Fix for MDEV-4009: main.delayed sporadically fails with "query 'REPLACE DELAYED t1 VALUES (5)' failed: 1317: Query execution was interrupted"

- Fixed broadcast without a proper mutex
- Don't break existing locks if we are just testing if we can get the lock

mysql-test/r/create_delayed.result:
  Added test case for failures with INSERT DELAYED with CREATE and DROP TABLE
mysql-test/t/create_delayed.test:
  Added test case for failures with INSERT DELAYED with CREATE and DROP TABLE
sql/mdl.cc:
  Don't break existing locks for timeout=0 (ie, just check if there are conflicting locks).
  This fixed the bug that INSERT DELAYED didn't work properly with CREATE TABLE
sql/sql_base.cc:
  One neads to hold the mutex before doing a mysql_cond_broadcast()
  This fixed the bug that INSERT DELAYED didn't work properly with DROP TABLE
sql/sql_insert.cc:
  Protect setting of mysys_var->current_mutex.

1 files changed, 3 insertions, 0 deletions

diff --git a/mysql-test/r/create_delayed.result b/mysql-test/r/create_delayed.result
new file mode 100644
index 00000000000..c36a8e60cbb
--- /dev/null
+++ b/mysql-test/r/create_delayed.result
@@ -0,0 +1,3 @@
+drop table if exists t1;
+Starting test
+# All done