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From: Elijah Newren <newren@gmail.com>
To: "Eckhard Maaß" <eckhard.s.maass@googlemail.com>
Cc: Junio C Hamano <gitster@pobox.com>,
	Ben Peart <peartben@gmail.com>,
	Ben Peart <Ben.Peart@microsoft.com>,
	"git@vger.kernel.org" <git@vger.kernel.org>,
	"peff@peff.net" <peff@peff.net>,
	"pclouds@gmail.com" <pclouds@gmail.com>,
	"vmiklos@frugalware.org" <vmiklos@frugalware.org>,
	Kevin Willford <kewillf@microsoft.com>,
	"Johannes.Schindelin@gmx.de" <Johannes.Schindelin@gmx.de>
Subject: Re: [PATCH v3 2/3] merge: Add merge.renames config setting
Date: Fri, 27 Apr 2018 13:23:20 -0700	[thread overview]
Message-ID: <CABPp-BHwM1jx2+VTxt7hga7v-E6gvHuxVNPqm-MPRXYe5CDVtA@mail.gmail.com> (raw)
In-Reply-To: <20180427183752.GA2799@esm>

On Fri, Apr 27, 2018 at 11:37 AM, Eckhard Maaß
<eckhard.s.maass@googlemail.com> wrote:
> On Fri, Apr 27, 2018 at 11:23:56AM +0900, Junio C Hamano wrote:
>> I think demoting from copy to rename-only is a good idea, at least
>> for now, because I do not believe we have figured out what we want
>> to happen when we detect copied files are involved in a merge.
>
> Does anyone know some threads concerning that topic? I tried to search
> for it, but somehow "merge copy detection" did not find me useful
> threads so far. I would be interested in that topic anyway, so I would
> like to know what the ideas are that floated so far.
>

I doubt it has ever been discussed before this thread.  But, if you're
curious, I'll try to dump a few thoughts.  Let's say we have branches
A and B, and:
   A: modifies file z
   B: copies z to y

Should the modifications to z done in A propagate to both z and y?  If
not, what good is copy detection?  If so, then there are several
ramifications...

- If B not only copied z but also first modified it, then do we have
  potential conflicts with both z and y -- possibly the exact same
  conflicts, making the user resolve them repeatedly?

- What if A copied z to x?  Do changes to z propagate to all three of
  z and x and y?  Do changes to either x or y affect z?  Do they
  affect each other?

- If A deleted z, does that give us a copy/delete conflict for y?  Do
  we also have to worry about copy/add conflicts?  copy/add/delete?
  rename/copy (multiple variants)?  copy/copy?

- Extra degrees of freedom may mean new conflict types:

  - The extra degrees of freedom from renames introduced multiple new
    conflict types (e.g. rename/add, rename/rename(1to2),
    rename/rename(2to1)).

  - Directory rename detection added more (rename/rename/rename(1to3),
    rename/rename(Nto1), add/add/add, directory/file/file, n-fold
    transitive rename, etc.), -- and forced us to specify various
    rules to limit the possibility space so that conflicts could be
    representable in the index and understandable by the user.

  - I suspect adding copies would add new types of conflicts we
    haven't thought of yet.

The more I think about it, the more I think that attempting to detect
copies in a merge algorithm just doesn't make sense.  Anything I can
think of that someone might attempt to use detected copies for would
just surprise users in a bad way...and it'd open up a big can of edge
and corner case problems as well.

  reply	other threads:[~2018-04-27 20:23 UTC|newest]

