Decoupling patch ids from metadata

On Wed, Dec 20, 2006 at 03:21:51PM +0100, Juliusz Chroboczek
wrote:
> >> Why does darcs allow me to create a corrupt
repo?  Is there any way
> >> to prevent this from happening in the first
place?
> 
> > It's been a wishlist request for a while (I
believe).  We don't want to
> > have a complete check by default, since that's an
O(N) process where N is
> > the length of the repository history (unless we
did something tricky), but
> > a check at the most recent patches would catch
most such mistakes, and be
> > cheap.
> 
> We've discussed this at FOSDEM last, and you know I
don't agree.
> 
> IMHO, the issue is with the way Darcs generates patch
ids -- for some
> reason, a patch id is generated as a hash of the patch
metadata.
> 
> Now if you look at the properties that a patch id must
have, there are
> only two:
> 
>   (1) a patch id must be invariant w.r.t. commutation;
>   (2) a patch id must be globally unique.
> 
> The current approach passes (2), but fails (1).  A
simpler approach
> would be to generte a patch id randomly; given a
sufficiently large
> space (and a little care in generating random numbers),
this would
> pass (1), and if the patch id is encoded in the patch
itself, it would
> also pass (2).
> 
> David, I would like to argue that making patch ids
arbitrary should be
> combined with the transition to hashed repositories. 
We would simply
> need to:
> 
>   - add the patch id to the on-disk patch when it's not
generated with
>     the current algorithm;
>   - add patch ids to inventories.
> 
> What do you think?

I think that there's no reason to couple this change with a
repository
format change.  All we need to do is add an extra line to
the "long
comment" portion of the current patch ID, and tell
darcs not to display it
to the user (unless perhaps they ask).  Then older darcs can
interact fine
with newer darcs, and there's no format transition at all.

This is also nice, in that it gives us a chance to introduce
a convention
for hiding darcs information in the long comment (e.g. lines
at the
beginning starting with "darcs-internal-" or
something like that.  So
perhaps we just have "darcs-internal-patchid:
DDGDSGDSG" as the first (or
last?) line of the long comment.  Then when we want to add a
hash of the
repo contents in tags (as in, the files and directories), we
could do that
in the same way "darcs-internal-pristine-hash:
xxxxxx".  We just need to
teach darcs to hide such lines (a one-line modification to
human_friendly),
and to generate them (which requires reading /dev/urandom or
something).
-- 
David Roundy
http://www.darcs.net

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Decoupling patch ids from metadata

Thread: Decoupling patch ids from metadata