug_sync	ON - current signals: 'something,from_function,from_myvar'
 SET DEBUG_SYNC= 'now WAIT_FOR anotherthing TIMEOUT 0 EXECUTE 0';
 SET DEBUG_SYNC= 'now HIT_LIMIT 1';
 ERROR HY000: debug sync point hit limit reached
 SET DEBUG_SYNC= 'RESET';
 SHOW VARIABLES LIKE 'DEBUG_SYNC';
 Variable_name	Value
-debug_sync	ON - current signal: ''
+debug_sync	ON - current signals: ''
 SET DEBUG_SYNC= 'p1abcd   SIGNAL s1 EXECUTE 2';
 SET DEBUG_SYNC= 'p2abc    SIGNAL s2 EXECUTE 2';
 SET DEBUG_SYNC= 'p9abcdef SIGNAL s9 EXECUTE 2';
@@ -190,23 +190,30 @@ SET DEBUG_SYNC= 'p3abcdef SIGNAL s3 EXECUTE 2';
 SET DEBUG_SYNC= 'p4a      TEST';
 SHOW VARIABLES LIKE 'DEBUG_SYNC';
 Variable_name	Value
-debug_sync	ON - current signal: 's4'
+debug_sync	ON - current signals: 's4'
 SET DEBUG_SYNC= 'p1abcd   TEST';
 SHOW VARIABLES LIKE 'DEBUG_SYNC';
 Variable_name	Value
-debug_sync	ON - current signal: 's1'
+debug_sync	ON - current signals: 's4,s1'
 SET DEBUG_SYNC= 'p7       TEST';
 SHOW VARIABLES LIKE 'DEBUG_SYNC';
 Variable_name	Value
-debug_sync	ON - current signal: 's7'
+debug_sync	ON - current signals: 's1,s7,s4'
 SET DEBUG_SYNC= 'p9abcdef TEST';
 SHOW VARIABLES LIKE 'DEBUG_SYNC';
 Variable_name	Value
-debug_sync	ON - current signal: 's9'
+debug_sync	ON - current signals: 's1,s7,s4,s9'
 SET DEBUG_SYNC= 'p3abcdef TEST';
 SHOW VARIABLES LIKE 'DEBUG_SYNC';
 Variable_name	Value
-debug_sync	ON - current signal: 's3'
+debug_sync	ON - current signals: 's1,s3,s4,s9,s7'
+SET DEBUG_SYNC= 'now WAIT_FOR s9';
+SET DEBUG_SYNC= 'now WAIT_FOR s1';
+SET DEBUG_SYNC= 'now WAIT_FOR s4';
+SET DEBUG_SYNC= 'now WAIT_FOR s7';
+SHOW VARIABLES LIKE 'DEBUG_SYNC';
+Variable_name	Value
+debug_sync	ON - current signals: 's3'
 SET DEBUG_SYNC= 'p1abcd   CLEAR';
 SET DEBUG_SYNC= 'p2abc    CLEAR';
 SET DEBUG_SYNC= 'p5abcde  CLEAR';
@@ -219,19 +226,19 @@ SET DEBUG_SYNC= 'p7       CLEAR';
 SET DEBUG_SYNC= 'p1abcd   TEST';
 SHOW VARIABLES LIKE 'DEBUG_SYNC';
 Variable_name	Value
-debug_sync	ON - current signal: 's3'
+debug_sync	ON - current signals: 's3'
 SET DEBUG_SYNC= 'p7       TEST';
 SHOW VARIABLES LIKE 'DEBUG_SYNC';
 Variable_name	Value
-debug_sync	ON - current signal: 's3'
+debug_sync	ON - current signals: 's3'
 SET DEBUG_SYNC= 'p9abcdef TEST';
 SHOW VARIABLES LIKE 'DEBUG_SYNC';
 Variable_name	Value
-debug_sync	ON - current signal: 's3'
+debug_sync	ON - current signals: 's3'
 SET DEBUG_SYNC= 'RESET';
 SHOW VARIABLES LIKE 'DEBUG_SYNC';
 Variable_name	Value
-debug_sync	ON - current signal: ''
+debug_sync	ON - current signals: ''
 CREATE USER mysqltest_1@localhost;
 GRANT SUPER ON *.* TO mysqltest_1@localhost;
 connect  con1,localhost,mysqltest_1,,;

