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Re: could `git merge --no-ff origin/master` be made more useful?




On 15 May 2018 at 00:58, Ævar Arnfjörð Bjarmason <avarab@xxxxxxxxx> wrote:
>
> On Mon, May 14 2018, demerphq wrote:
>
>> The first time I tried to use --no-ff I tried to do something like this:
>>
>>   git checkout master
>>   git commit -a -m'whatever'
>>   git commit -a -m'whatever2'
>>   git merge --no-ff origin/master
>>
>> and was disappointed when "it didn't work" and git told me there was
>> nothing to do as the branch was up to date. (Which I found a bit
>> confusing.)
>>
>> I realize now my expectations were incorrect, and that the argument to
>> merge needs to resolve to a commit that is ahead of the current
>> commit, and in the above sequence it is the other way around. So to do
>> what I want I can do:
>>
>>   git checkout master
>>   git checkout -b topic
>>   git commit -a -m'whatever'
>>   git commit -a -m'whatever2'
>>   git checkout master
>>   git merge --no-ff topic
>>
>> and iiuir this works because 'master' would be behind 'topic' in this case.
>>
>> But I have a few questions, 1) is there is an argument to feed to git
>> merge to make the first recipe work like the second? And 2) is this
>> asymmetry necessary with --no-ff?
>
> I've been bitten my this myself, but found that it's documented as the
> very first thing in git-merge:
>
>     Incorporates changes from the named commits (since the time their
>     histories diverged from the current branch) into the current
>     branch[...].
>
> Since origin/master hasn't diverged from your current branch (unlike the
> other way around), the merge with --no-ff is a noop.

Yeah, I got it, but only after rereading a lot of times.

>
>> More specifically would something horrible break if --no-ff
>> origin/trunk detected that the current branch was ahead of the named
>> branch and "swapped"  the implicit order of the two so that the first
>> recipe could behave like the second
>
> If it worked like that then the user who sets merge.ff=false in his
> config and issues a "git pull" after making a commit on his local master
> would create a merge commit.
>
> This old E-Mail of Junio's discusses that edge case & others in detail:
> https://public-inbox.org/git/7vty1zfwmd.fsf@xxxxxxxxxxxxxxxxxxxxxxxx/

Thanks I skimmed, but it is long so I will review later.

I see the point about the config option for no-ff.

But what about an option like --reverse? Assuming we are on a local
branch master then

  git merge --no-ff --reverse origin/master

would treat origin/master as the "current" branch, and "master" as the
merged in branch, and create the appropriate merge commit. Which as
far as I can tell is tree-wise identical to creating a topic branch
instead of hacking on the local master.

>> Anyway, even if the above makes no sense, would it be hard to make the
>> message provided by git merge in the first recipe a bit more
>> suggestive of what is going on? For instance if it had said "Cannot
>> --no-ff merge, origin/master is behind master" it would have been much
>> more clear what was going on.
>
> I can't spot any reason for why we couldn't have something like this POC
> (would be properly done through advice.c):
>
>     diff --git a/builtin/merge.c b/builtin/merge.c
>     index 9db5a2cf16..920f67d9f8 100644
>     --- a/builtin/merge.c
>     +++ b/builtin/merge.c
>     @@ -1407,6 +1407,8 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
>                      * but first the most common case of merging one remote.
>                      */
>                     finish_up_to_date(_("Already up to date."));
>     +               if (fast_forward == FF_NO)
>     +                       fprintf(stderr, "did you mean this the other way around?\n");
>                     goto done;
>             } else if (fast_forward != FF_NO && !remoteheads->next &&
>                             !common->next &&
>
> But that should probably be reworked to be smart about whether --no-ff
> or merge.ff=false was specified, i.e. do we want to yell this at the
> user who's just set that at his config default, or the user who's
> specified --no-ff explicitly, or both? I don't know.

Yes, all those points make sense.

Yves


-- 
perl -Mre=debug -e "/just|another|perl|hacker/"