Bug#41098: Query Cache returns wrong result with concurrent insert

The problem is that select queries executed concurrently with
a concurrent insert on a MyISAM table could be cached if the
select started after the query cache invalidation but before
the unlock of tables performed by the concurrent insert. This
race could happen because the concurrent insert was failing
to prevent cache of select queries happening at the same time.

The solution is to add a 'uncacheable' status flag to signal
that a concurrent insert is being performed on the table and
that queries executing at the same time shouldn't cache the
results.

mysql-test/r/query_cache_debug.result:
  Add test case result for Bug#41098
mysql-test/t/disabled.def:
  Re-enable test case.
mysql-test/t/query_cache_debug.test:
  Add test case for Bug#41098
sql/sql_cache.cc:
  Debug sync point for regression testing purposes.
sql/sql_insert.cc:
  Remove meaningless query cache invalidate. There is already
  a preceding invalidate for queries that started before the
  concurrent insert.
storage/myisam/ha_myisam.cc:
  Check for a active concurrent insert.
storage/myisam/mi_locking.c:
  Signal the start of a concurrent insert. Flag is zeroed once
  the state is updated back.
storage/myisam/myisamdef.h:
  Add flag to signal a active concurrent insert.

1 files changed, 49 insertions, 0 deletions

diff --git a/mysql-test/r/query_cache_debug.result b/mysql-test/r/query_cache_debug.result
index c90165368e3..b03a71d3fec 100644
--- a/mysql-test/r/query_cache_debug.result
+++ b/mysql-test/r/query_cache_debug.result
@@ -22,3 +22,52 @@ Qcache_queries_in_cache	0
 set global query_cache_size= 0;
 use test;
 drop table t1;
+SET @old_concurrent_insert= @@GLOBAL.concurrent_insert;
+SET @old_query_cache_size= @@GLOBAL.query_cache_size;
+DROP TABLE IF EXISTS t1, t2;
+CREATE TABLE t1 (a INT);
+CREATE TABLE t2 (a INT);
+INSERT INTO t1 VALUES (1),(2),(3);
+SET GLOBAL concurrent_insert= 1;
+SET GLOBAL query_cache_size= 1024*512;
+SET GLOBAL query_cache_type= ON;
+# Switch to connection con1
+SET SESSION debug='+d,wait_after_query_cache_invalidate';
+# Send concurrent insert, will wait in the query cache table invalidate
+INSERT INTO t1 VALUES (4);
+# Switch to connection default
+# Wait for concurrent insert to reach the debug point
+# Switch to connection con2
+# Send SELECT that shouldn't be cached
+SELECT * FROM t1;
+a
+1
+2
+3
+# Switch to connection default
+# Notify the concurrent insert to proceed
+SELECT ID FROM INFORMATION_SCHEMA.PROCESSLIST
+WHERE STATE = 'wait_after_query_cache_invalidate' INTO @thread_id;
+KILL QUERY @thread_id;
+# Switch to connection con1
+# Gather insert result
+SHOW STATUS LIKE "Qcache_queries_in_cache";
+Variable_name	Value
+Qcache_queries_in_cache	0
+# Test that it's cacheable
+SELECT * FROM t1;
+a
+1
+2
+3
+4
+SHOW STATUS LIKE "Qcache_queries_in_cache";
+Variable_name	Value
+Qcache_queries_in_cache	1
+# Disconnect
+# Restore defaults
+RESET QUERY CACHE;
+DROP TABLE t1,t2;
+SET GLOBAL concurrent_insert= DEFAULT;
+SET GLOBAL query_cache_size= DEFAULT;
+SET GLOBAL query_cache_type= DEFAULT;