Bug#50561: ALTER PARTITIONS does not have adequate lock, breaks with

concurrent I_S query

There were two problem:
1) MYSQL_LOCK_IGNORE_FLUSH also ignored name locks
2) there was a race between abort_and_upgrade_locks and
   alter_close_tables
   (i.e. remove_table_from_cache and
    close_data_files_and_morph_locks)

Which allowed the table to be opened with MYSQL_LOCK_IGNORE_FLUSH flag
resulting in renaming a partition that was already in use,
which could cause the table to be unusable.

Solution was to not allow IGNORE_FLUSH to skip waiting for
a named locked table.

And to not release the LOCK_open mutex between the
calls to remove_table_from_cache and
close_data_files_and_morph_locks by merging the functions
abort_and_upgrade_locks and alter_close_tables.

sql/sql_show.cc

@@ -30,6 +30,7 @@
 #include "event_data_objects.h"
 #endif
 #include <my_dir.h>
+#include "debug_sync.h"
 
 #define STR_OR_NIL(S) ((S) ? (S) : "<nil>")
 
@@ -3447,6 +3448,7 @@ int get_all_tables(THD *thd, TABLE_LIST *tables, COND *cond)
             lex->sql_command= SQLCOM_SHOW_FIELDS;
             show_table_list->i_s_requested_object=
               schema_table->i_s_requested_object;
+            DEBUG_SYNC(thd, "before_open_in_get_all_tables");
             res= open_normal_and_derived_tables(thd, show_table_list,
                                                 MYSQL_LOCK_IGNORE_FLUSH);
             lex->sql_command= save_sql_command;