Bug #45796: invalid memory reads and writes when altering merge

            and base tables 

myrg_attach_children() could reuse a buffer that was allocated 
previously based on a definition of a child table. The problem 
was that the child's definition might have been changed, so 
reusing the buffer could lead to crashes or valgrind errors 
under some circumstances. 
 
Fixed by changing myrg_attach_children() so that the 
rec_per_key_part buffer is reused only when the child table
have not changed, and reallocated otherwise (the old buffer is 
deallocated if necessary).

include/myisammrg.h

@@ -88,7 +88,8 @@ extern MYRG_INFO *myrg_parent_open(const char *parent_name,
                                    void *callback_param);
 extern int myrg_attach_children(MYRG_INFO *m_info, int handle_locking,
                                 MI_INFO *(*callback)(void*),
-                                void *callback_param);
+                                void *callback_param,
+                                my_bool *need_compat_check);
 extern int myrg_detach_children(MYRG_INFO *m_info);
 extern int myrg_panic(enum ha_panic_function function);
 extern int myrg_rfirst(MYRG_INFO *file,uchar *buf,int inx);

mysql-test/r/merge.result

@@ -2127,4 +2127,18 @@ SELECT * FROM m1;
 ERROR HY000: Unable to open underlying table which is differently defined or of non-MyISAM type or doesn't exist
 DROP VIEW v1;
 DROP TABLE m1, t1;
+#
+# Bug #45796: invalid memory reads and writes when altering merge and 
+#             base tables
+#
+CREATE TABLE t1(c1 INT) ENGINE=MyISAM;
+CREATE TABLE m1(c1 INT) ENGINE=MERGE UNION=(t1);
+ALTER TABLE m1 ADD INDEX idx_c1(c1);
+SELECT * FROM m1;
+ERROR HY000: Unable to open underlying table which is differently defined or of non-MyISAM type or doesn't exist
+ALTER TABLE t1 ADD INDEX idx_c1(c1);
+SELECT * FROM m1;
+c1
+DROP TABLE m1;
+DROP TABLE t1;
 End of 5.1 tests

mysql-test/t/merge.test

@@ -1535,4 +1535,24 @@ SELECT * FROM m1;
 
 DROP VIEW v1;
 DROP TABLE m1, t1;
+
+--echo #
+--echo # Bug #45796: invalid memory reads and writes when altering merge and 
+--echo #             base tables
+--echo #
+
+CREATE TABLE t1(c1 INT) ENGINE=MyISAM;
+CREATE TABLE m1(c1 INT) ENGINE=MERGE UNION=(t1);
+ALTER TABLE m1 ADD INDEX idx_c1(c1);
+# Open the MERGE table and allocate buffers based on children's definition.
+--error ER_WRONG_MRG_TABLE
+SELECT * FROM m1;
+# Change the child table definition.
+ALTER TABLE t1 ADD INDEX idx_c1(c1);
+# Check that old buffers are not reused
+SELECT * FROM m1;
+
+DROP TABLE m1;
+DROP TABLE t1;
+
 --echo End of 5.1 tests

storage/myisammrg/ha_myisammrg.cc

@@ -545,7 +545,8 @@ int ha_myisammrg::attach_children(void)
 
   if (myrg_attach_children(this->file, this->test_if_locked |
                            current_thd->open_options,
-                           myisammrg_attach_children_callback, this))
+                           myisammrg_attach_children_callback, this,
+                           (my_bool *) &need_compat_check))
   {
     DBUG_PRINT("error", ("my_errno %d", my_errno));
     DBUG_RETURN(my_errno ? my_errno : -1);

storage/myisammrg/myrg_open.c

@@ -365,11 +365,14 @@ MYRG_INFO *myrg_parent_open(const char *parent_name,
     The callback returns the MyISAM table handle of the child table.
     Check table definition match.
 
-  @param[in]    m_info          MERGE parent table structure
-  @param[in]    handle_locking  if contains HA_OPEN_FOR_REPAIR, warn about
-                                incompatible child tables, but continue
-  @param[in]    callback        function to call for each child table
-  @param[in]    callback_param  data pointer to give to the callback
+  @param[in]    m_info            MERGE parent table structure
+  @param[in]    handle_locking    if contains HA_OPEN_FOR_REPAIR, warn about
+                                  incompatible child tables, but continue
+  @param[in]    callback          function to call for each child table
+  @param[in]    callback_param    data pointer to give to the callback
+  @param[in]    need_compat_check pointer to ha_myisammrg::need_compat_check
+                                  (we need this one to decide if previously
+                                  allocated buffers can be reused).
 
   @return status
     @retval     0               OK
@@ -382,7 +385,7 @@ MYRG_INFO *myrg_parent_open(const char *parent_name,
 
 int myrg_attach_children(MYRG_INFO *m_info, int handle_locking,
                          MI_INFO *(*callback)(void*),
-                         void *callback_param)
+                         void *callback_param, my_bool *need_compat_check)
 {
   ulonglong  file_offset;
   MI_INFO    *myisam;
@@ -423,6 +426,11 @@ int myrg_attach_children(MYRG_INFO *m_info, int handle_locking,
       m_info->reclength= myisam->s->base.reclength;
       min_keys=  myisam->s->base.keys;
       key_parts= myisam->s->base.key_parts;
+      if (*need_compat_check && m_info->rec_per_key_part)
+      {
+        my_free((char *) m_info->rec_per_key_part, MYF(0));
+        m_info->rec_per_key_part= NULL;
+      }
       if (!m_info->rec_per_key_part)
       {
         if(!(m_info->rec_per_key_part= (ulong*)