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Re: Reintegrate-like merges and diff_ignore_ancestry

From: Ivan Zhakov <ivan_at_visualsvn.com>
Date: Fri, 1 Feb 2013 15:57:41 +0400

On Fri, Feb 1, 2013 at 1:49 AM, Ivan Zhakov <ivan_at_visualsvn.com> wrote:
> On Wed, Jan 23, 2013 at 9:39 PM, Paul Burba <ptburba_at_gmail.com> wrote:
>> On Wed, Jan 23, 2013 at 9:51 AM, Julian Foad <julianfoad_at_btopenworld.com> wrote:
>>> Ivan Zhakov wrote:
>>>
>>>> I was testing recent changes in ra_serf update editor and noticed that
>>>> reintegrate-like merges for long living branches are extremely slow.
>>>> Client requests server for diff between branches with respect to
>>>> ancestry and servers reports no-op txdelta for every file that was
>>>> changed in original branch. Then for every such file client retrieves
>>>> text and properties.
>>>>
>>>> For example try to reintegrate ev2-export branch back to trunk.
> [...]
>
> Hi Bert and Paul!
>
> On Wed, Jan 23, 2013 at 6:51 PM, Bert Huijben <bert_at_qqmail.nl> wrote:
>> Is there a way the diff code can see that it receives a no-op txdelta?
>>
>> In that case it can just skip retrieving the file as it won't be able to produce a difference anyway.
>>
> It would be really hard to implement on client, because no-op txdelta
> transmitted as full txdelta with instructions to copy all source data.
>
> But we can detect such no-op txdelta on server side. See my plan bellow.
>
>>> Thanks Ivan, that's very interesting. I'll take a look, since I have just been working on that code.
>>>
>>> I exposed this as a separate option because the
>> two meanings of the previous 'ignore_ancestry' option were conflated, but I don't deeply understand what happens when this option is specified. I know what it's
>> supposed to mean at a basic level: "diff a pair of unrelated nodes as if they are related". What I don't know is how well it's implemented and whether it's really useful when merging.
>>>
>>>> What is the purpose of diff_ignore_ancestry for merges? Can we default
>>>> it to FALSE?
>>>
>>> I assume one of the purposes is if your work flow has been such
>> that a file is sometimes replaced (without copy-from)
>>
>> Any replacement is a problem, regardless of copy-from. For example:
>>
>
> [...]
>
>>
>>>, and sometimes a new file added on one branch has been added on the
>> other branch by a simple add (without copy-from). In that sort of work flow,
>> the 'diff_ignore_ancestry' would cause Subversion to do diffs between
>> different versions of a file, and that might well help in merging the changes.
>> Without the 'diff_ignore_ancestry' option, Subversion would treat the file as
>> 'replaced', and so you'd be likely to get a tree conflict when you try to merge it.
>>
> Thanks for demonstrating use-case. That what I was looking for. We
> cannot change to ignore_ancestry = TRUE by default, but I'm going to
> try the following:
> 1. Add test case for replacement merge that you demonstrated (it would
> be useful anyway)
>
Test added in r1441400.

> 2. Modify delta reporter on server side to do not send deltas if files
> have the same content, regardless ancestry (see
> subversion/libsvn_repos/reporter.c:696 delta_files()) Patch attached.
>
The new test for merge replacements also passes with my patch. So I'm
going to cleanup it and commit after further testing.

> 3. Test it and think a little bit more is it safe change or not :)
>

-- 
Ivan Zhakov
Received on 2013-02-01 12:58:42 CET

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