On using receive.denyNonFastForwards and advice.pushNonFastForward

On using receive.denyNonFastForwards and advice.pushNonFastForward

[Hilco Wijbenga]

Hi all,

I was wondering how hard it would be to make "git push" more adamant
about not pushing non-ff updates. So I wanted to see the effects of
receive.denyNonFastForwards and advice.pushNonFastForward. (By the
way, why is one plural and the other singular? That doesn't seem
consistent?)

HERE=$(pwd) &&
git init --bare remote-repo &&
cd remote-repo/ &&
git config --add receive.denyNonFastForwards true &&
cd .. &&
git clone file://$HERE/remote-repo local-repo &&
cd local-repo/ &&
git config --add advice.pushNonFastForward true &&
echo "1" > one.txt &&
git add -A . && git commit -m 1 && git push origin master &&
git checkout -b next &&
echo "a" > two.txt &&
git add -A . && git commit -m 2 &&
git checkout master &&
echo "2" > one.txt &&
git add -A . && git commit -m 3 && git push origin master &&
git merge next &&
git push

To my surprise there was neither warning nor error. Does this last
push really qualify as a FF update? Apparently, linear history and
FF-only updates are not the same thing?

Cheers,
Hilco

Re: On using receive.denyNonFastForwards and advice.pushNonFastForward

[Christopher Tiwald]

From: Christopher Tiwald <christiwald@gmail.com>
To: Hilco Wijbenga <hilco.wijbenga@gmail.com>
Cc: Git Users <git@vger.kernel.org>
Bcc: 
Subject: Re: On using receive.denyNonFastForwards and
 advice.pushNonFastForward
Reply-To: 
In-Reply-To: <CAE1pOi1M-fdMJtZw9MNL2R6zWvpXvWVo4ros_NSCQtLmQb6TOQ@mail.gmail.com>

On Sun, Jul 08, 2012 at 02:26:50PM -0700, Hilco Wijbenga wrote:
> I was wondering how hard it would be to make "git push" more adamant
> about not pushing non-ff updates. So I wanted to see the effects of
> receive.denyNonFastForwards and advice.pushNonFastForward. (By the
> way, why is one plural and the other singular? That doesn't seem
> consistent?)

'advice.pushNonFastForward' doesn't control whether or not
the remote accepts non-fast-forward updates. Rather, it controls whether
or not advice displays when 'git push' errors because the user attempted
a non-fast-forward update. As of 1.7.11 (f25950f3), it was supplemented
with 'advice.pushNonFFCurrent', 'advice.pushNonFFDefault', and
'pushNonFFMatching'. Setting the original 'advice.pushNonFastForward'
config option to 'false' will disable all three of these situational
hints. None of them will affect the actual operation of 'git push'.

As for this series of commands, none of these are non-fast-forward
updates (i.e. a situation where a pushed branch tip is behind its remote
counterpart):

> HERE=$(pwd) &&
> git init --bare remote-repo &&
> cd remote-repo/ &&
> git config --add receive.denyNonFastForwards true &&
> cd .. &&
> git clone file://$HERE/remote-repo local-repo &&
> cd local-repo/ &&
> git config --add advice.pushNonFastForward true &&
> echo "1" > one.txt &&
> git add -A . && git commit -m 1 && git push origin master &&

This is the inital push to the remote:

$ git log --graph --oneline
* 32bbda2 1

$ git diff master origin/master ;# noop

> git checkout -b next &&
> echo "a" > two.txt &&
> git add -A . && git commit -m 2 &&
> git checkout master &&
> echo "2" > one.txt &&
> git add -A . && git commit -m 3 && git push origin master &&

This is a standard fast-forward update to the remote:

$ git log --graph --oneline
* 0176f87 3
* 32bbda2 1

$ git diff master origin/master ;# noop

> git merge next &&
> git push

This is also a standard fast-forward update to the remote:

$ git log --graph --oneline
*   b881618 Merge branch 'next'
|\
| * 843a285 2
* | 0176f87 3
|/
* 32bbda2 1

$ git diff master origin/master ;# noop

If you want to see a true non-fast-forward error from this point, try
this:

git reset --hard HEAD~1 &&
echo "non-ff" > one.txt &&
git add . && git commit -m 4 && git push origin master

You should get something like this (the advice will change based on your
version of git):

[master cf28ce8] 4
1 file changed, 1 insertion(+), 1 deletion(-)
To /tmp/remote-repo
 ! [rejected]        master -> master (non-fast-forward)
error: failed to push some refs to '/tmp/remote-repo'
hint: Updates were rejected because the tip of your current branch is behind
hint: its remote counterpart. Merge the remote changes (e.g. 'git pull')
hint: before pushing again.
hint: See the 'Note about fast-forwards' in 'git push --help' for
details.

The "hint" lines at the end are configured by the 'advice' options
described above.

--
Christopher Tiwald

Re: On using receive.denyNonFastForwards and advice.pushNonFastForward

[Matthieu Moy]

Hilco Wijbenga <hilco.wijbenga@gmail.com> writes:

> I was wondering how hard it would be to make "git push" more adamant
> about not pushing non-ff updates. So I wanted to see the effects of
> receive.denyNonFastForwards

This changes the behavior only if you use --force (or some magic refspec
starting with +). If you never used --force or +refspecs, you never did
a non-fast-forward push.

> and advice.pushNonFastForward.

Activated by default, so if you never did anything about it, it was
already active.

> Apparently, linear history and FF-only updates are not the same thing?

No, they are not. non-FF means you're trying to push a commit which is
not a direct descendant of the remote one, i.e. you're trying to
override the remote history with yours. See 'NOTE ABOUT FAST-FORWARDS'
in "man git-push".

-- 
Matthieu Moy
http://www-verimag.imag.fr/~moy/