MDEV-34541 Clean up spider self reference check

SPIDER_CONN::loop_check_meraged_first is useless, because all
SPIDER_CONN_LOOP_CHECKs are in SPIDER_CONN::loop_check_queue, which in
spider_db_conn::fin_loop_check() is iterated over.

This fixes the use-after-free issue when there are three spider tables
sharing the same remote, and their corresponding
SPIDER_CONN_LOOP_CHECKs getting merged in
spider_conn_queue_and_merge_loop_check()

This also fixes MDEV-34555

storage/spider/mysql-test/spider/bugfix/r/mdev_34541.result

+for master_1
+for child2
+for child3
+SET SESSION spider_same_server_link=1;
+SET sql_mode='';
+set @old_table_open_cache=@@global.table_open_cache;
+set global table_open_cache=10;
+set spider_same_server_link= 1;
+CREATE SERVER srv FOREIGN DATA WRAPPER mysql
+OPTIONS (SOCKET "$MASTER_1_MYSOCK", DATABASE 'test',user 'root');
+CREATE TABLE t1 (c INT) ENGINE=InnoDB;
+CREATE TABLE t2 (c INT) ENGINE=InnoDB;
+CREATE TABLE t3 (c INT) ENGINE=InnoDB;
+CREATE TABLE ta (c INT) ENGINE=Spider COMMENT='WRAPPER "mysql",SRV "srv",TABLE "t"';
+CREATE TABLE t5 (c INT) ENGINE=Spider COMMENT='WRAPPER "mysql",SRV "srv",TABLE "t"';
+CREATE TABLE t6 (c INT KEY) ENGINE=InnoDB PARTITION BY RANGE (c) (PARTITION p VALUES LESS THAN (5));
+CREATE TABLE t7 (a INT) ENGINE=Spider COMMENT='WRAPPER "mysql",SRV "srv",TABLE "t"';
+CREATE TABLE t8 (c INT) ENGINE=Spider COMMENT='WRAPPER "mysql",SRV "srv",TABLE "t"';
+SELECT * FROM t8;
+ERROR HY000: Remote table 'test.t' is not found
+CREATE TEMPORARY TABLE t7 (c INT) ENGINE=InnoDB SELECT * FROM t7;
+ERROR HY000: Remote table 'test.t' is not found
+CALL foo;
+ERROR 42000: PROCEDURE test.foo does not exist
+CREATE TEMPORARY TABLE t7 (c INT) ENGINE=InnoDB;
+SELECT * FROM t7 JOIN t6 ON tc=t0.c;
+ERROR 42S22: Unknown column 'tc' in 'on clause'
+SHOW TABLE STATUS;
+drop table ta, t8, t7, t6, t5, t3, t2, t1;
+drop table t7;
+drop server srv;
+set global table_open_cache=@old_table_open_cache;
+for master_1
+for child2
+for child3

storage/spider/mysql-test/spider/bugfix/r/mdev_34555.result

+for master_1
+for child2
+for child3
+set spider_same_server_link= 1;
+CREATE SERVER srv FOREIGN DATA WRAPPER mysql
+OPTIONS (SOCKET "$MASTER_1_MYSOCK", DATABASE '',user 'Spider', password 'foo');
+CREATE TABLE tSpider (a INT) ENGINE=Spider COMMENT='WRAPPER "mysql",SRV "srv",TABLE "t"';
+CREATE TABLE t2 (c INT,c2 CHAR(1)) ENGINE=Spider COMMENT='WRAPPER "mysql",SRV "srv",TABLE "t"';
+XA START 'a';
+SELECT * FROM information_schema.table_constraints;
+SELECT SLEEP (1);
+SLEEP (1)
+0
+SELECT * FROM t2;
+ERROR HY000: Unable to connect to foreign data source: srv
+SELECT SLEEP (1);
+SLEEP (1)
+0
+SELECT * FROM t2;
+ERROR HY000: Unable to connect to foreign data source: srv
+SELECT SLEEP (1);
+SLEEP (1)
+0
+SELECT * FROM t2;
+ERROR HY000: Unable to connect to foreign data source: srv
+xa end 'a';
+xa rollback 'a';
+drop table tSpider, t2;
+drop server srv;
+for master_1
+for child2
+for child3

