difftool fails with dangling symlinks, writes to targets outside of the repository

From: Philippe <bqobctqyemmeacqxbrsaury3l4ka@quarantine.de>
Date: Nov 22, 2019, 12:37 UTC
Message-ID: <7e04799d-c03c-ac28-604d-ec4d30029d44@quarantine.de>

Hi all,

I'm using a git repository for configuration management which contains several dangling symlinks (which only make sense on the to be deployed system, fwiw). Now, using git 2.23, I ran into a problem regarding the difftool, which fails when such a symlink is involved in a changeset.

The best way to describe that problem is probably to provide an example.

Show 16 quoted lines

> #!/bin/bash
> tmp=$(mktemp -d)
> trap "rm -rf ${tmp}" 0 1 2 15
> 
> set -x
> 
> cd ${tmp}
> git init
> ln -s ../doesnot/exist myfile
> git add myfile ; git commit -m "initial checkin" myfile
> 
> git checkout -b mybranch
> rm -f myfile ; echo "foo" > myfile
> git commit -am "typechange"
> 
> git difftool -d master..mybranch

The last call fails with:

> fatal: could not open '/tmp/git-difftool.2wHaR9/left/myfile' for writing: No such file or directory

Neither specifying "--symlinks" nor "--no-symlinks" resolves that.

Playing around with this issue lead me to another strange behaviour. Difftool might overwrite symlink targets that are not part of the repository. Please take a look at the following script:

Show 21 quoted lines

> #!/bin/bash
> tmp=$(mktemp -d)
> trap "rm -rf ${tmp}" 0 1 2 15
> 
> set -x
> 
> echo "hello world" > /tmp/testfile
> cat /tmp/testfile
> 
> cd ${tmp}
> git init
> ln -s /tmp/testfile myfile
> git add myfile ; git commit -m "initial checkin" myfile
> 
> git checkout -b mybranch
> rm -f myfile ; echo "foo" > myfile
> git commit -am "typechange"
> 
> git difftool -d master..mybranch
> 
> cat /tmp/testfile

Here, difftool overwrites the /tmp/testfile's content "hello world" with "/tmp/testfile".

Did I run into a bug or is it a layer 8 problem?

Regards,

Philippe