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Re: Towards a Shelving MVP

From: Johan Corveleyn <jcorvel_at_gmail.com>
Date: Mon, 28 Aug 2017 22:41:15 +0200

On Mon, Aug 28, 2017 at 9:05 PM, Julian Foad <julianfoad_at_apache.org> wrote:
> Branko ─îibej wrote:
>> On the topic of storing patches, I'd like to propose an alternative
>> implementation.
>>
>> Instead of saving a set of patches, save two sets of (untranslated
>> pristine) files:
>>
>> * All currently modified files
>> * Their pristine, unmodified versions (these are of course already in
>> the pristine store and would only need an extra reference in the WC
>> DB so that 'svn cleanup' doesn't delete them).
>
>> The idea is that 'unshelve' could, instead of applying patches to the
>> current working copy, invoke our built-in diff3 algorithm with whole
>> (untranslated!) files. This is likely to produce much better results and
>> saner conflict info than applying patches, which have limited context
>> information.
>
> Thanks for the thoughts.
>
> Three-way merging should indeed produce better results than patching
> when applying the results to a base other than the original base. We
> know that patching is sufficient for many use cases, but we also know
> there are cases that where a proper merge would be superior, and they
> might turn out to be common in certain usage patterns. I would certainly
> be happier if we could do that.

There is one big disadvantage of storing the complete modified files,
and that's storage. If I'm making a small edit to a 100 MB file,
instead of storing a patch of 500 bytes, I have to store 100 MB per
shelved change.

I'm not an expert, but do you really need the modified file itself, if
you have the patch and a reference to the base file (pristine)? Why
store both F (pristine) and F' (modified file), if I can reconstruct
F' out of F + P (patch). So I suggest:

* Store the patch
* Keep the pristine on which the patch was based (keep a reference to
it in the pristine store, like in Brane's suggestion)

We can still perform the 3-way merge by first reconstructing F and F'
out of F and P.

And even if the pristine gets lost (either because the patch was
transferred to another user; or because the user executed the
not-yet-existing command 'svn cleanup --vacuum-shelve-pristines' to
reclaim diskspace) the patch will still be usable, although without
the 3-way merging.

-- 
Johan
Received on 2017-08-28 22:41:38 CEST

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