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Re: Outsider impression about merge tracking

From: Folker Schamel <schamel23_at_spinor.com>
Date: 2007-11-20 11:16:22 CET

Folker Schamel wrote:
> Hi Karl!
>
>> Folker Schamel <schamel23@spinor.com> writes:
>>> I'm reluctant sending this mail, because I have a huge respect
>>> for the Subversion developers. But maybe sometimes some outsider view
>>> is helpful:
>>>
>>> I fear that subversion team is going to slap some premature
>>> merge-tracking
>>> feature out.
>>
>> Thanks for your concerns! Of course we worry about that too, and are
>> trying to make merging Do The Right Thing as much as possible.
>>
>>> Which in practice will causes serious headaches and dissatisfaction
>>> for users
>>> due to many open issues. (Also a good documentation of the limitations
>>> of merge-tracking won't prevent this.)
>>>
>>> And for such kind of non-trivial algorithmic problems, usually you
>>> cannot
>>> fix broken implementations afterwards, but the only chance is to
>>> (re)implement a clean solution from the beginning.
>>
>> Well, your statements are true in general, but they'll only be helpful
>> if you give specific examples. Which is more work, of course; it's
>> also the only way to get constructive results.
>>
>>> Is commercial pressure driving the subversion code base in a dead end?
>>
>> No. Better merging is something Subversion has needed for a long
>> time; it's an area where we lag behind other version control systems
>> (both proprietary and open source). We need it.
>>
>> While there are certainly corporations that want to see the feature
>> finished, and some of them are helping to develop it (e.g., CollabNet,
>> Google, et al), I don't see commercial pressure causing any problems
>> here. The community is setting the deadlines and making the technical
>> decisions in public forums, as always.
>>
>> I too am worried, but mainly about technical issues that I think are
>> fixeable (e.g., issue #2897, etc). Can you be more specific?
>
> I have the questions of Mark in mind:
> http://svn.haxx.se/dev/archive-2007-11/0729.shtml
> I think both
> i) supporting reflected merges without filtering out a conflict
> resolution, and
> ii) merging between branches
> is important, otherwise people will complain a lot.
> Currently people complain that Subversion has no merge-tracking.
> Without solving i) and ii) people will complain that merge-tracking
> of Subversion is fundamentally broken.
>
> Technically, I came to two conclusions:
> a) Finding an algorithm solving both i) and ii) basically seems to be
> not too difficult in principle (see below). However:
> b) This is *in principle not possible* with the mergeinfo scheme
> you are currently using.
>
> Therefore my conclusion:
> Dead end.
>
> ***** Details about a) and b):
>
> Issue i) can be solved by
> http://svn.haxx.se/dev/archive-2007-11/0766.shtml
> while ii) (combined with i) is not possible with the current mergeinfo
> scheme:
> Currently, when merging from branchA -> branchB -> branchC,
> then mergeinfo of branchC contains both the revisions committed
> to branchA and branchB, eliminating the information that
> merges von branchA came indirectly via branchB.
> But this information seems to be *unavoidable* necessary to solve ii).
> I think mergeinfo of branchC must only refer to branchB,
> only indirectly referring to branchA by the mergeinfo stored at branchB.
> So that mergeinfo data builds a graph.
> And that graph must be processed efficiently.
> (Some longer time ago, Daniel Rall pointed out correctly that
> merging is a graph problem.)
> In other words, the current mergeinfo scheme has not enough information
> locally.
>
> A working merge algorithm for "merge FROM -r X:Y TO" could be for example:
> for each r in X:Y in reverse order:
> Check if the mergeinfo *tree* of FROM and TO have a common subtree
> r_sub@where_sub.
> In this case unmerge that change: merge -c -r_sub where_sub
> Add r@FROM to mergeinfo of TO. (But do *not* add mergeinfo of FROM.)
> Do a classical merge FROM -rX:Y TO.
> This is the algorithm I found which is closed to Subversion philosophy.
> But also this still requires a fundamentally different mergeinfo
> property handling (*not* storing branchA in mergeinfo of branchC),
> and probably a completely different database for that info
> for efficiently doing the tree search. Otherwise merge tracking
> would be horrible inefficient.
>
> Of course, there may also exist different algorithms solving i) and ii).
> But I claim that there is no algorithm (which is efficient enough)
> with the current mergeinfo data scheme (where mergeinfo of branchC
> contains branchA) solving i) and ii) for principle reasons.

Clarification:
I suppose fixing *only* i) or fixing *only* ii) can be made
with the current scheme.
But I claim that fixing *both* i) and ii) is not possible
with the current scheme.
Does anyone know an algorithm?

Cheers,
Folker

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Received on Tue Nov 20 11:38:35 2007

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