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Re: Thought experiment - follow logs back before r1 into previous repository

From: Julian Foad <julian.foad_at_wandisco.com>
Date: Fri, 30 Sep 2011 12:17:09 +0100

To record my own opinion: I think it's a fine idea that users in that
situation should be able to do that sort of thing but I don't think that
functionality belongs in "svn" as I think it's an uncommon use case and
can't be cleanly and generally supported -- it's rather a hack. If we
supported third-party client-side plug-ins that's where it would
belong ... but we don't have any plans to do so.

- Julian

I (Julian Foad) wrote:
> I want to share with you an idea that came to me from a customer. I'm
> not at all proposing that anybody should do this, I'm just curious what
> you think.
> Imagine, if you will, that we are coders working in a Subversion
> repository that has grown very large and that for IT reasons a decision
> has been made to freeze the repository -- make it read-only -- and a new
> repository has been created, taking a snapshot of the old HEAD and
> importing that as the new r1. We are to continue our development work
> in the new repository.
> Those of you who are "old" enough svn devs, think back to when
> Subversion became self-hosting, starting with a snapshow of the head of
> the CVS repository. All the prior history was back before r1,
> inaccessible via Subversion. Was that a big problem? No, it wasn't,
> and I know that the snapshot approach is often recommended as a
> pragmatic and perfectly reasonable way to migrate from one VCS to
> another. But maybe this time there will be hundreds of developers
> working in dozens of projects[1].
> As Subversion devs today we might like to say "no, don't do that, let's
> find a better solution to whatever problem was forcing us to re-start
> with an imported snapshot". But imagine that's already been discussed
> and this is the best way forward and now we simply have to get on with
> using the new repository.
> Q: What simple modifications could "we" (anybody) make to our
> Subversion clients that would help us to work more effectively in this
> scenario? The customer I got this idea from is more interested in
> TortoiseSVN than in "svn" and asked me a somewhat different question,
> but I think this is the general idea that's of wider interest.
> A: What do you think?
> Maybe one of the most useful things we could do is teach "svn
> log" (when running in the usual 'backwards' direction) to run a
> follow-on log in the old repo if and when reaching r1. Perhaps we'd set
> a revprop on (new) r0 or r1 pointing to the old repo URL so that this
> info is configured in a single place. The two sets of revision numbers
> in the output would be confusing so we may want to consider tagging the
> old and/or the new revnums with some marker as well as inserting an "And
> now from the old repository:" message.
> I think teaching "svn blame" to view the old repo would be harder:
> it would require more intrusive code changes in svn_client_blame().
> It's not theoretically difficult to do, of course, but perhaps the
> code-to-value ratio would not be worth having in libsvn_client ... hmm,
> unless we re-architect the blame code so that it's fed diffs from the
> client layer instead of fetching them itself, then it could be done
> really cleanly. The output format would just need a minor tweak to
> distinguish old from new revs.
> I think teaching "svn diff" to do general cross-repo diffs would not
> be feasible with the current diff implementation. However, one of my
> goals is to generalize the diff code further so it could support such
> things (cross-repo, unversioned local tree, etc.). That would be useful
> in theory, but in practice I can't see it really being used very often
> in this start-again scenario. But any single-rev diff is easily
> supported because the cut-over revision is present in both repos. (We
> can assume that the tree in old_at_OLD_HEAD is identical to new_at_1.) So
> maybe we'd want to make single-rev diffs and all same-repo diffs easier
> by tweaking "svn diff" to follow the specified path back into a revision
> in the old repo, a bit like what I said above for "svn log", if some
> special switch is specified.
> Any other commands or work flows that might be really useful? I
> wouldn't dream of trying to make "svn up" go back to the old repo, that
> would certainly be over the top. And I wouldn't expect "svn cat", "svn
> proplist" etc. to be worth bothering with, unless all such simple
> read-only commands get the same functionality "for free".
> Mad or genius? (And I know it wouldn't be worth bothering in a small
> repository; let's assume it's a big and busy project with lots of
> interesting history.)
> - Julian
> [1] I'm just making up numbers here; I don't know what sort of numbers
> the customer that brought up this idea has.
Received on 2011-09-30 13:17:46 CEST

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