[darcs-devel] Re: summary of my recent spurt of patches, and request for suggestions

[Simon Marlow]

David Roundy wrote:
> On Mon, Dec 04, 2006 at 04:34:48PM +0000, Simon Marlow wrote:
> 
>>David Roundy wrote:
>>
>>
>>>I've been working hard on getting support for the new hashed inventory
>>>format into good shape.  If you aren't familiar with the benefits of the
>>>new format (which I've talked about with at least some of you in person),
>>>suffice to say that I see it as a precursor to working out the new way of
>>>dealing with conflicts.
>>
>>As an interested bystander, I'd really like to hear a brief description of 
>>what a "hashed inventory" is, and what benefits it brings.  Not a 12-page 
>>paper, just a quick outline will do fine, I don't want to distract you from 
>>the hacking frenzy :)
> 
> 
> A hashed inventory is a modification of the darcs repository format, which
> essentially replaces the _darcs/inventory file (which is human-readable, if
> not human-modifiable, so if you're not familiar with it, you could take a
> look) with a _darcs/hashed_inventory file.  The difference is that a hash
> of the contents of each patch is stored, along with the identifier of the
> patch, as is currently stored.  This hash is then used as the filename in
> _darcs/patches/.  This has several benefits.
> 
> At the most obvious level, we've now got some extra information for
> checking the consistency of a repository (helpful if, e.g. an http proxy
> modifies files in transit).
> 
> The next advantage is that by cryptographically signing the hashed
> inventory, you cryptographically sign the entire contents of the repository
> (unless someone cracks sha1).  This is potentially valuable to high-profile
> projects, or projects that use untrusted mirrors.
> 
> Next, because the filename for patches now depends on patch contents, all
> darcs commands will be atomic (except with respect to the pristine
> cache--but atomic with respect to remote access), including those that
> currently aren't, such as amend-record and obliterate.
> 
> With hashed inventories it will be possible to implement "lazy" partial
> repositories, in which darcs downloads patch files as needed to do the
> commands you ask, since we'll have the hash with which to verify that the
> patch files haven't been commuted (and therefore are still in the proper
> context for our use).
> 
> Finally, as I mentioned above, the refactoring for this change should help
> with our plans for new conflict handling, which will probably require that
> we break the current picture of one patch file per named patch (which
> wouldn't work in the current scheme where the patch filename is determined
> by the name of the patch).

Great, thanks David!

Simon