From 117b76a562431090ca9f0b29702f921872065970 Mon Sep 17 00:00:00 2001
From: "konstantin@mysql.com" <>
Date: Tue, 27 Jun 2006 00:47:52 +0400
Subject:  A fix and a test case for  Bug#19022 "Memory bug when switching db
 during trigger execution"  Bug#17199 "Problem when view calls function from
 another database."  Bug#18444 "Fully qualified stored function names don't
 work correctly in             SELECT statements"

 Documentation note: this patch introduces a change in behaviour of prepared
 statements.

 This patch adds a few new invariants with regard to how THD::db should
 be used. These invariants should be preserved in future:

  - one should never refer to THD::db by pointer and always make a deep copy
    (strmake, strdup)
  - one should never compare two databases by pointer, but use strncmp or
    my_strncasecmp
  - TABLE_LIST object table->db should be always initialized in the parser or
    by creator of the object.

    For prepared statements it means that if the current database is changed
    after a statement is prepared, the database that was current at prepare
    remains active. This also means that you can not prepare a statement that
    implicitly refers to the current database if the latter is not set.
    This is not documented, and therefore needs documentation. This is NOT a
    change in behavior for almost all SQL statements except:
     - ALTER TABLE t1 RENAME t2
     - OPTIMIZE TABLE t1
     - ANALYZE TABLE t1
     - TRUNCATE TABLE t1 --
     until this patch t1 or t2 could be evaluated at the first execution of
     prepared statement.

     CURRENT_DATABASE() still works OK and is evaluated at every execution
     of prepared statement.

     Note, that in stored routines this is not an issue as the default
     database is the database of the stored procedure and "use" statement
     is prohibited in stored routines.

  This patch makes obsolete the use of check_db_used (it was never used in the
  old code too) and all other places that check for table->db and assign it
  from THD::db if it's NULL, except the parser.

 How this patch was created: THD::{db,db_length} were replaced with a
 LEX_STRING, THD::db. All the places that refer to THD::{db,db_length} were
 manually checked and:
  - if the place uses thd->db by pointer, it was fixed to make a deep copy
  - if a place compared two db pointers, it was fixed to compare them by value
    (via strcmp/my_strcasecmp, whatever was approproate)
 Then this intermediate patch was used to write a smaller patch that does the
 same thing but without a rename.

 TODO in 5.1:
   - remove check_db_used
   - deploy THD::set_db in mysql_change_db

 See also comments to individual files.
---
 sql/sql_class.h | 43 ++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 42 insertions(+), 1 deletion(-)

(limited to 'sql/sql_class.h')

diff --git a/sql/sql_class.h b/sql/sql_class.h
index 0ddba0e6f05..b63f88d7210 100644
--- a/sql/sql_class.h
+++ b/sql/sql_class.h
@@ -1570,6 +1570,47 @@ public:
   void restore_sub_statement_state(Sub_statement_state *backup);
   void set_n_backup_active_arena(Query_arena *set, Query_arena *backup);
   void restore_active_arena(Query_arena *set, Query_arena *backup);
+
+  /*
+    Initialize the current database from a NULL-terminated string with length
+  */
+  void set_db(const char *new_db, uint new_db_len)
+  {
+    if (new_db)
+    {
+      /* Do not reallocate memory if current chunk is big enough. */
+      if (db && db_length >= new_db_len)
+        memcpy(db, new_db, new_db_len+1);
+      else
+      {
+        safeFree(db);
+        db= my_strdup_with_length(new_db, new_db_len, MYF(MY_WME));
+      }
+      db_length= db ? new_db_len: 0;
+    }
+  }
+  void reset_db(char *new_db, uint new_db_len)
+  {
+    db= new_db;
+    db_length= new_db_len;
+  }
+  /*
+    Copy the current database to the argument. Use the current arena to
+    allocate memory for a deep copy: current database may be freed after
+    a statement is parsed but before it's executed.
+  */
+  bool copy_db_to(char **p_db, uint *p_db_length)
+  {
+    if (db == NULL)
+    {
+      my_message(ER_NO_DB_ERROR, ER(ER_NO_DB_ERROR), MYF(0));
+      return TRUE;
+    }
+    *p_db= strmake(db, db_length);
+    if (p_db_length)
+      *p_db_length= db_length;
+    return FALSE;
+  }
 };
 
 
@@ -1915,7 +1956,7 @@ typedef struct st_sort_buffer {
 
 class Table_ident :public Sql_alloc
 {
- public:
+public:
   LEX_STRING db;
   LEX_STRING table;
   SELECT_LEX_UNIT *sel;
-- 
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