# Overview

This is a sketching-out page for ‘seamless SVN integration’ proposed in http://bugs.darcs.net/issue1627.

# The dream

I want Darcs to be able to just treat any SVN repository as a Darcs repository. You should be able to pull revisions as patches and push patches as revisions. You should also be able to talk to the same SVN repository from multiple Darcs repos which in turn should be able to exchange patches between each other.

No need for any silly git-svn style tricks… just seamlessness

# Desiderata

• darcs clone/pull/push svn://foo just works

• SVN revisions should be able to flow from one Darcs repo to another

SVN ----------+
|            |
| r1-r4      | r1-r2
V            V
Eric ------- Nicolas
-> r3-r4
-> e2, e6
<- n4
• Darcs should know not to try to push SVN revisions back

• Ability to cherry-pick SVN revisions within Darcs (r5 without r3)
• In the end, basically, SVN repository behaves just like an append-only darcs repository.

# Open problems

• How import the copying changes.
• How import the renaming changes (since they are encoded as copy;delete).
• How to deal with svn properties.
• How to deal with SVN automagically rewriting line terminators
• How to deal with conflicts?

# Still being sketched out

## Bidirectional mapping between SVN revisions and Darcs patches

Use SVN properties and maybe some Darcs metadata in the patch log Ignore-this: SVN-foo

You only need to mark foreign-originated patches in such a way: revisions in subversion that came to existence as darcs patches first, and darcs patches that originated as SVN revisions. Either way, you need to make the conversion process from subversion to darcs fully deterministic (see the caution in the “replace” patch discussion below), which means that there’s no need to keep any extra metadata in subversion about those – a given revision will always produce an identical patch (in identical context) within darcs.

One more thing to note is that you want to include SVN UUID and the conversion root in the darcs metadata. So say:

Ignore-This: SVN cf2b9d22-acb5-11dc-bc8c-05e83ce5dbec /trunk r1200

this should uniquely identify a given pristine state (as stored by subversion). It may make sense to actually name both revisions, the delta of which has been converted into a darcs patch: r1200:r1201 or so (or you could say that r1200 always means r1199 -> r1200, or something).

STATUS: still need to check discussion on mailing list and tracker

## SVN copying

MAYBE: I propose that we model these as addfile+hunk patches that depend on all named patches that touch the file being copied. This is basically us relying on the fact that SVN doesn’t do commutation.

STATUS: Nicolas said this might work.

## darcs replace patches

• PROPOSAL A: have a separate SVN format that disallows darcs replace patches
• PROPOSAL B1: emit a warning like “SVN repositories do not support darcs replace patches… are you sure you want to create this?”
• PROPOSAL B2: make B1 refuse to create them by default, but allow an override
• PROPOSAL C: allow them, but transform them to hunk patches when pushing to SVN (and use metadata to tell Darcs that it’s really a replace patch). This relies on lack of commutation in SVN.

A generalisation of this problem is that even hunks are not “safe” within subversion. As a matter of fact, subversion stores tree snapshots, not patches. So there are basically two approaches to the generalised problem: either store darcs patches in svn revprops in their entirety, or alternatively, only store “fancy” patches that way and rely on re-diffing the states to obtain hunks. It should be noted, that it is unsafe to let subversion compute the diff – it is imperative that we fetch the two versions and do the diff ourselves. In this case, it is also imperative that the diffing algorithm is frozen in darcs, otherwise the result would be incorrect.

Moreover, if we want multiple “get”s of the subversion repository to be compatible among each other, we have to freeze the diffing algorithm of darcs and make it part of the repository format contract. It would be very unfortunate if two different versions of future darcs would produce slightly incompatible copies of the same subversion repository, leading to extremely subtle bugs. The same caution applies to any snapshot-based repository format.

# Confirmed details

## Branches

SVN does not have a model of branches; they’re just subdirectories.

We should treat each branch/directory as a repository of its own, so that

• darcs clone svn://gforge.inria.fr/projectname/trunk and
• darcs clone svn://gforge.inria.fr/projectname

give you two completely repositories that don’t exchange patches. Tough luck; we’re keeping it simple.

On the darcs clone layer, we could follow one superficial convention so that doing darcs clone svn://gforge.inria.fr/projectname/trunk gives you a repository called projectname instead of trunk.