Re: [PATCH 2/3] mem-pool: use more standard initialization and finalization

[Junio C Hamano <gitster@pobox.com>]

"Elijah Newren via GitGitGadget" <gitgitgadget@gmail.com> writes:

> written.  mem_pool_init() does essentially the following (simplified
> for purposes of explanation here):
>
>     void mem_pool_init(struct mem_pool **pool...)
>     {
>         *pool = xcalloc(1, sizeof(*pool));
>
> It seems weird to require that mem_pools can only be accessed through a
> pointer.

Yup, if the _init() were to also allocate, I would expect it to be
more like

	struct mem_pool *mem_pool_create(...)
	{
		struct mem_pool *pool = xcalloc(1, sizeof(*pool));
		...
		return pool;
	}

It also is OK to let the caller pass uninitialized region of memory,
which is how we usually arrange _init() to work.  It seems that that
is the approach this patch takes.

> -void mem_pool_init(struct mem_pool **mem_pool, size_t initial_size)
> +void mem_pool_init(struct mem_pool *mem_pool, size_t initial_size)
>  {
> -	struct mem_pool *pool;
> -
> -	if (*mem_pool)
> -		return;
> -
> -	pool = xcalloc(1, sizeof(*pool));
> -
> -	pool->block_alloc = BLOCK_GROWTH_SIZE;
> +	mem_pool->mp_block = NULL;
> +	mem_pool->pool_alloc = 0;
> +	mem_pool->block_alloc = BLOCK_GROWTH_SIZE;
>  
>  	if (initial_size > 0)
> -		mem_pool_alloc_block(pool, initial_size, NULL);
> -
> -	*mem_pool = pool;
> +		mem_pool_alloc_block(mem_pool, initial_size, NULL);

It used to be that this function both knew and took control of all
the bits in *pool memory by using xcalloc().  Any field the function
assigned to of course got explicitly the value the function wanted
it to have, and all other fields were left to 0.

It may happen to be still the case (i.e. the assignments we see in
this function cover all the fields defined), but don't we need some
provision to make sure it will hold to be true in the future?

Starting it with "memset(pool, 0, sizeof(*pool)" would be one way.

You'd standardize to s/mem_pool/pool/ in [3/3]; shouldn't this be
written with pool to begin with, instead of reintroducing mem_pool
that is of different type from the original?

> -	if (!*pool_ptr)
> -		mem_pool_init(pool_ptr, 0);
> +	if (!*pool_ptr) {
> +		*pool_ptr = xmalloc(sizeof(**pool_ptr));
> +		mem_pool_init(*pool_ptr, 0);

This one gives an uninitialized chunk of memory to the _init(); an
example of the caller that the earlier comment may matter.

> +	istate->ce_mem_pool = xmalloc(sizeof(*istate->ce_mem_pool));
>  	if (istate->version == 4) {
> -		mem_pool_init(&istate->ce_mem_pool,
> +		mem_pool_init(istate->ce_mem_pool,
>  				estimate_cache_size_from_compressed(istate->cache_nr));
>  	} else {
> -		mem_pool_init(&istate->ce_mem_pool,
> +		mem_pool_init(istate->ce_mem_pool,
>  				estimate_cache_size(mmap_size, istate->cache_nr));
>  	}

Likewise.

Thanks.