Pitfalls in auto-fast-forwarding heads that are not checked out?

Pitfalls in auto-fast-forwarding heads that are not checked out?

[Martin Langhoff]

I am building a small git wrapper around puppet, and one of the
actions it performs is auto-fastforwarding of branches without
checking them out.

In simplified code... we ensure that we are on a head called master,
and in some cases "ppg commit", will commit to master and...

  ## early on
  # sanity-check we are on master
  headname=$(git rev-parse --symbolic-full-name --revs-only HEAD)
  if [ "$headname" -ne "refs/heads/headname" ]; then
      echo >&2 "ERROR: can only issue --immediate commit from the
master branch!"
      exit 1
  fi

  ## then
  git commit -bla blarg baz

  ## and then...

  # ensure we can ff
  head_sha1=$(git rev-parse --revs-only master)
  mb=$(git merge-base $production_sha1 refs/heads/master)
  if [[ "$mb" -ne "$production_sha1" ]]; then
      echo >&2 "ERROR: cannot fast-forward master to production"
      exit 1
  fi
  $GIT_EXEC_PATH/git-update-ref -m "ppg immediate commit"
refs/heads/production $head_sha1 $production_sha1 || exit 1

Are there major pitfalls in this approach? I cannot think of any, but
git has stayed away from updating my local tracking branches; so maybe
there's a reason for that...

cheers,


m
--
 martin.langhoff@gmail.com
 -  ask interesting questions
 - don't get distracted with shiny stuff  - working code first
 ~ http://docs.moodle.org/en/User:Martin_Langhoff

Re: Pitfalls in auto-fast-forwarding heads that are not checked out?

[Johannes Sixt]

Am 04.05.2013 00:46, schrieb Martin Langhoff:
> I am building a small git wrapper around puppet, and one of the
> actions it performs is auto-fastforwarding of branches without
> checking them out.
> 
> In simplified code... we ensure that we are on a head called master,
> and in some cases "ppg commit", will commit to master and...
> 
>   ## early on
>   # sanity-check we are on master
>   headname=$(git rev-parse --symbolic-full-name --revs-only HEAD)
>   if [ "$headname" -ne "refs/heads/headname" ]; then

You mean "refs/heads/master" and "!=" here because -ne is numeric
comparison in a shell script.

>       echo >&2 "ERROR: can only issue --immediate commit from the
> master branch!"
>       exit 1
>   fi
> 
>   ## then
>   git commit -bla blarg baz
> 
>   ## and then...
> 
>   # ensure we can ff
>   head_sha1=$(git rev-parse --revs-only master)
>   mb=$(git merge-base $production_sha1 refs/heads/master)
>   if [[ "$mb" -ne "$production_sha1" ]]; then

Your approach looks OK (but note again the incorrect "-ne").

Since git 1.8.0 you can express this check as

    if git merge-base --is-ancestor $production_sha1 refs/heads/master

>       echo >&2 "ERROR: cannot fast-forward master to production"

	echo >&2 "ERROR: cannot fast-forward production to master"

>       exit 1
>   fi
>   $GIT_EXEC_PATH/git-update-ref -m "ppg immediate commit"
> refs/heads/production $head_sha1 $production_sha1 || exit 1
> 
> Are there major pitfalls in this approach?

I don't think there are.

> I cannot think of any, but
> git has stayed away from updating my local tracking branches; so maybe
> there's a reason for that...

I don't understand what you are saying here. What is "that"?

-- Hannes

Re: Pitfalls in auto-fast-forwarding heads that are not checked out?

[Martin Langhoff]

On Sat, May 4, 2013 at 3:34 AM, Johannes Sixt <j6t@kdbg.org> wrote:
> You mean "refs/heads/master" and "!=" here because -ne is numeric
> comparison in a shell script.

thanks! Yeah, I fixed those up late last night :-)

> Since git 1.8.0 you can express this check as
>
>     if git merge-base --is-ancestor $production_sha1 refs/heads/master

Ah, that's great! Unfortunate it's not there in earlier / more widely
used releases of git.

>> Are there major pitfalls in this approach?
>
> I don't think there are.

Thanks...

>> I cannot think of any, but
>> git has stayed away from updating my local tracking branches; so maybe
>> there's a reason for that...
>
> I don't understand what you are saying here. What is "that"?

When I do git pull, git is careful to only update the branch I have
checked out (if appropriate). It leaves any other branches that track
branches on the remote that has just been fetched untouched. I always
thought that at some point git pull would learn to evaluate those
branches and auto-merge them if the merge is a ff.

I would find that a natural bit of automation in git pull. Of course
it would mean a change of semantics, existing scripts could be
affected.

cheers,



m
--
 martin.langhoff@gmail.com
 -  ask interesting questions
 - don't get distracted with shiny stuff  - working code first
 ~ http://docs.moodle.org/en/User:Martin_Langhoff

Re: Pitfalls in auto-fast-forwarding heads that are not checked out?

[John Szakmeister]

On Sat, May 4, 2013 at 7:35 AM, Martin Langhoff
<martin.langhoff@gmail.com> wrote:
[snip]
> When I do git pull, git is careful to only update the branch I have
> checked out (if appropriate). It leaves any other branches that track
> branches on the remote that has just been fetched untouched. I always
> thought that at some point git pull would learn to evaluate those
> branches and auto-merge them if the merge is a ff.
>
> I would find that a natural bit of automation in git pull. Of course
> it would mean a change of semantics, existing scripts could be
> affected.

I agree.  I've been using this script for quite a while now:

<https://github.com/jszakmeister/etc/blob/master/git-addons/git-ffwd>

I've been pretty happy with it.  It's not of my own design, I picked
up from StackOverflow:

<http://stackoverflow.com/a/9076361/683080>

And made a couple of minor tweaks to cope with my configuration (I
have merge setup to not fast-forward merge by default).

-John

Re: Pitfalls in auto-fast-forwarding heads that are not checked out?

[Jonathan Nieder]

Martin Langhoff wrote:
> On Sat, May 4, 2013 at 3:34 AM, Johannes Sixt <j6t@kdbg.org> wrote:

>> Since git 1.8.0 you can express this check as
>>
>>     if git merge-base --is-ancestor $production_sha1 refs/heads/master
>
> Ah, that's great! Unfortunate it's not there in earlier / more widely
> used releases of git.

Another trick is to use "git push":

	git push . $production_sha1:refs/heads/master

This advances the "master" branch if

 (1) it is not the current branch, and
 (2) that update is a fast-forward

It can be convenient sometimes.

Hope that helps,
Jonathan

Re: Pitfalls in auto-fast-forwarding heads that are not checked out?

[Martin Langhoff]

On Sat, May 4, 2013 at 2:51 PM, Jonathan Nieder <jrnieder@gmail.com> wrote:
> Another trick is to use "git push":
>         git push . $production_sha1:refs/heads/master

Great trick -- thanks! In use in ppg now :-)



m
--
 martin.langhoff@gmail.com
 -  ask interesting questions
 - don't get distracted with shiny stuff  - working code first
 ~ http://docs.moodle.org/en/User:Martin_Langhoff