Re: git merge --no-commit <branch>; does commit

From: Jakub Narebski <jnareb@gmail.com>
Date: Dec 15, 2007, 19:18 UTC
Message-ID: <m37ijfrcm5.fsf@roke.D-201>
In-Reply-To: <alpine.LSU.0.99999.0712151905280.4341@castor.milkiway.cos>

Michael Dressel <MichaelTiloDressel@t-online.de> writes:

Show 22 quoted lines

> On Fri, 14 Dec 2007, Alex Riesen wrote:
> 
>> Michael Dressel, Thu, Dec 13, 2007 22:28:30 +0100:
>>> On Thu, 13 Dec 2007, Alex Riesen wrote:
>>>> On 13/12/2007, Michael Dressel wrote:
>>>>> git merge --no-commit <branch> does "create" a commit. At lesat the
>>>>> head and index are moved to the new commit fetched from <branch>. Maybe
>>>>> that is because git was able to do a fast forward?
>>>>
>>>> Yes. Because fast-forward is what it called: fast-forward.
>>>> It does not do any commits at all.
>>>>
>>>
>>> It looks like I misunderstood the meaning of --no-commit. I have to use
>>> --squash in this case.
>>>
>>
>> Maybe. Or maybe you misunderstood the meaning of --squash, which also
>> is not a merge.
> 
> Since "git merge --squash <branch>" does a merge of <branch> into the
> working tree why would you not call it a merge?

A few illustrations. Commits A, B, C were made on 'trunk', commits a, b, c were made on 'branch'. You are on 'trunk', running "git merge <options> branch". We assume that there are no conflicts.

1. A non fast-forward case

   1---2---3---A---B---C   <-- trunk    <-- HEAD
            \
             \-a---b---c   <-- branch

1.1. "git merge branch"

   1---2---3---A---B---C--M   <-- trunk  <-- HEAD
            \            /
             \-a---b---c/     <-- branch

M is _merge_ commits, with two parents: C and c. It was created by "git merge branch".

1.2. "git merge --no-commit branch"

                        /-------- trunk  <-- HEAD
                       v
   1---2---3---A---B---C--*
            \            /
             \-a---b---c/     <-- branch
                       ^                           
                        \-------- MERGE_HEAD

The merge commit '*' is prepared, but not yet committed, just as if there were merge conflict during merge. "git commit" would commit a merge (and would tell you that it does a merge commit).

1.3. "git merge --no-commit --no-ff branch"

Like in 1.2, because it is non fast-forward case.

1.4. "git merge --squash branch"

   1---2---3---A---B---C--[abc]'   <-- trunk <-- HEAD
            \
             \-a---b---c           <-- branch

"[abc]'" is a _single parent_ commit (it is not a merge commit!) which incorporates changes made on branch as if it were made in a single, large (!) commit.

1.5. "git rebase trunk branch" (or "git pull --rebase branch")

   1---2---3---A---B---C              <-- trunk    
                        \
                         \-a'--b'--c' <-- branch  <-- HEAD

Rebase is step-by-step process, replying commits a, b, c on top of trunk, trying for the same _changes_. a', b', c' are newly created commits, introducing the same changes as a, b, c if there were no rebase conflicts.

I think that "git pull --rebase branch" goes a step further and actually does after-rebase fast-forward merge, and does not change branch, but I might be mistaken.

   1---2---3---A---B---C---a'--b'--c'  <-- trunk <-- HEAD
            \
             \-a---b---c               <-- branch

2. Fast forward case; there are no commits A, B, C, and we
start from the following situation:

   1---2---3               <-- trunk    <-- HEAD
            \
             \-a---b---c   <-- branch

2.1. "git merge branch"

   1---2---3            /----- trunk    <-- HEAD
            \          v
             \-a---b---c   <-- branch

Fast forward results in simply moving the head of trunk. It does not create a commit, hence:

2.2. "git merge --no-commit branch"

Like in 2.1, because fast-forwarding does not create a commit.

2.3. "git merge --no-commit --no-ff branch"

   1---2---3               <-- trunk    <-- HEAD
            \
             \-a---b---c   <-- branch

(probably with some error message).

Test of understanding the concepts: what does "git merge --squash --no-commit" does?

-- 
Jakub Narebski
Poland
ShadeHawk on #git