Bug#31347 Increase in memory usage after many DROP USER statements

Dropping users causes huge increase in memory usage because field values were
allocated on the server memory root for temporary usage but never deallocated.

This patch changes the target memory root to be that of the thread handler
instead since this root is cleared between each statement.

sql/sql_acl.cc

@@ -311,7 +311,7 @@ static my_bool acl_load(THD *thd, TABLE_LIST *tables)
       continue;
     }
 
-    const char *password= get_field(&mem, table->field[2]);
+    const char *password= get_field(thd->mem_root, table->field[2]);
     uint password_len= password ? strlen(password) : 0;
     set_user_salt(&user, password, password_len);
     if (user.salt_len == 0 && password_len != 0)
@@ -364,7 +364,7 @@ static my_bool acl_load(THD *thd, TABLE_LIST *tables)
       /* Starting from 4.0.2 we have more fields */
       if (table->s->fields >= 31)
       {
-        char *ssl_type=get_field(&mem, table->field[next_field++]);
+        char *ssl_type=get_field(thd->mem_root, table->field[next_field++]);
         if (!ssl_type)
           user.ssl_type=SSL_TYPE_NONE;
         else if (!strcmp(ssl_type, "ANY"))
@@ -378,11 +378,11 @@ static my_bool acl_load(THD *thd, TABLE_LIST *tables)
         user.x509_issuer=  get_field(&mem, table->field[next_field++]);
         user.x509_subject= get_field(&mem, table->field[next_field++]);
 
-        char *ptr = get_field(&mem, table->field[next_field++]);
+        char *ptr = get_field(thd->mem_root, table->field[next_field++]);
         user.user_resource.questions=ptr ? atoi(ptr) : 0;
-        ptr = get_field(&mem, table->field[next_field++]);
+        ptr = get_field(thd->mem_root, table->field[next_field++]);
         user.user_resource.updates=ptr ? atoi(ptr) : 0;
-        ptr = get_field(&mem, table->field[next_field++]);
+        ptr = get_field(thd->mem_root, table->field[next_field++]);
         user.user_resource.conn_per_hour= ptr ? atoi(ptr) : 0;
         if (user.user_resource.questions || user.user_resource.updates ||
             user.user_resource.conn_per_hour)
@@ -391,7 +391,7 @@ static my_bool acl_load(THD *thd, TABLE_LIST *tables)
         if (table->s->fields >= 36)
         {
           /* Starting from 5.0.3 we have max_user_connections field */
-          ptr= get_field(&mem, table->field[next_field++]);
+          ptr= get_field(thd->mem_root, table->field[next_field++]);
           user.user_resource.user_conn= ptr ? atoi(ptr) : 0;
         }
         else
@@ -4865,6 +4865,7 @@ static int handle_grant_table(TABLE_LIST *tables, uint table_no, bool drop,
   byte user_key[MAX_KEY_LENGTH];
   uint key_prefix_length;
   DBUG_ENTER("handle_grant_table");
+  THD *thd= current_thd;
 
   if (! table_no) // mysql.user table
   {
@@ -4932,17 +4933,18 @@ static int handle_grant_table(TABLE_LIST *tables, uint table_no, bool drop,
           DBUG_PRINT("info",("scan error: %d", error));
           continue;
         }
-        if (! (host= get_field(&mem, host_field)))
+        if (! (host= get_field(thd->mem_root, host_field)))
           host= "";
-        if (! (user= get_field(&mem, user_field)))
+        if (! (user= get_field(thd->mem_root, user_field)))
           user= "";
 
 #ifdef EXTRA_DEBUG
         DBUG_PRINT("loop",("scan fields: '%s'@'%s' '%s' '%s' '%s'",
                            user, host,
-                           get_field(&mem, table->field[1]) /*db*/,
-                           get_field(&mem, table->field[3]) /*table*/,
-                           get_field(&mem, table->field[4]) /*column*/));
+                           get_field(thd->mem_root, table->field[1]) /*db*/,
+                           get_field(thd->mem_root, table->field[3]) /*table*/,
+                           get_field(thd->mem_root,
+                                     table->field[4]) /*column*/));
 #endif
         if (strcmp(user_str, user) ||
             my_strcasecmp(system_charset_info, host_str, host))