storage/spider/mysql-test/spider/bugfix/t/mdev_34541.test

+--disable_query_log
+--disable_result_log
+--source ../../t/test_init.inc
+--enable_result_log
+--enable_query_log
+
+SET SESSION spider_same_server_link=1;
+SET sql_mode='';
+
+set @old_table_open_cache=@@global.table_open_cache;
+set global table_open_cache=10;
+
+set spider_same_server_link= 1;
+evalp CREATE SERVER srv FOREIGN DATA WRAPPER mysql
+OPTIONS (SOCKET "$MASTER_1_MYSOCK", DATABASE 'test',user 'root');
+
+CREATE TABLE t1 (c INT) ENGINE=InnoDB;
+CREATE TABLE t2 (c INT) ENGINE=InnoDB;
+CREATE TABLE t3 (c INT) ENGINE=InnoDB;
+CREATE TABLE ta (c INT) ENGINE=Spider COMMENT='WRAPPER "mysql",SRV "srv",TABLE "t"';
+CREATE TABLE t5 (c INT) ENGINE=Spider COMMENT='WRAPPER "mysql",SRV "srv",TABLE "t"';
+CREATE TABLE t6 (c INT KEY) ENGINE=InnoDB PARTITION BY RANGE (c) (PARTITION p VALUES LESS THAN (5));
+CREATE TABLE t7 (a INT) ENGINE=Spider COMMENT='WRAPPER "mysql",SRV "srv",TABLE "t"';
+CREATE TABLE t8 (c INT) ENGINE=Spider COMMENT='WRAPPER "mysql",SRV "srv",TABLE "t"';
+--error 12702
+SELECT * FROM t8;
+--error 12702
+CREATE TEMPORARY TABLE t7 (c INT) ENGINE=InnoDB SELECT * FROM t7;
+--error ER_SP_DOES_NOT_EXIST
+CALL foo;
+CREATE TEMPORARY TABLE t7 (c INT) ENGINE=InnoDB;
+--error ER_BAD_FIELD_ERROR
+SELECT * FROM t7 JOIN t6 ON tc=t0.c;
+--disable_result_log
+SHOW TABLE STATUS;
+--enable_result_log
+
+# we need to drop t7 twice
+drop table ta, t8, t7, t6, t5, t3, t2, t1;
+drop table t7;
+drop server srv;
+set global table_open_cache=@old_table_open_cache;
+
+--disable_query_log
+--disable_result_log
+--source ../../t/test_deinit.inc
+--enable_result_log
+--enable_query_log

storage/spider/mysql-test/spider/bugfix/t/mdev_34555.test

+--disable_query_log
+--disable_result_log
+--source ../../t/test_init.inc
+--enable_result_log
+--enable_query_log
+set spider_same_server_link= 1;
+evalp CREATE SERVER srv FOREIGN DATA WRAPPER mysql
+OPTIONS (SOCKET "$MASTER_1_MYSOCK", DATABASE '',user 'Spider', password 'foo');
+CREATE TABLE tSpider (a INT) ENGINE=Spider COMMENT='WRAPPER "mysql",SRV "srv",TABLE "t"';
+CREATE TABLE t2 (c INT,c2 CHAR(1)) ENGINE=Spider COMMENT='WRAPPER "mysql",SRV "srv",TABLE "t"';
+XA START 'a';
+--disable_result_log
+--error 0,ER_CONNECT_TO_FOREIGN_DATA_SOURCE
+SELECT * FROM information_schema.table_constraints;
+--enable_result_log
+SELECT SLEEP (1);
+--error ER_CONNECT_TO_FOREIGN_DATA_SOURCE
+SELECT * FROM t2;
+SELECT SLEEP (1);
+--error ER_CONNECT_TO_FOREIGN_DATA_SOURCE
+SELECT * FROM t2;
+SELECT SLEEP (1);
+--error ER_CONNECT_TO_FOREIGN_DATA_SOURCE
+SELECT * FROM t2;
+xa end 'a';
+xa rollback 'a';
+drop table tSpider, t2;
+drop server srv;
+--disable_query_log
+--disable_result_log
+--source ../../t/test_deinit.inc
+--enable_result_log
+--enable_query_log

storage/spider/spd_conn.h

@@ -26,24 +26,62 @@
 #define SPIDER_SIMPLE_CHECKSUM_TABLE      4
 #endif
 
+/*
+  The SPIDER_CONN_LOOP_CHECK has been added to the loop_check queue to
+  check for self-reference.
+*/
 #define SPIDER_LOP_CHK_QUEUED             (1 << 0)
+/*
+  The SPIDER_CONN_LOOP_CHECK is a merge of multiple
+  SPIDER_CONN_LOOP_CHECKs with the same data node table
+*/
 #define SPIDER_LOP_CHK_MERAGED            (1 << 1)
+/*
+  The SPIDER_CONN_LOOP_CHECK has been ignored because it has already
+  been marked as checked
+*/
 #define SPIDER_LOP_CHK_IGNORED            (1 << 2)
 
