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.cc

@@ -1151,27 +1151,41 @@ void spider_conn_queue_UTC_time_zone(
   DBUG_VOID_RETURN;
 }
 
+/*
+  Construct merged values and insert into the loop check queue
+
+  Search the loop_check_queue for the data node table, and if one does
+  not exist, construct the merged value in the same format as the
+  right hand side. Otherwise, merge the right hand side of the
+  existing SPIDER_CONN_LOOP_CHECK with the right hand side of lcptr
+  into one right hand side. In either case, add the
+  SPIDER_CONN_LOOP_CHECK to the loop check queue
+*/
 int spider_conn_queue_and_merge_loop_check(
   SPIDER_CONN *conn,
   SPIDER_CONN_LOOP_CHECK *lcptr
 ) {
   int error_num = HA_ERR_OUT_OF_MEM;
-  char *tmp_name, *from_name, *cur_name, *to_name, *full_name, *from_value,
+  char *tmp_name, *cur_name, *to_name, *full_name, *from_value,
     *merged_value;
   SPIDER_CONN_LOOP_CHECK *lcqptr, *lcrptr;
   DBUG_ENTER("spider_conn_queue_and_merge_loop_check");
   DBUG_PRINT("info", ("spider conn=%p", conn));
 #ifdef SPIDER_HAS_HASH_VALUE_TYPE
-  if (unlikely(!(lcqptr = (SPIDER_CONN_LOOP_CHECK *)
+  if (!(lcqptr = (SPIDER_CONN_LOOP_CHECK *)
     my_hash_search_using_hash_value(&conn->loop_check_queue,
     lcptr->hash_value_to,
-    (uchar *) lcptr->to_name.str, lcptr->to_name.length))))
+    (uchar *) lcptr->to_name.str, lcptr->to_name.length)))
 #else
   if (unlikely(!(lcqptr = (SPIDER_CONN_LOOP_CHECK *) my_hash_search(
     &conn->loop_check_queue,
     (uchar *) lcptr->to_name.str, lcptr->to_name.length))))
 #endif
   {
+    /*
+      Construct the right hand side:
+      -<mac>-<pid>-<cur_table>-<from_value>
+    */
     DBUG_PRINT("info", ("spider create merged_value and insert"));
     lcptr->merged_value.length = spider_unique_id.length +
       lcptr->cur_name.length + lcptr->from_value.length + 1;
@@ -1194,10 +1208,10 @@ int spider_conn_queue_and_merge_loop_check(
     }
     lcptr->flag |= SPIDER_LOP_CHK_QUEUED;
   } else {
+    /* Merge lcptr and lcqptr into a newly created lcrptr. */
     DBUG_PRINT("info", ("spider append merged_value and replace"));
     if (unlikely(!spider_bulk_malloc(spider_current_trx, 271, MYF(MY_WME),
       &lcrptr, (uint) (sizeof(SPIDER_CONN_LOOP_CHECK)),
-      &from_name, (uint) (lcqptr->from_name.length + 1),
       &cur_name, (uint) (lcqptr->cur_name.length + 1),
       &to_name, (uint) (lcqptr->to_name.length + 1),
       &full_name, (uint) (lcqptr->full_name.length + 1),
@@ -1209,13 +1223,13 @@ int spider_conn_queue_and_merge_loop_check(
     )) {
       goto error_alloc_loop_check_replace;
     }
+    /*
+      TODO: the new lcrptr has the same cur_name, to_name, full_name
+      and from_value as lcqptr, but they do not seem to be relevant.
+    */
 #ifdef SPIDER_HAS_HASH_VALUE_TYPE
     lcrptr->hash_value_to = lcqptr->hash_value_to;
-    lcrptr->hash_value_full = lcqptr->hash_value_full;
 #endif
-    lcrptr->from_name.str = from_name;
-    lcrptr->from_name.length = lcqptr->from_name.length;
-    memcpy(from_name, lcqptr->from_name.str, lcqptr->from_name.length + 1);
     lcrptr->cur_name.str = cur_name;
     lcrptr->cur_name.length = lcqptr->cur_name.length;
     memcpy(cur_name, lcqptr->cur_name.str, lcqptr->cur_name.length + 1);
@@ -1228,8 +1242,14 @@ int spider_conn_queue_and_merge_loop_check(
     lcrptr->from_value.str = from_value;
     lcrptr->from_value.length = lcqptr->from_value.length;
     memcpy(from_value, lcqptr->from_value.str, lcqptr->from_value.length + 1);
+    /*
+      The merged_value of lcrptr is a concatenation of that of lcqptr
+      and constructed merged_value from lcptr.
+    */
     lcrptr->merged_value.str = merged_value;
-    lcrptr->merged_value.length = lcqptr->merged_value.length;
+    lcrptr->merged_value.length =
+      lcqptr->merged_value.length + spider_unique_id.length +
+      lcptr->cur_name.length + 1 + lcptr->from_value.length;
     memcpy(merged_value,
       lcqptr->merged_value.str, lcqptr->merged_value.length);
     merged_value += lcqptr->merged_value.length;
@@ -1273,9 +1293,6 @@ int spider_conn_queue_and_merge_loop_check(
       goto error_hash_insert_queue;
     }
     lcptr->flag = SPIDER_LOP_CHK_MERAGED;
-    lcptr->next = NULL;
-    if (!conn->loop_check_meraged_first)
-      conn->loop_check_meraged_first = lcptr;
   }
   DBUG_RETURN(0);
 
