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Confused about "degenerate" combined diff for merge commits

Subject: Confused about "degenerate" combined diff for merge commits

## tl;dr

2 messages between Aug 15, 2010 and Aug 15, 2010.

replies: 1people: 2as markdown or json

Stefan Haller· Aug 15, 2010, 17:51 UTC · lore

(Git newbie alert. Please be patient with me if I'm missing obvious things...)

I'm converting a moderately large repository from Subversion to git. As part of the process, I'm experimenting with correctly representing merges in the resulting git repository. In Subversion, we used commit logs such as "Merged r1234:1279 from branch_xyz", so in most cases it should be possible to use this information to create the proper parents in git. I'm currently trying how well this works by using grafts; but that's not the question.

Now, I'm using "gitk --all --merges" to look at my synthesized merge commits; and I assume that I manufactured them correctly if the resulting combined diff is empty. I only expect diff output for merge conflicts with a non-trivial resolution, i.e. neither ours nor theirs was used to resolve the conflict. Is this assumption correct so far?

However, for some of these merge commits I see diff output such as this:

diff --cc Src/ClipBoard.cpp index 4357ea0,4357ea0..3fad79a

--- a/Src/ClipBoard.cpp
+++ b/Src/ClipBoard.cpp
@@@ -71,7 -71,7 +71,7 @@@ AClipBoard* AClipBoard::SNew(
  
  AClipBoard* AClipBoard::SClipBoard()
  {
--  static AClipBoard* spClipBoard = SNew();
++  static TPtr<AClipBoard> spClipBoard = SNew();
    return spClipBoard;
  }
  
It looks like both merge parents had an identical diff here, so this is
not a conflict; why does diff --cc even show this to me?

-- 
Stefan Haller
Berlin, Germany
http://www.haller-berlin.de/

Thomas Rast· Aug 15, 2010, 19:54 UTC · re: Stefan Haller · lore

Re: Confused about "degenerate" combined diff for merge commits

Stefan Haller wrote:

Show 17 quoted lines

> However, for some of these merge commits I see diff output such as this:
> 
> diff --cc Src/ClipBoard.cpp
> index 4357ea0,4357ea0..3fad79a
> --- a/Src/ClipBoard.cpp
> +++ b/Src/ClipBoard.cpp
> @@@ -71,7 -71,7 +71,7 @@@ AClipBoard* AClipBoard::SNew(
>   
>   AClipBoard* AClipBoard::SClipBoard()
>   {
> --  static AClipBoard* spClipBoard = SNew();
> ++  static TPtr<AClipBoard> spClipBoard = SNew();
>     return spClipBoard;
>   }
>   
> It looks like both merge parents had an identical diff here, so this is
> not a conflict; why does diff --cc even show this to me?

The --cc output has one column per parent, and if you removed all other +/-/space columns, then (for that hunk) you'd have a unified diff with that parent.

So I'd say the above means that both parents had

  static AClipBoard* spClipBoard = SNew();

but your merge result was

  static TPtr<AClipBoard> spClipBoard = SNew();

This is usually called an "evil merge" in git terms: it is introducing changes that are neither coming from one side, nor part of a benign merge resolution.

-- 
Thomas Rast
trast@{inf,student}.ethz.ch