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Fix remaining cases of Bug #3596: fix possible races caused by an obsolete value of thd->query_length in SHOW PROCESSLIST and SHOW INNODB STATUS; this fix depends on the fact that thd->query is always set to NULL before setting it to point to a new query
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heikki@hundin.mysql.fi
2004-05-14 16:48:56 +03:00
parent b4ae2577f0
commit 02f51ccf2d
7 changed files with 33 additions and 5 deletions
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19
sql/sql_class.h
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@ -360,7 +360,24 @@ 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.
*/
char *query; // Points to the current query,
/*
A pointer to the stack frame of handle_one_connection(),