@@ -1296,7 +1313,6 @@ int spider_conn_queue_and_merge_loop_check(
 int spider_conn_reset_queue_loop_check(
   SPIDER_CONN *conn
 ) {
-  int error_num;
   SPIDER_CONN_LOOP_CHECK *lcptr;
   DBUG_ENTER("spider_conn_reset_queue_loop_check");
   uint l = 0;
@@ -1318,30 +1334,8 @@ int spider_conn_reset_queue_loop_check(
     ++l;
   }
 
-  lcptr = conn->loop_check_ignored_first;
-  while (lcptr)
-  {
-    lcptr->flag = 0;
-    if ((error_num = spider_conn_queue_and_merge_loop_check(conn, lcptr)))
-    {
-      goto error_queue_and_merge;
-    }
-    lcptr = lcptr->next;
-  }
-  conn->loop_check_meraged_first = NULL;
   pthread_mutex_unlock(&conn->loop_check_mutex);
   DBUG_RETURN(0);
-
-error_queue_and_merge:
-  lcptr = lcptr->next;
-  while (lcptr)
-  {
-    lcptr->flag = 0;
-    lcptr = lcptr->next;
-  }
-  conn->loop_check_meraged_first = NULL;
-  pthread_mutex_unlock(&conn->loop_check_mutex);
-  DBUG_RETURN(error_num);
 }
 
