git-pull sets write bit, git-push does not

git-pull sets write bit, git-push does not

[Alexander E Genaud]

git-pull sets write bit, git-push does not

Hello,

Background: I am using Git locally with ClearCase upstream. I
initialized a Git repository on top of a ClearCase snapshot view,
while my work branches are in a clone. As ClearCase is particular
about the write bit, I have come to depend on an undocumented feature
of Git. Namely, that git-push preserves read only permissions, while
git-pull sets modified files writable.

Can git-push be relied upon to preserve the write bit (readonly)? Why
is git-pull different? Is it a side effect of the plumbing?

Thanks,
Alex

http://genaud.net/2008/08/clearcase-globally-git-locally/


Simplified case:

echo --
echo create a repo r1 with files A and B committed
echo --
mkdir r1
cd r1
echo A > A
echo B > B
git init
git add .
git commit -m init

echo --
echo create an identical repo r2 whose files are readonly
echo --
cp -r ../r1 ../r2
chmod u-w ../r2/[AB]

echo --
echo push a modification of A from r1 to r2
echo --
echo AA > A
git commit -a -m modA
git push ../r2

echo --
echo pull a modification of B from r1 to r2
echo --
echo BB > B
git commit -a -m modB
cd ../r2
git pull ../r1

echo --
echo notice that pushed A remains readonly
echo while pulled B has become writable
echo --
ls -l


-- 
[ alex@genaud.net ][ http://genaud.net ]
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Re: git-pull sets write bit, git-push does not

[Matt Pearson]

On Sat, Aug 2, 2008 at 6:32 PM, Alexander E Genaud <alex@genaud.net> wrote:
> git-pull sets write bit, git-push does not
>
> Hello,
>
> Background: I am using Git locally with ClearCase upstream. I
> initialized a Git repository on top of a ClearCase snapshot view,
> while my work branches are in a clone. As ClearCase is particular
> about the write bit, I have come to depend on an undocumented feature
> of Git. Namely, that git-push preserves read only permissions, while
> git-pull sets modified files writable.
>
> Can git-push be relied upon to preserve the write bit (readonly)? Why
> is git-pull different? Is it a side effect of the plumbing?

Push and pull are not opposite operations: the opposite of push is
fetch. This is because pull updates the working copy, while push does
not (like fetch, it only modifies the ref).

>
> Thanks,
> Alex
>
> http://genaud.net/2008/08/clearcase-globally-git-locally/
>
>
> Simplified case:
>
> echo --
> echo create a repo r1 with files A and B committed
> echo --
> mkdir r1
> cd r1
> echo A > A
> echo B > B
> git init
> git add .
> git commit -m init
>
> echo --
> echo create an identical repo r2 whose files are readonly
> echo --
> cp -r ../r1 ../r2
> chmod u-w ../r2/[AB]
>
> echo --
> echo push a modification of A from r1 to r2
> echo --
> echo AA > A
> git commit -a -m modA
> git push ../r2
>
> echo --
> echo pull a modification of B from r1 to r2
> echo --
> echo BB > B
> git commit -a -m modB
> cd ../r2
> git pull ../r1
>
> echo --
> echo notice that pushed A remains readonly
> echo while pulled B has become writable
> echo --
> ls -l
>
After running this:
$ cat A
A
$ cat ../r1/A
AA

The working copy in r2 was not updated with the changes you pushed to
it (both the content and the mode change).

Re: git-pull sets write bit, git-push does not

[Matt Pearson]

On Sat, Aug 2, 2008 at 7:18 PM, Matt Pearson <404emailnotfound@gmail.com> wrote:
> The working copy in r2 was not updated with the changes you pushed to
> it (both the content and the mode change).

Hm, your test case is more complicated than I originally thought. I
get that result with the git from Ubuntu Hardy, but current master
dies on the pull complaining B is not uptodate (has local changes). I
guess this was a bug that was fixed, but there's no way I'm going to
bisect or look at release notes over half a year (or more) of changes
:)

What it mainly comes down to is that git has only two possible file
modes: 644 and 755. These are the only ones that will ever be stored
in the object database. It seems like older git would ignore
permission changes to the working copy, and reset the permissions to
the "normalized" version when updating a file that was modified by the
pull.

Here's a blow-by-blow explanation. I'll use numbers to refer to the
commits, where 1 is the initial commit, 2 is the commit you pushed,
and 3 is the one you pulled. After the cp -r, both repos have a clean
working tree; the chmod dirties the working tree of r2.

The push causes both repos to have 2 be the HEAD commit, but r2 keeps
the dirty working tree changes, including (what now appears to be,
from git's POV) a change in the content of A from 'AA' to the 'A' in
the initial commit. (The content change will appear as a staged change
when running status in r2, because the index isn't updated either)

When pulling 3, git thinks that these changes were made on top of 2,
and you want to keep them. So it doesn't modify the contents of A
because A was not changed in commit 3---if it was, you'd get a "not
uptodate" message.

B, however, *was* changed in 3, so it tries to apply those changes.
With the older version, it seems to ignore the permissions change and
simply fast-forward to the state B was in after commit 3 (content
'BB', mode 644). With the newer version, it does see that the mode has
been changed, and aborts due to a conflict. In fact, it doesn't seem
to like any mode change---it still dies with a conflict if I change
the chmod to 'a+x'.

Hope that clears it up better---figuring this out helped me learn some
stuff about git. Now I'll wait for someone more knowledgeable than I
to tell me I'm wrong. :)

Matt