On 17/05/10 00:02, Benjamin R. Haskell wrote:
On Sun, 16 May 2010, Tony Mechelynck wrote:
On 16/05/10 21:26, James Vega wrote:
On Sun, May 16, 2010 at 06:09:15PM +0200, Tony Mechelynck wrote:
I don't know if Mercurial hashes are carried over (the short hash
for Bram's vim73 parent to my latest merge is listed as
c6f1aa1e9f32 with the description "Add 'relativenumber' patch from
Markus Heidelberg") and anyway they aren't taken over in the
resulting executable.
The hashes are global to all clones of a given repository. That is,
any commit which is shared among clones will have the same hash.
That's how Mercurial knows which changesets have been applied. The
changeset index is local to each clone simply as a convenience so
you don't have to type out full hashes.
Ah, I see. I wonder how Mercurial makes sure that a changeset I import
will never have a hash (not even a 12-nybble "short hash") already
used by an earlier "local commit".
It doesn't (and can't) mathematically guarantee no collisions in the
full hash. The input space is far larger [text of any length] than the
output space [non-negative integers from 0 to 2^160-1].
Saying that "it will never happen" because the apriori probability is
thought to be low strikes me as the wrong way to set up server
software (see Murphy's law). Oh, well, I'll worry about it when I see
it happen.
It's not just that "the apriori probability is thought to be low", it's
that it's thought to be astronomically low.
There's a Mercurial FAQ[1] that sums it up well:
"""
7.11. What about hash collisions? What about weaknesses in SHA1?
The SHA1 hashes are large enough that the odds of accidental hash
collision are negligible for projects that could be handled by the human
race. The known weaknesses in SHA1 are currently still not practical to
attack, and Mercurial will switch to SHA256 hashing before that becomes
a realistic concern.
Collisions with the "short hashes" are not a concern as they're always
checked for ambiguity and are still long enough that they're not likely
to happen for reasonably-sized projects (< 1M changes).
"""
The "short hash" isn't used in "mission-critical" contexts. (As with
the local 'revision numbers', it's merely for convenience.)
As with the crypto-law conversation, probably getting a bit offtopic for
vim-dev, but if you're interested, some more reading in the links
below[2][3].
Ah, thanks, I'll make sure to keep this post where I can easily get back
to it. :-)
Best regards,
Tony.
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