A fix and a test case for Bug#47648 "main.merge fails sporadically".

If a prepared statement used both a MyISAMMRG table and a stored 
function or trigger, execution could fail with "No such table"
error or crash. 
The error would come from a failure of the MyISAMMRG engine
to meet the expectations of the prelocking algorithm, 
in particular maintain lex->query_tables_own_last pointer
in sync with lex->query_tables_last pointer/the contents
of lex->query_tables. When adding merge children, the merge
engine would extend the table list. Then, when adding 
prelocked tables, the prelocking algorithm would use a pointer
to the last merge child to assign to lex->query_tables_own_last.
Then, when merge children were removed at the end of
open_tables(), lex->query_tables_own_last
was not updated, and kept pointing
to a removed merge child.

The fix ensures that query_tables_own_last is always in
sync with lex->query_tables_last.

This is a regression introduced by WL#4144 and present only
in next-4284 tree and 6.0.

mysql-test/r/merge.result:
  Update results (Bug#47648).
mysql-test/t/merge.test:
  Add a test case for Bug#47648.
  Update the result file to reflect a fix of another bug
  in MyISAMMRG code: not maintaining lex->query_tables_own_last
  allowed a stored function or trigger to modify a merge table
  which was already updated by the main statement.
  It is not allowed for other storage engines, and should
  not be allowed for MyISAMMRG.
storage/myisammrg/ha_myisammrg.cc:
  When adding children to the list of tables to open,
  make sure that we properly set lex->query_tables_own_last.
  When removing the children, update lex->query_tables_own_last
  if necessary.

1 files changed, 21 insertions, 0 deletions

diff --git a/storage/myisammrg/ha_myisammrg.cc b/storage/myisammrg/ha_myisammrg.cc
index 60de6d361e5..066290ab515 100644
--- a/storage/myisammrg/ha_myisammrg.cc
+++ b/storage/myisammrg/ha_myisammrg.cc
@@ -458,6 +458,18 @@ int ha_myisammrg::add_children_list(void)
   */
   if (thd->lex->query_tables_last == &parent_l->next_global)
     thd->lex->query_tables_last= this->children_last_l;
+  /*
+    The branch below works only when re-executing a prepared
+    statement or a stored routine statement:
+    We've just modified query_tables_last. Keep it in sync with
+    query_tables_last_own, if it was set by the prelocking code.
+    This ensures that the check that prohibits double updates (*)
+    can correctly identify what tables belong to the main statement.
+    (*) A double update is, e.g. when a user issues UPDATE t1 and
+    t1 has an AFTER UPDATE trigger that also modifies t1.
+  */
+  if (thd->lex->query_tables_own_last == &parent_l->next_global)
+    thd->lex->query_tables_own_last= this->children_last_l;
 
 end:
   DBUG_RETURN(0);
@@ -888,6 +900,15 @@ int ha_myisammrg::detach_children(void)
     if (thd->lex->query_tables_last == this->children_last_l)
       thd->lex->query_tables_last= this->children_l->prev_global;
 
+    /*
+      If the statement requires prelocking, and prelocked
+      tables were added right after merge children, modify the
+      last own table pointer to point at prev_global of the merge
+      parent.
+    */
+    if (thd->lex->query_tables_own_last == this->children_last_l)
+      thd->lex->query_tables_own_last= this->children_l->prev_global;
+
     /* Terminate child list. So it cannot be tried to remove again. */
     *this->children_last_l= NULL;
     this->children_l->prev_global= NULL;