MDEV-34716 spider: some trivial cleanups and documentation

- document tmp_share, which are temporary spider shares with only one
link (no ha)
- simplify spider_get_sys_tables_connect_info() where link_idx is
always 0

storage/spider/spd_copy_tables.cc

@@ -246,6 +246,7 @@ int spider_udf_get_copy_tgt_tables(
   char table_key[MAX_KEY_LENGTH];
   SPIDER_COPY_TABLE_CONN *table_conn = NULL, *src_table_conn_prev = NULL,
     *dst_table_conn_prev = NULL;
+  /* This share has only one link. */
   SPIDER_SHARE *tmp_share;
   char **tmp_connect_info;
   uint *tmp_connect_info_length;

storage/spider/spd_include.h

@@ -1465,6 +1465,7 @@ typedef struct st_spider_mon_table_result
 
 typedef struct st_spider_table_mon
 {
+  /* This share has only one link. */
   SPIDER_SHARE               *share;
   uint32                     server_id;
   st_spider_table_mon_list   *parent;
@@ -1491,6 +1492,7 @@ typedef struct st_spider_table_mon_list
   SPIDER_TABLE_MON           *current;
   volatile int               mon_status;
 
+  /* This share has only one link */
   SPIDER_SHARE               *share;
 
   pthread_mutex_t            caller_mutex;
@@ -1504,6 +1506,7 @@ typedef struct st_spider_table_mon_list
 
 typedef struct st_spider_copy_table_conn
 {
+  /* This share has only one link. */
   SPIDER_SHARE               *share;
   int                        link_idx;
   SPIDER_CONN                *conn;

storage/spider/spd_table.cc

@@ -946,6 +946,7 @@ int spider_free_share_alloc(
   DBUG_RETURN(0);
 }
 
+/* Free a tmp_share, which has only one link */
 void spider_free_tmp_share_alloc(
   SPIDER_SHARE *share
 ) {
@@ -5892,6 +5893,7 @@ int spider_open_all_tables(
   THD *thd = trx->thd;
   TABLE *table_tables;
   int error_num, *need_mon, mon_val;
+  /* This share has only one link */
   SPIDER_SHARE tmp_share;
   char *db_name, *table_name;
   uint db_name_length, table_name_length;
@@ -5973,9 +5975,6 @@ int spider_open_all_tables(
         table_name_length
       )) ||
       (error_num = spider_create_conn_keys(&tmp_share)) ||
-/*
-      (error_num = spider_db_create_table_names_str(&tmp_share)) ||
-*/
       (error_num = spider_create_tmp_dbton_share(&tmp_share))
     ) {
       spider_sys_index_end(table_tables);
@@ -7367,6 +7366,10 @@ bool spider_check_pk_update(
   DBUG_RETURN(FALSE);
 }
 
+/*
+  Set fields of a tmp share which has only one link. For use in
+  monitoring, spider_copy_tables udf etc.
+*/
 void spider_set_tmp_share_pointer(
   SPIDER_SHARE *tmp_share,
   char **tmp_connect_info,

storage/spider/spd_trx.cc

@@ -2746,6 +2746,7 @@ int spider_internal_xa_rollback_by_xid(
   int error_num;
   char xa_key[MAX_KEY_LENGTH];
   char xa_member_key[MAX_KEY_LENGTH];
+  /* This share has only one link */
   SPIDER_SHARE tmp_share;
   char *tmp_connect_info[SPIDER_TMP_SHARE_CHAR_PTR_COUNT];
   uint tmp_connect_info_length[SPIDER_TMP_SHARE_UINT_COUNT];