MDEV-11896 thd_get_error_context_description race condition

Change the semantics of thd_get_error_context_description().
It no longer expects the caller to lock LOCK_thread_count
(no caller ever did that anyway), but takes care of it internally.

include/service_versions.h

@@ -31,4 +31,4 @@
 #define VERSION_my_sha1                 0x0100
 #define VERSION_logger                  0x0100
 #define VERSION_thd_autoinc             0x0100
-#define VERSION_thd_error_context       0x0100
+#define VERSION_thd_error_context       0x0200

sql/sql_class.cc

@@ -726,12 +726,6 @@ extern "C"
   @param length length of buffer
   @param max_query_len how many chars of query to copy (0 for all)
  
-  @req LOCK_thread_count
-  
-  @note LOCK_thread_count mutex is not necessary when the function is invoked on
-   the currently running thread (current_thd) or if the caller in some other
-   way guarantees that access to thd->query is serialized.
- 
   @return Pointer to string
 */
 
@@ -744,6 +738,9 @@ char *thd_get_error_context_description(THD *thd, char *buffer,
   const Security_context *sctx= &thd->main_security_ctx;
   char header[256];
   int len;
+
+  mysql_mutex_lock(&LOCK_thread_count);
+
   /*
     The pointers thd->query and thd->proc_info might change since they are
     being modified concurrently. This is acceptable for proc_info since its
@@ -799,6 +796,7 @@ char *thd_get_error_context_description(THD *thd, char *buffer,
     }
     mysql_mutex_unlock(&thd->LOCK_thd_data);
   }
+  mysql_mutex_unlock(&LOCK_thread_count);
 
   if (str.c_ptr_safe() == buffer)
     return buffer;