+/* Used for self-reference check. */
 typedef struct st_spider_conn_loop_check
 {
+  /*
+    Could be 0, SPIDER_LOP_CHK_QUEUED, SPIDER_LOP_CHK_MERAGED, or
+    SPIDER_LOP_CHK_IGNORED
+  */
   uint               flag;
 #ifdef SPIDER_HAS_HASH_VALUE_TYPE
+  /* hash value of to_name, used for the hash conn->loop_checked */
   my_hash_value_type hash_value_to;
-  my_hash_value_type hash_value_full;
 #endif
-  LEX_CSTRING        from_name;
+  /*
+    The fully qualified name of the current spider table, which will
+    also be used to construct the user var name to set in the data
+    node
+  */
   LEX_CSTRING        cur_name;
+  /*
+    The fully qualified data node table name, also used as key in
+    conn->loop_check_queue
+  */
   LEX_CSTRING        to_name;
+  /*
+    A concatenation of from_value, cur_name and to_name, used as key
+    in hash conn->loop_checked
+  */
   LEX_CSTRING        full_name;
+  /*
+    The first component of the uservar value on the current server,
+    consisting of information of a table that uses the current spider
+    table as a data node
+  */
   LEX_CSTRING        from_value;
+  /*
+    The uservar value to set in the data node, a concatenation of info
+    of tables, mac addresses and process ids of tables that use the
+    current spider table as the data node
+  */
   LEX_CSTRING        merged_value;
-  st_spider_conn_loop_check *next;
 } SPIDER_CONN_LOOP_CHECK;
 
 uchar *spider_conn_get_key(

storage/spider/spd_db_include.cc

@@ -66,7 +66,7 @@ spider_db_conn::spider_db_conn(
   DBUG_VOID_RETURN;
 }
 
-int spider_db_conn::fin_loop_check()
+int spider_db_conn::fin_loop_check() /* reset flags of all relevant SPIDER_CONN_LOOP_CHECKs */
 {
   st_spider_conn_loop_check *lcptr;
   DBUG_ENTER("spider_db_conn::fin_loop_check");
@@ -82,20 +82,6 @@ int spider_db_conn::fin_loop_check()
     }
     my_hash_reset(&conn->loop_check_queue);
   }
-  lcptr = conn->loop_check_ignored_first;
-  while (lcptr)
-  {
-    lcptr->flag = 0;
-    lcptr = lcptr->next;
-  }
-  conn->loop_check_ignored_first = NULL;
-  lcptr = conn->loop_check_meraged_first;
-  while (lcptr)
-  {
-    lcptr->flag = 0;
-    lcptr = lcptr->next;
-  }
-  conn->loop_check_meraged_first = NULL;
   DBUG_RETURN(0);
 }
 

storage/spider/spd_db_mysql.cc

@@ -5283,6 +5283,22 @@ int spider_db_mbase_util::append_time_zone(
   DBUG_RETURN(0);
 }
 
+/*
+  iterate over loop_check_queue and build queries for loop check
+
+  The query is in the form of
+
+  set @`spider_lc_.<to_table>` =
+  '-<mac1>-<pid1>-<from_table1>--<mac2>-<pid2>-<from_table2>-...'
+
+  where from_table1, from_table2 etc. are spider tables whose direct
+  or indirect remote data nodes contain to_table.
+
+  e.g. if t0->t1->t2, then the query could be:
+
+  set @`spider_lc_./test/t2` =
+  '-234567890abc-def012-./test/t1--1234567890ab-cdef01-./test/t0-'
+  */
 int spider_db_mbase_util::append_loop_check(
   spider_string *str,
   SPIDER_CONN *conn

storage/spider/spd_include.h

@@ -793,19 +793,24 @@ typedef struct st_spider_conn
   SPIDER_IP_PORT_CONN *ip_port_conn;
 
   pthread_mutex_t    loop_check_mutex;
+  /*
+    A hash of SPIDER_CONN_LOOP_CHECK, indexed by
+    SPIDER_CONN_LOOP_CHECK::full_name
+  */
   HASH               loop_checked;
   uint               loop_checked_id;
   const char         *loop_checked_func_name;
   const char         *loop_checked_file_name;
   ulong              loop_checked_line_no;
+  /*
+    A hash of SPIDER_CONN_LOOP_CHECK, indexed by
+    SPIDER_CONN_LOOP_CHECK::to_name
+  */
   HASH               loop_check_queue;
   uint               loop_check_queue_id;
   const char         *loop_check_queue_func_name;
   const char         *loop_check_queue_file_name;
   ulong              loop_check_queue_line_no;
-  SPIDER_CONN_LOOP_CHECK *loop_check_ignored_first;
-  SPIDER_CONN_LOOP_CHECK *loop_check_ignored_last;
-  SPIDER_CONN_LOOP_CHECK *loop_check_meraged_first;
 } SPIDER_CONN;
 
 typedef struct st_spider_lgtm_tblhnd_share