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Re: [PATCH 0/5] tree-walk object_id refactor

On Thu, Jan 10, 2019 at 04:25:46AM +0000, brian m. carlson wrote:

> There are a small number of places in our codebase where we cast a
> buffer of unsigned char to a struct object_id pointer. When we have
> GIT_MAX_RAWSZ set to 32 (because we have SHA-256), one of these places
> (the buffer for tree objects) can lead to us copying too much data when
> using SHA-1 as the hash, since there are only 20 bytes to read.
> This was not expected to be a problem before future code was introduced,
> but due to a combination of series the issue became noticeable.
> This series introduces a refactor to avoid referencing the struct
> object_id directly from a buffer and instead storing an additional
> struct object_id (and an int) in struct name_entry and referring to
> that.

I think this is really the only safe and sane solution. We resisted it
because of the cost of the extra copies (especially the
update_tree_entry() one). But I don't know that anybody actually
measured it. Do you have any performance numbers before/after this