Re: #30201 [Com]: crash with query cache when killing a select

Hi,

> --- a/sql/sql_cache.cc	2007-05-10 17:54:53 +05:00
> +++ b/sql/sql_cache.cc	2007-08-22 11:07:03 +05:00
> @@ -706,6 +706,12 @@ void query_cache_end_of_result(THD *thd)
>    Query_cache_block *query_block;
>    DBUG_ENTER("query_cache_end_of_result");
>  

Ok, query_cache_abort checks null-pointers and removes query cache blocks from store.

>    /* See the comment on double-check locking usage above. */
>      DBUG_VOID_RETURN;
> @@ -727,13 +733,10 @@ void query_cache_end_of_result(THD *thd)
>      DUMP(&query_cache);
>      BLOCK_LOCK_WR(query_block);
> -    Query_cache_block *last_result_block= header->result()->prev;
> -    ulong allign_size= ALIGN_SIZE(last_result_block->used);
> -    ulong len= max(query_cache.min_allocation_unit, allign_size);
> -    if (last_result_block->length >= query_cache.min_allocation_unit + len)
> -      query_cache.split_block(last_result_block,len);

>  
> -#ifndef DBUG_OFF
>      if (header->result() == 0)
>      {
>        DBUG_PRINT("error", ("end of data whith no result. query '%s'",
> @@ -747,7 +750,13 @@ void query_cache_end_of_result(THD *thd)
>        */
>        goto end;
>      }
> -#endif
> +

> +
>      header->found_rows(current_thd->limit_found_rows);
>      header->result()->type= Query_cache_block::RESULT;
>      header->writer(0);

Ok: check if we have a result block before we reference it. But if there is a query cache block in the store which doesn't have any result set this is inconvenient later on in send_result_to_client:

.
.
.

  if (result_block == 0 || result_block->type != Query_cache_block::RESULT)   {
    /* The query is probably yet processed */     DBUG_PRINT("qcache", ("query found, but no data or data incomplete"));     BLOCK_UNLOCK_RD(query_block);
    goto err_unlock;
  }

.
.
.

Better to remove this query block at once when we detected the error. Please change from:

    if (header->result() == 0)
    {

      DBUG_PRINT("error", ("end of data whith no result. query '%s'",
                           header->query()));
      query_cache.wreck(__LINE__, "");  <-- disables the query cache which is a bit unnecessary

      /*
        We do not need call of BLOCK_UNLOCK_WR(query_block); here because
        query_cache.wreck() switched query cache off but left content
        untouched for investigation (it is debugging method).
      */
      goto end;

Change to:

    if (header->result() == 0)
    {

      DBUG_PRINT("error", ("End of data with no result blocks; Query '%s' removed from cache.",
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                           header->query()));
	
      BLOCK_UNLOCK_WR(query_block);
      query_cache.free_query(query);
      
      goto end;

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Cheers,

Kristofer

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