Re: [IGNORETHIS/PATCH] Choosing the sha1 prefix of your commits

On Thu, Oct 20, 2011 at 5:57 PM, Junio C Hamano [off-list ref] wrote:

Jeff King [off-list ref] writes:

quoted

Agreed. Having hidden cruft makes birthday collision attacks easier (or
it will, if sha1 ever gets broken to that point).  Unfortunately, there
is a _ton_ of code which assumes that commit messages are
NUL-terminated, as they always have been since e871b64 (2005-05-25).

I think that commit is irrelevant, as long as read_sha1_file() returns the
contents as <ptr,len> pair, which has been the case forever. It's just the
matter of propagating the length back up the callchain.

A naïve implementation to add "len" member to struct commit would increase
the size of the in-core commit object by sizeof(unsigned long), which we
may want to avoid. Traversals that care nothing but the topology of the
history would have to waste that memory and these things tend to add up
(8-byte ulong * 250k commits = 2MB).

Or write commit_length(struct commit *c) that calculates SHA-1 from
c->buf and checks against c->object.sha1. If it does not match,
consider NUL part of the message and move to the next NUL. This
function would be expensive so we may not call frequently though.

Or store length in ((int*)c->buf)[-1].
-- 
Duy