Allow export --force

After https://github.com/postmarketOS/pmbootstrap/pull/250 has been merged I've found a situation where flasher export crashes:

When the symlink already exists but points to an non-existing file, the os.path.exists condition is False and then "ln -s" fails.

Reproduce:

pmb flasher export
cd /tmp/postmarketOS-export
ln -sf /unexisting initramfs-motorola-titan  # adjust name
pmb flasher export

Proposed solution:

We could allow a --force argument that ignores if file already exists and also forces the symlink creation ("ln -sf")

Created by: PureTryOut

Hmm, not sure if that is the correct thing to do. It would mean the script still crashes before you find out what's wrong. It's probably preferable to prevent the crash from happening in the first place, instead of using a workaround.

By * postmarketOS Bot on 2017-07-28T12:50:02

Sorry, I didn't explain that the crash comes from executing ln -s when the link already exists:

$ ln -s /etc/passwd p  # first time succeeds
$ ln -s /etc/passwd p  # second time fails
ln: failed to create symbolic link 'p': File exists

In the actual code, this situation is not handled because os.path.exists fails (there exists a link but points to an unexisting file) and ln -s fails because there is a link already

By Pablo Castellano on 2017-07-28T16:37:00

Instead of adding a new option to export, why not just make export clean up old (and/or broken) links by default before trying to create new ones?

By clayton craft on 2017-07-28T17:22:18

Created by: ollieparanoid

@PabloCastellano made PR #279, which includes a patch for this issue. I suggest commenting the patch there.

By * postmarketOS Bot on 2017-07-28T18:00:07