diff options
| -rw-r--r-- | sql/sql_class.h | 35 |
1 files changed, 17 insertions, 18 deletions
diff --git a/sql/sql_class.h b/sql/sql_class.h index aaa81fbe165..50b0c566c89 100644 --- a/sql/sql_class.h +++ b/sql/sql_class.h @@ -544,6 +544,23 @@ public: 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 @@ public: 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; /* |