when we update thd->db in replication, it's safer to update thd->db_length too.

This does not fix any known bug, but is still a good idea.

sql/log_event.cc

@@ -1806,7 +1806,7 @@ int Query_log_event::exec_event(struct st_relay_log_info* rli)
 {
   int expected_error, actual_error= 0;
   init_sql_alloc(&thd->mem_root, thd->variables.query_alloc_block_size,0);
-  thd->db= (char*) rewrite_db(db);
+  thd->db= (char*) rewrite_db(db); // thd->db_length is set later if needed
 
   /*
     InnoDB internally stores the master log position it has processed so far;
@@ -1836,6 +1836,11 @@ int Query_log_event::exec_event(struct st_relay_log_info* rli)
   {
     thd->set_time((time_t)when);
     thd->current_tablenr = 0;
+    /*
+      We cannot use db_len from event to fill thd->db_length, because
+      rewrite_db() may have changed db.
+    */ 
+    thd->db_length= thd->db ? strlen(thd->db) : 0;
     thd->query_length= q_len;
     thd->query= (char *) query;
     VOID(pthread_mutex_lock(&LOCK_thread_count));
@@ -1930,7 +1935,7 @@ end:
   VOID(pthread_mutex_lock(&LOCK_thread_count));
   thd->db= 0;	                        // prevent db from being freed
   thd->query= 0;			// just to be sure
-  thd->query_length= 0;
+  thd->query_length= thd->db_length =0;
   VOID(pthread_mutex_unlock(&LOCK_thread_count));
   close_thread_tables(thd);      
   free_root(&thd->mem_root,MYF(MY_KEEP_PREALLOC));
@@ -1968,7 +1973,7 @@ int Load_log_event::exec_event(NET* net, struct st_relay_log_info* rli,
 {
   char *load_data_query= 0;
   init_sql_alloc(&thd->mem_root, thd->variables.query_alloc_block_size, 0);
-  thd->db= (char*) rewrite_db(db);
+  thd->db= (char*) rewrite_db(db); // thd->db_length is set later if needed
   DBUG_ASSERT(thd->query == 0);
   clear_all_errors(thd, rli);
 
@@ -2001,6 +2006,7 @@ int Load_log_event::exec_event(NET* net, struct st_relay_log_info* rli,
   {
     thd->set_time((time_t)when);
     thd->current_tablenr = 0;
+    thd->db_length= thd->db ? strlen(thd->db) : 0;
     VOID(pthread_mutex_lock(&LOCK_thread_count));
     thd->query_id = query_id++;
     VOID(pthread_mutex_unlock(&LOCK_thread_count));
@@ -2117,7 +2123,7 @@ Slave: load data infile on table '%s' at log position %s in log \
   VOID(pthread_mutex_lock(&LOCK_thread_count));
   thd->db= 0;
   thd->query= 0;
-  thd->query_length= 0;
+  thd->query_length= thd->db_length= 0;
   VOID(pthread_mutex_unlock(&LOCK_thread_count));
   close_thread_tables(thd);
   if (load_data_query)

sql/slave.cc

@@ -1124,6 +1124,7 @@ slaves can't replicate a 5.0 or newer master.";
   Used by fetch_master_table (used by LOAD TABLE tblname FROM MASTER and LOAD
   DATA FROM MASTER). Drops the table (if 'overwrite' is true) and recreates it
   from the dump. Honours replication inclusion/exclusion rules.
+  db must be non-zero (guarded by assertion).
 
   RETURN VALUES
     0           success
@@ -1134,8 +1135,8 @@ static int create_table_from_dump(THD* thd, NET* net, const char* db,
 				  const char* table_name, bool overwrite)
 {
   ulong packet_len = my_net_read(net); // read create table statement
-  char *query;
-  char* save_db;
+  char *query, *save_db;
+  uint32 save_db_length;
   Vio* save_vio;
   HA_CHECK_OPT check_opt;
   TABLE_LIST tables;
@@ -1193,9 +1194,13 @@ static int create_table_from_dump(THD* thd, NET* net, const char* db,
   thd->proc_info = "Creating table from master dump";
   // save old db in case we are creating in a different database
   save_db = thd->db;
+  save_db_length= thd->db_length;
   thd->db = (char*)db;
+  DBUG_ASSERT(thd->db);
+  thd->db_length= strlen(thd->db);
   mysql_parse(thd, thd->query, packet_len); // run create table
   thd->db = save_db;		// leave things the way the were before
+  thd->db_length= save_db_length;
   thd->options = save_options;
   
   if (thd->query_error)
@@ -2689,7 +2694,7 @@ err:
 		  IO_RPL_LOG_NAME, llstr(mi->master_log_pos,llbuff));
   VOID(pthread_mutex_lock(&LOCK_thread_count));
   thd->query = thd->db = 0; // extra safety
-  thd->query_length = 0;
+  thd->query_length= thd->db_length= 0;
   VOID(pthread_mutex_unlock(&LOCK_thread_count));
   if (mysql)
   {
@@ -2838,7 +2843,7 @@ the slave SQL thread with \"SLAVE START\". We stopped at log \
  err:
   VOID(pthread_mutex_lock(&LOCK_thread_count));
   thd->query = thd->db = 0; // extra safety
-  thd->query_length = 0;
+  thd->query_length= thd->db_length= 0;
   VOID(pthread_mutex_unlock(&LOCK_thread_count));
   thd->proc_info = "Waiting for slave mutex on exit";
   pthread_mutex_lock(&rli->run_lock);

sql/sql_db.cc

@@ -385,6 +385,15 @@ err:
 }
 
 
+/*
+  Changes the current database.
+
+  NOTES
+    Do as little as possible in this function, as it is not called for the
+    replication slave SQL thread (for that thread, setting of thd->db is done
+    in ::exec_event() methods of log_event.cc).
+*/
+
 bool mysql_change_db(THD *thd,const char *name)
 {
   int length, db_length;