 int spider_conn_queue_loop_check(
@@ -1352,7 +1346,7 @@ int spider_conn_queue_loop_check(
   int error_num = HA_ERR_OUT_OF_MEM;
   uint conn_link_idx = spider->conn_link_idx[link_idx], buf_sz;
   char path[FN_REFLEN + 1];
-  char *tmp_name, *from_name, *cur_name, *to_name, *full_name, *from_value,
+  char *tmp_name, *cur_name, *to_name, *full_name, *from_value,
     *merged_value;
   user_var_entry *loop_check;
   char *loop_check_buf;
@@ -1363,6 +1357,17 @@ int spider_conn_queue_loop_check(
   LEX_CSTRING lex_str, from_str, to_str;
   DBUG_ENTER("spider_conn_queue_loop_check");
   DBUG_PRINT("info", ("spider conn=%p", conn));
+  /*
+    construct loop check user var name (left hand side) into
+    lex_str. It is of the format
+
+    spider_lc_<spider_table_name>
+
+    So if the spider table name is ./test/t1, then the constructed
+    user var name is:
+
+    spider_lc_./test/t1
+  */
   lex_str.length = top_share->path.length + SPIDER_SQL_LOP_CHK_PRM_PRF_LEN;
   buf_sz = lex_str.length + 2;
   loop_check_buf = (char *) my_alloca(buf_sz);
@@ -1388,6 +1393,16 @@ int spider_conn_queue_loop_check(
   } else {
     lex_str.str = loop_check->value;
     lex_str.length = loop_check->length;
+    /*
+      Validate that there are at least four dashes in the user var
+      value: -<mac_addr>-<proc_id>-<table_name>-
+
+      Note: if the value is merged from multiple values, such as
+
+      "-<mac1>-<pid1>-<table_name1>--<mac2>-<pid2>-<table_name2>--<mac3>-<pid3>-<table_name3>-"
+
+      then only the first component is put into from_str
+    */
     DBUG_PRINT("info", ("spider from_str=%s", lex_str.str));
     if (unlikely(!(tmp_name = strchr(loop_check->value, '-'))))
     {
@@ -1415,12 +1430,23 @@ int spider_conn_queue_loop_check(
     }
     else
     {
+      /*
+        Validation passed. Put the first component of rhs in from_str
+      */
       from_str.str = lex_str.str;
       from_str.length = tmp_name - lex_str.str + 1;
     }
   }
   my_afree(loop_check_buf);
-
+  /*
+    construct loop_check_buf as <from_str>-<cur>-<to_str> e.g.
+    "-<mac>-<pid>-./test/t0--./test/t1-./test/t2", later used as
+    full_name
+
+    from_str is the first component in the user var value (RHS) or
+    empty if user var value is empty, cur is the spider table, to_str
+    is the remote data node table
+  */
   to_str.length = build_table_filename(path, FN_REFLEN,
     share->tgt_dbs[conn_link_idx] ? share->tgt_dbs[conn_link_idx] : "",
     share->tgt_table_names[conn_link_idx], "", 0);
@@ -1457,7 +1483,7 @@ int spider_conn_queue_loop_check(
   lcptr = (SPIDER_CONN_LOOP_CHECK *) my_hash_search(
     &conn->loop_checked, (uchar *) loop_check_buf, buf_sz - 1);
 #endif
-  if (unlikely(
+  if (
     !lcptr ||
     (
       !lcptr->flag &&
@@ -1466,7 +1492,7 @@ int spider_conn_queue_loop_check(
         memcmp(lcptr->from_value.str, lex_str.str, lex_str.length)
       )
     )
-  ))
+  )
   {
     if (unlikely(lcptr))
     {
@@ -1482,7 +1508,6 @@ int spider_conn_queue_loop_check(
     DBUG_PRINT("info", ("spider alloc_lcptr"));
     if (unlikely(!spider_bulk_malloc(spider_current_trx, 272, MYF(MY_WME),
       &lcptr, (uint) (sizeof(SPIDER_CONN_LOOP_CHECK)),
-      &from_name, (uint) (from_str.length + 1),
       &cur_name, (uint) (top_share->path.length + 1),
       &to_name, (uint) (to_str.length + 1),
       &full_name, (uint) (buf_sz),
@@ -1495,9 +1520,6 @@ int spider_conn_queue_loop_check(
       goto error_alloc_loop_check;
     }
     lcptr->flag = 0;
-    lcptr->from_name.str = from_name;
-    lcptr->from_name.length = from_str.length;
-    memcpy(from_name, from_str.str, from_str.length + 1);
     lcptr->cur_name.str = cur_name;
     lcptr->cur_name.length = top_share->path.length;
     memcpy(cur_name, top_share->path.str, top_share->path.length + 1);
@@ -1510,12 +1532,19 @@ int spider_conn_queue_loop_check(
     lcptr->from_value.str = from_value;
     lcptr->from_value.length = lex_str.length;
     memcpy(from_value, lex_str.str, lex_str.length + 1);
+    /*
+      merged_value will only be populated later, in
+      spider_conn_queue_and_merge_loop_check()
+    */
     lcptr->merged_value.str = merged_value;
 #ifdef SPIDER_HAS_HASH_VALUE_TYPE
-    lcptr->hash_value_to = my_calc_hash(&conn->loop_checked,
+    lcptr->hash_value_to = my_calc_hash(&conn->loop_check_queue,
       (uchar *) to_str.str, to_str.length);
-    lcptr->hash_value_full = hash_value;
 #endif
+    /*
+      Mark as checked. It will be added to loop_check_queue in
+      spider_conn_queue_and_merge_loop_check() below for checking
+    */
 #ifdef HASH_UPDATE_WITH_HASH_VALUE
     if (unlikely(my_hash_insert_with_hash_value(&conn->loop_checked,
       lcptr->hash_value_full, (uchar *) lcptr)))
@@ -1527,19 +1556,11 @@ int spider_conn_queue_loop_check(
       goto error_hash_insert;
     }
   } else {
+    /* Already marked as checked, ignore and return. */
     if (!lcptr->flag)
     {
       DBUG_PRINT("info", ("spider add to ignored list"));
       lcptr->flag |= SPIDER_LOP_CHK_IGNORED;
-      lcptr->next = NULL;
-      if (!conn->loop_check_ignored_first)
-      {
-        conn->loop_check_ignored_first = lcptr;
-        conn->loop_check_ignored_last = lcptr;
-      } else {
-        conn->loop_check_ignored_last->next = lcptr;
-        conn->loop_check_ignored_last = lcptr;
-      }
     }
     pthread_mutex_unlock(&conn->loop_check_mutex);
     my_afree(loop_check_buf);

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