Re: [PATCH] midx: reduce memory pressure while writing bitmaps

> From: Derrick Stolee <derrickstolee@github.com>
> [...]
> It is unfortunate that the lifetime of the 'entries' array is less
> clear. To make this simpler, I added a few things to try and prevent an
> accidental reference:
>
>  1. Using FREE_AND_NULL() we will at least get a segfault from reading a
>     NULL pointer instead of a use-after-free.
>
>  2. 'entries_nr' is also set to zero to make any loop that would iterate
>     over the entries be trivial.
>
>  3. Set the 'ctx' pointer to NULL within write_midx_bitmap() so it does
>     not get another reference later. This requires adding a local copy
>     of 'pack_order' giving us a reference that we can use later in the
>     method.
>
>  4. Add significant comments in write_midx_bitmap() and
>     write_midx_internal() to add warnings for future authors who might
>     accidentally add references to this cleared memory.
> [...]
> +	/*
> +	 * Remove the ctx.entries to reduce memory pressure.
> +	 * Nullify 'ctx' to help avoid adding new references to ctx->entries.
> +	 */
> +	FREE_AND_NULL(ctx->entries);
> +	ctx->entries_nr = 0;
> +	pack_order = ctx->pack_order;
> +	ctx = NULL;