MDEV-5748 Assertion `status_var.memory_used == 0' fails on disconnect after...

MDEV-5748 Assertion `status_var.memory_used == 0' fails on disconnect after opening an OQGRAPH table

make sure MY_THREAD_SPECIFIC is not set for memroot that can be
transferred between threads

storage/oqgraph/ha_oqgraph.cc

@@ -538,7 +538,11 @@ int ha_oqgraph::open(const char *name, int mode, uint test_if_locked)
 
   origid= destid= weight= 0;
 
+  // Here we're abusing init_tmp_table_share() which is normally only works for thread-local shares.
   init_tmp_table_share( thd, share, table->s->db.str, table->s->db.length, options->table_name, "");
+  // because of that, we need to reinitialize the memroot (to reset MY_THREAD_SPECIFIC flag)
+  DBUG_ASSERT(share->mem_root.used == NULL); // it's still empty
+  init_sql_alloc(&share->mem_root, TABLE_ALLOC_BLOCK_SIZE, 0, MYF(0));
 
   // What I think this code is doing:
   // * Our OQGRAPH table is `database_blah/name`

storage/oqgraph/mysql-test/oqgraph/connections_mdev5748.result

+CREATE TABLE oq_backing (
+origid INT UNSIGNED NOT NULL,
+destid INT UNSIGNED NOT NULL,
+weight DOUBLE NOT NULL,
+PRIMARY KEY (origid, destid),
+KEY (destid)
+);
+CREATE TABLE oq_table (
+latch VARCHAR(32) NULL,
+origid BIGINT UNSIGNED NULL,
+destid BIGINT UNSIGNED NULL,
+weight DOUBLE NULL,
+seq BIGINT UNSIGNED NULL,
+linkid BIGINT UNSIGNED NULL,
+KEY (latch, origid, destid) USING HASH,
+KEY (latch, destid, origid) USING HASH
+) ENGINE=OQGRAPH
+data_table='oq_backing' origid='origid' destid='destid' weight='weight';
+flush tables;
+show fields in oq_table;
+Field	Type	Null	Key	Default	Extra
+latch	varchar(32)	YES	MUL	NULL	
+origid	bigint(20) unsigned	YES		NULL	
+destid	bigint(20) unsigned	YES		NULL	
+weight	double	YES		NULL	
+seq	bigint(20) unsigned	YES		NULL	
+linkid	bigint(20) unsigned	YES		NULL	
+show tables;
+Tables_in_test
+oq_backing
+oq_table
+drop table oq_table, oq_backing;

storage/oqgraph/mysql-test/oqgraph/connections_mdev5748.test

+#
+# MDEV-5748 Assertion `status_var.memory_used == 0' fails on disconnect after opening an OQGRAPH table
+#
+
+# try to open oqgraph table in one connection and use in another:
+
+--connect (con1,localhost,root,,)
+
+CREATE TABLE oq_backing (
+  origid INT UNSIGNED NOT NULL,
+  destid INT UNSIGNED NOT NULL,
+  weight DOUBLE NOT NULL,
+  PRIMARY KEY (origid, destid),
+  KEY (destid)
+);
+
+CREATE TABLE oq_table (
+  latch VARCHAR(32) NULL,
+  origid BIGINT UNSIGNED NULL,
+  destid BIGINT UNSIGNED NULL,
+  weight DOUBLE NULL,
+  seq BIGINT UNSIGNED NULL,
+  linkid BIGINT UNSIGNED NULL,
+  KEY (latch, origid, destid) USING HASH,
+  KEY (latch, destid, origid) USING HASH
+) ENGINE=OQGRAPH
+data_table='oq_backing' origid='origid' destid='destid' weight='weight';
+
+flush tables;
+show fields in oq_table;
+--disconnect con1
+
+--connection default
+show tables;
+
+drop table oq_table, oq_backing;
+