Thread overview: 65+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-04-20 13:36 [PATCH v1 0/2] add additional config settings for merge Ben Peart
2018-04-20 13:36 ` [PATCH v1 1/2] merge: Add merge.renames config setting Ben Peart
2018-04-20 17:02   ` Elijah Newren
2018-04-20 17:26     ` Elijah Newren
2018-04-23 12:57       ` Ben Peart
2018-04-20 17:59     ` Ben Peart
2018-04-20 18:34       ` Elijah Newren
2018-04-21  4:23         ` Junio C Hamano
2018-04-23 16:00           ` Ben Peart
2018-04-23 23:23             ` Junio C Hamano
2018-04-24 11:58               ` Johannes Schindelin
2018-04-24 17:47                 ` Elijah Newren
2018-04-25  8:20                   ` Johannes Schindelin
2018-04-22 12:07         ` Eckhard Maaß
2018-04-23 13:15           ` Ben Peart
2018-04-23 21:32             ` Eckhard Maaß
2018-04-24 16:53               ` Ben Peart
2018-04-23 13:22         ` Ben Peart
2018-04-20 13:36 ` [PATCH v1 2/2] merge: Add merge.aggressive " Ben Peart
2018-04-20 17:22   ` Elijah Newren
2018-04-24 16:45     ` Ben Peart
2018-04-24 17:36       ` Elijah Newren
2018-04-24 23:57       ` Junio C Hamano
2018-04-25 14:47         ` Ben Peart
2018-04-20 17:34 ` [PATCH v1 0/2] add additional config settings for merge Elijah Newren
2018-04-20 18:19   ` Ben Peart
2018-04-24 17:11 ` [PATCH v2 " Ben Peart
2018-04-24 17:11   ` [PATCH v2 1/2] merge: Add merge.renames config setting Ben Peart
2018-04-24 18:11     ` Elijah Newren
2018-04-24 18:59     ` Elijah Newren
2018-04-24 20:31       ` Ben Peart
2018-04-25 16:01         ` Elijah Newren
2018-04-24 17:11   ` [PATCH v2 2/2] merge: Add merge.aggressive " Ben Peart
2018-04-25  0:13   ` [PATCH v2 0/2] add additional config settings for merge Junio C Hamano
2018-04-25 15:22     ` Ben Peart
2018-04-26  1:48       ` Junio C Hamano
2018-04-26 20:52 ` [PATCH v3 0/3] add merge.renames config setting Ben Peart
2018-04-26 20:52   ` [PATCH v3 1/3] merge: update documentation for {merge,diff}.renameLimit Ben Peart
2018-04-26 23:11     ` Elijah Newren
2018-04-26 23:23       ` Jonathan Tan
2018-04-26 20:52   ` [PATCH v3 2/3] merge: Add merge.renames config setting Ben Peart
2018-04-26 22:52     ` Elijah Newren
2018-04-27  0:54       ` Ben Peart
2018-04-27  2:23         ` Junio C Hamano
2018-04-27  3:28           ` Elijah Newren
2018-04-27  7:23             ` Johannes Schindelin
2018-04-27 14:32               ` Elijah Newren
2018-04-27 18:37           ` Eckhard Maaß
2018-04-27 20:23             ` Elijah Newren [this message]
2018-04-30  8:03               ` Eckhard Maaß
2018-04-30 16:54                 ` Elijah Newren
2018-04-27  4:17         ` Elijah Newren
2018-04-27 18:19         ` Elijah Newren
2018-04-30 13:11           ` Ben Peart
2018-04-30 16:12             ` Re: Elijah Newren
2018-05-02 14:33               ` Re: Ben Peart
2018-04-26 20:52   ` [PATCH v3 3/3] merge: pass aggressive when rename detection is turned off Ben Peart
2018-04-26 23:00     ` Elijah Newren
2018-04-26 22:08   ` [PATCH v3 0/3] add merge.renames config setting Elijah Newren
2018-05-02 16:01 ` [PATCH v4 0/3] add additional config settings for merge Ben Peart
2018-05-02 16:01   ` [PATCH v4 1/3] merge: update documentation for {merge,diff}.renameLimit Ben Peart
2018-05-02 16:01   ` [PATCH v4 2/3] merge: Add merge.renames config setting Ben Peart
2018-05-04  3:07     ` Junio C Hamano
2018-05-02 16:01   ` [PATCH v4 3/3] merge: pass aggressive when rename detection is turned off Ben Peart
2018-05-02 17:20   ` [PATCH v4 0/3] add additional config settings for merge Elijah Newren

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