mysql-test/main/debug_sync.test

@@ -298,6 +298,16 @@ SET DEBUG_SYNC= 'p9abcdef TEST';
 SHOW VARIABLES LIKE 'DEBUG_SYNC';
 SET DEBUG_SYNC= 'p3abcdef TEST';
 SHOW VARIABLES LIKE 'DEBUG_SYNC';
+
+#
+# Wait for all signals currently active except s3.
+#
+SET DEBUG_SYNC= 'now WAIT_FOR s9';
+SET DEBUG_SYNC= 'now WAIT_FOR s1';
+SET DEBUG_SYNC= 'now WAIT_FOR s4';
+SET DEBUG_SYNC= 'now WAIT_FOR s7';
+SHOW VARIABLES LIKE 'DEBUG_SYNC';
+
 #
 # Clear the actions.
 #
@@ -320,7 +330,7 @@ SHOW VARIABLES LIKE 'DEBUG_SYNC';
 SET DEBUG_SYNC= 'p9abcdef TEST';
 SHOW VARIABLES LIKE 'DEBUG_SYNC';
 #
-# Now cleanup. Actions are clear already, but signal needs to be cleared.
+# Now cleanup. Actions are clear already, but s3 signal needs to be cleared.
 #
 SET DEBUG_SYNC= 'RESET';
 SHOW VARIABLES LIKE 'DEBUG_SYNC';

mysql-test/suite/innodb/r/mdev-14846.result

@@ -31,11 +31,12 @@ pk	f1	f2	f3
 3	t	q	1
 5	z	t	NULL
 SET DEBUG_SYNC='now SIGNAL default_dml';
+SET DEBUG_SYNC='now SIGNAL con2_dml';
 connection default;
 SET DEBUG_SYNC='now WAIT_FOR default_dml';
 UPDATE t3 AS alias1 LEFT JOIN t3 AS alias2 ON ( alias1.f1 <> alias1.f2 ) SET alias1.f3 = 59 WHERE ( EXISTS ( SELECT t1.f3 FROM t1 WHERE t1.f1 = alias1.f1 ) ) OR alias2.f1 = 'h';
 connect  con2,localhost,root,,test;
-set debug_sync='now WAIT_FOR default_dml';
+set debug_sync='now WAIT_FOR con2_dml';
 SET DEBUG_SYNC='now SIGNAL con1_dml2';
 disconnect con2;
 connection con1;

mysql-test/suite/innodb/t/mdev-14846.test

@@ -35,14 +35,15 @@ SET DEBUG_SYNC='now WAIT_FOR con1_dml';
 begin;
 SELECT * FROM t1 for update; # Holds x lock of all records in the table t1
 SET DEBUG_SYNC='now SIGNAL default_dml';
+SET DEBUG_SYNC='now SIGNAL con2_dml';
 
 --connection default
 SET DEBUG_SYNC='now WAIT_FOR default_dml';
 --send UPDATE t3 AS alias1 LEFT JOIN t3 AS alias2 ON ( alias1.f1 <> alias1.f2 ) SET alias1.f3 = 59 WHERE ( EXISTS ( SELECT t1.f3 FROM t1 WHERE t1.f1 = alias1.f1 ) ) OR alias2.f1 = 'h'
 # It holds the lock of all record in t3 and tries to acquire record lock for the table  t1.
 
 --connect (con2,localhost,root,,test)
-set debug_sync='now WAIT_FOR default_dml';
+set debug_sync='now WAIT_FOR con2_dml';
 let $wait_condition=
 select count(*) > 0 from information_schema.innodb_lock_waits;
 --source include/wait_condition.inc

mysql-test/suite/sys_vars/r/debug_sync_basic.result

@@ -2,17 +2,17 @@ select @@global.debug_sync;
 ERROR HY000: Variable 'debug_sync' is a SESSION variable
 select @@session.debug_sync;
 @@session.debug_sync
-ON - current signal: ''
+ON - current signals: ''
 show global variables like "debug_sync";
 Variable_name	Value
 show session variables like "debug_sync";
 Variable_name	Value
-debug_sync	ON - current signal: ''
+debug_sync	ON - current signals: ''
 select * from information_schema.global_variables where variable_name="debug_sync";
 VARIABLE_NAME	VARIABLE_VALUE
 select * from information_schema.session_variables where variable_name="debug_sync";
 VARIABLE_NAME	VARIABLE_VALUE
-DEBUG_SYNC	ON - current signal: ''
+DEBUG_SYNC	ON - current signals: ''
 set @@session.debug_sync=1;
 ERROR 42000: Incorrect argument type to variable 'debug_sync'
 set @@session.debug_sync=1.1;

mysql-test/suite/sys_vars/r/sysvars_debug.result

@@ -77,7 +77,7 @@ READ_ONLY	YES
 COMMAND_LINE_ARGUMENT	OPTIONAL
 GLOBAL_VALUE_PATH	NULL
 VARIABLE_NAME	DEBUG_SYNC
-SESSION_VALUE	ON - current signal: ''
+SESSION_VALUE	ON - current signals: ''
 GLOBAL_VALUE	NULL
 GLOBAL_VALUE_ORIGIN	COMPILE-TIME
 DEFAULT_VALUE