BUG#18054998 - BACKPORT FIX FOR BUG#11765785 to 5.5

This is a backport of the patch of bug#11765785. Commit message
by Prabakaran Thirumalai from bug#11765785 is reproduced below:
Description:
------------
Global Query ID (global_query_id ) is not incremented for PING and 
statistics command. These two query types are filtered before 
incrementing the global query id. This causes race condition and 
results in duplicate query id for different queries originating from 
different connections.
      
Analysis:
---------
sqlparse.cc::dispath_command() is the only place in code which sets 
thd->query_ id to global_query_id and then increments it based on the 
query type. In all other places it is incremented first and then 
assigned to thd->query_id.
      
This is done such that global_query_id is not incremented for PING 
and statistics commands in dispatch_command() function.
      
Fix:
----
As per suggestion from Serg, "There is no reason to skip query_id for 
the PING and STATISTICS command.", removing the check which filters 
PING and statistics commands.
      
Instead of using get_query_id() and next_query_id() which can still 
cause race condition if context switch happens soon after executing 
get_query_id(), changing the code to use next_query_id() instead of 
get_query_id() as it is done in other parts of code which deals with 
global_query_id.
      
Removed get_query_id() function and forced next_query_id() caller 
to use the return value by specifying warn_unused_result attribute.

sql/mysqld.h

@@ -438,7 +438,7 @@ extern my_atomic_rwlock_t global_query_id_lock;
 void unireg_end(void) __attribute__((noreturn));
 
 /* increment query_id and return it.  */
-inline query_id_t next_query_id()
+inline __attribute__((warn_unused_result)) query_id_t next_query_id()
 {
   query_id_t id;
   my_atomic_rwlock_wrlock(&global_query_id_lock);
@@ -447,16 +447,6 @@ inline query_id_t next_query_id()
   return (id+1);
 }
 
-inline query_id_t get_query_id()
-{
-  query_id_t id;
-  my_atomic_rwlock_wrlock(&global_query_id_lock);
-  id= my_atomic_load64(&global_query_id);
-  my_atomic_rwlock_wrunlock(&global_query_id_lock);
-  return id;
-}
-
-
 /*
   TODO: Replace this with an inline function.
  */

sql/sql_class.h

@@ -3667,7 +3667,9 @@ public:
 /**
   Skip the increase of the global query id counter. Commonly set for
   commands that are stateless (won't cause any change on the server
-  internal states).
+  internal states). This is made obsolete as query id is incremented for
+  ping and statistics commands as well because of race condition 
+  (Bug#58785).
 */
 #define CF_SKIP_QUERY_ID        (1U << 0)
 

sql/sql_parse.cc

@@ -237,8 +237,8 @@ void init_update_queries(void)
   /* Initialize the server command flags array. */
   memset(server_command_flags, 0, sizeof(server_command_flags));
 
-  server_command_flags[COM_STATISTICS]= CF_SKIP_QUERY_ID | CF_SKIP_QUESTIONS;
-  server_command_flags[COM_PING]=       CF_SKIP_QUERY_ID | CF_SKIP_QUESTIONS;
+  server_command_flags[COM_STATISTICS]=   CF_SKIP_QUESTIONS;
+  server_command_flags[COM_PING]=         CF_SKIP_QUESTIONS;
   server_command_flags[COM_STMT_PREPARE]= CF_SKIP_QUESTIONS;
   server_command_flags[COM_STMT_CLOSE]=   CF_SKIP_QUESTIONS;
   server_command_flags[COM_STMT_RESET]=   CF_SKIP_QUESTIONS;
@@ -901,9 +901,7 @@ bool dispatch_command(enum enum_server_command command, THD *thd,
     thd->security_ctx->master_access|= SHUTDOWN_ACL;
     command= COM_SHUTDOWN;
   }
-  thd->set_query_id(get_query_id());
-  if (!(server_command_flags[command] & CF_SKIP_QUERY_ID))
-    next_query_id();
+  thd->set_query_id(next_query_id());
   inc_thread_running();
 
   if (!(server_command_flags[command] & CF_SKIP_QUESTIONS))