Commit 63af3d3a authored by unknown's avatar unknown
Browse files

Move Heikki's comment about thd->query comment to proper place 

(bad merge from 4.0)


sql/sql_class.h:
  Move a comment to proper place (bad merge from 4.0)
parent 9b8586bf

sql/sql_class.h

+17 −18
Original line number Diff line number Diff line
@@ -544,6 +544,23 @@ class Statement: public Item_arena 
    Points to the query associated with this statement. It's const, but

    we need to declare it char * because all table handlers are written

    in C and need to point to it.



    Note that (A) if we set query = NULL, we must at the same time set

    query_length = 0, and protect the whole operation with the

    LOCK_thread_count mutex. And (B) we are ONLY allowed to set query to a

    non-NULL value if its previous value is NULL. We do not need to protect

    operation (B) with any mutex. To avoid crashes in races, if we do not

    know that thd->query cannot change at the moment, one should print

    thd->query like this:

      (1) reserve the LOCK_thread_count mutex;

      (2) check if thd->query is NULL;

      (3) if not NULL, then print at most thd->query_length characters from

      it. We will see the query_length field as either 0, or the right value

      for it.

    Assuming that the write and read of an n-bit memory field in an n-bit

    computer is atomic, we can avoid races in the above way. 

    This printing is needed at least in SHOW PROCESSLIST and SHOW INNODB

    STATUS.

  */

  char *query;

  uint32 query_length;                          // current query length
@@ -684,24 +701,6 @@ class THD :public ilink, 
  struct  rand_struct rand;		// used for authentication

  struct  system_variables variables;	// Changeable local variables

  pthread_mutex_t LOCK_delete;		// Locked before thd is deleted

  /* 

    Note that (A) if we set query = NULL, we must at the same time set

    query_length = 0, and protect the whole operation with the

    LOCK_thread_count mutex. And (B) we are ONLY allowed to set query to a

    non-NULL value if its previous value is NULL. We do not need to protect

    operation (B) with any mutex. To avoid crashes in races, if we do not

    know that thd->query cannot change at the moment, one should print

    thd->query like this:

      (1) reserve the LOCK_thread_count mutex;

      (2) check if thd->query is NULL;

      (3) if not NULL, then print at most thd->query_length characters from

      it. We will see the query_length field as either 0, or the right value

      for it.

    Assuming that the write and read of an n-bit memory field in an n-bit

    computer is atomic, we can avoid races in the above way. 

    This printing is needed at least in SHOW PROCESSLIST and SHOW INNODB

    STATUS.

  */

  /* all prepared statements and cursors of this connection */

  Statement_map stmt_map; 

  /*