Re: [Qemu-block] [PATCH 04/12] hbitmap: Fix merge when b is empty, and result is not an alias of a

[Max Reitz]

On 20.06.19 03:03, John Snow wrote:
> Nobody calls the function like this currently, but we neither prohibit
> or cope with this behavior. I decided to make the function cope with it.
> 
> Signed-off-by: John Snow <address@hidden>
> ---
>  util/hbitmap.c | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
> 
> diff --git a/util/hbitmap.c b/util/hbitmap.c
> index 7905212a8b..45d1725daf 100644
> --- a/util/hbitmap.c
> +++ b/util/hbitmap.c
> @@ -781,8 +781,9 @@ bool hbitmap_can_merge(const HBitmap *a, const HBitmap *b)
>  }
>  
>  /**
> - * Given HBitmaps A and B, let A := A (BITOR) B.
> - * Bitmap B will not be modified.
> + * Given HBitmaps A and B, let R := A (BITOR) B.
> + * Bitmaps A and B will not be modified,
> + *     except when bitmap R is an alias of A or B.
>   *
>   * @return true if the merge was successful,
>   *         false if it was not attempted.
> @@ -797,7 +798,9 @@ bool hbitmap_merge(const HBitmap *a, const HBitmap *b, 
> HBitmap *result)
>      }
>      assert(hbitmap_can_merge(b, result));
>  
> -    if (hbitmap_count(b) == 0) {
> +    if ((!hbitmap_count(a) && result == b) ||
> +        (!hbitmap_count(b) && result == a) ||
> +        (!hbitmap_count(a) && !hbitmap_count(b))) {
>          return true;
>      }

The rest of this function completely overwrites the @result bitmap.
Therefor, @result does not need to be cleared when calling this function.

Therfore, hbitmap_merge(hbitmap_alloc(), hbitmap_alloc(), output) should
actually clear @output, I think.

Max

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