Subject: Re: [PATCH 0/3] Reject non-ff pulls by default

From: Richard Hansen <[email-masked]>
Date: Thu, 05 Sep 2013 11:20:43 -0400

On 2013-09-04 18:59, Junio C Hamano wrote:
> "Philip Oakley" <[email-masked]> writes:
> 
>> From: "Junio C Hamano" <[email-masked]>
>>> John Keeping <[email-masked]> writes:
>>>
>>>> I think there are two distinct uses for pull, which boil down to:
>>>>
>>>>     (1) git pull
>>> ...
>>> Peff already covered (1)---it is highly doubtful that a merge is
>>> "almost always wrong".  In fact, if that _were_ the case, we should
>>> simply be defaulting to rebase, not failing the command and asking
>>> between merge and rebase like jc/pull-training-wheel topic did.
>>>
>>> We simply do not know what the user wants, as it heavily depends on
>>> the project, so we ask the user to choose one (and stick to it).
>>
>> We only offer a limited list. It won't be sufficient for all use
>> cases. It wasn't for me.
> 
> Very interesting. Tell us more.

I'm a bit late to the discussion, but I wanted to chime in.  I detest
'git pull' and discourage everyone I meet from using it.  See:
<http://stackoverflow.com/questions/15316601/why-is-git-pull-considered-harmful>
for my reasons.

Instead, I encourage people to do this:

   git config --global alias.up '!git remote update -p; git merge
--ff-only @{u}'

and tell them to run 'git up' whenever they would be tempted to use a
plain 'git pull'.

I usually work with a central repository with topic branches.  I follow
this rule of thumb:
  * When merging a "same-named" branch (e.g., origin/foo into foo, foo
    into origin/foo), it should always be a fast-forward.  This may
    require rebasing.
  * When merging a "differently-named" branch (e.g., feature.xyz into
    master), it should never be a fast-forward.

In distributed workflows, I think of 'git pull <collaborator-repo>
<their-branch>' as merging a differently-named branch (I wouldn't be
merging if they hadn't told me that a separate feature they were working
on is complete), so I generally want the merge commit.  But when I do a
'git pull' without extra arguments, I'm updating a same-named branch so
I never want a merge.

When merging a differently-named branch, I prefer the merge --no-ff to
be preceded by a rebase to get a nice, pretty graph:

       * merge feature.xyz  <- master
       |\
       | * xyz part 3/3
       | * xyz part 2/3
       | * xyz part 1/3
       |/
       * merge feature.foo
       |\
       | * foo part 2/2
       | * foo part 1/2
       |/
       * merge feature.bar
       |\
       ...

The explicit merge has several benefits:
  * It clearly communicates to others that the feature is done.
  * It makes it easier to revert the entire feature by reverting the
    merge if necessary.
  * It allows our continuous integration tool to skip over the
    work-in-progress commits and test only complete features.
  * It makes it easier to review the entire feature in one diff.
  * 'git log --first-parent' shows a high-level summary of the changes
    over time, while a normal 'git log' shows the details.

> 
> When "git pull" stops because what was fetched in FETCH_HEAD does
> not fast-forward, then what did _you_ do (and with the knowledge you
> currently have, what would you do)?

I stop and review what's going on, then make a decision:
  * usually it's a rebase
  * sometimes it's a rebase --onto (because the branch was
    force-updated to undo a particularly bad commit)
  * sometimes it's a rebase -p (because there's an explicit merge of a
    different branch that I want to keep)
  * sometimes it's a reset --hard (my changes were made obsolete by a
    different upstream change)
  * sometimes it's a merge
  * sometimes I do nothing.  This is a fairly regular pattern:  I'm in
    the middle of working on something that I know will conflict with
    some changes that were just pushed upstream, and I want to finish
    my changes before starting the rebase.  My collaborator contacts me
    and asks, "Would you take a look at the changes I just pushed?"  If
    I type 'git pull' out of habit to get the commits, then I'll make a
    mess of my work-in-progress work tree.  If I type 'git up' out of
    habit, then the merge --ff-only will fail as expected and I can
    quickly review the commits without messing with my work tree or
    HEAD.

Even if I always rebase or always merge, I want to briefly review what
changed in the remote branch *before* I start the rebase.  This helps me
understand the conflicts I might encounter.

Thus, ff-only always works for me.  I might have to type a second merge
or rebase command, but that's OK -- it gives me an opportunity to think
about what I want first.  Non-ff merges are rare enough that the
interruption isn't annoying at all.

> In a single project, would you
> choose to sometimes rebase and sometimes merge, and if so, what is
> the choice depend on?  "When I am on these selected branches, I want
> to merge, but on other branches I want to rebase?"

My choice depends on the circumstances of the divergence.  It's never as
simple as branch X always has this policy while branch Y has that policy.

> Are there cases where you do not want to either rebase nor merge?
> If so what do you want to do after "git pull" fetches from the other
> side?  Nothing?
> 
> 	Side note: a knee-jerk response to a "yes" answer to the
> 	last question from me has always been "then why are you
> 	running 'git pull' in the first place.

Habit/muscle memory/I'm tired and not thinking 100% clearly.

-Richard