ShellDriver.put_ssh_key not working on read-only filesystem

[rohieb]

Doing ShellDriver.put_ssh_key() on a read-only file system only results in a stacktrace for me:

   INFO: Key not on target, mounting...
Traceback (most recent call last):
  File "test-xmodem.py", line 14, in <module>
    sd.put_ssh_key("../projectroot/root/.ssh/development_rsa.pub")
  File "/ptx/work/dude/WORK_B/rhi/realtimeci-DistroKit/labgrid-tests/env/lib/python3.5/site-packages/labgrid-0.1.1.dev1+g0897126-py3.5.egg/labgrid/binding.py", line 87, in wrapper
    return func(self, *_args, **_kwargs)
  File "/ptx/work/dude/WORK_B/rhi/realtimeci-DistroKit/labgrid-tests/env/lib/python3.5/site-packages/labgrid-0.1.1.dev1+g0897126-py3.5.egg/labgrid/driver/shelldriver.py", line 206, in put_ssh_key
    self._put_ssh_key(key)
  File "/ptx/work/dude/WORK_B/rhi/realtimeci-DistroKit/labgrid-tests/env/lib/python3.5/site-packages/labgrid-0.1.1.dev1+g0897126-py3.5.egg/labgrid/step.py", line 192, in wrapper
    _result = func(*_args, **_kwargs)
  File "/ptx/work/dude/WORK_B/rhi/realtimeci-DistroKit/labgrid-tests/env/lib/python3.5/site-packages/labgrid-0.1.1.dev1+g0897126-py3.5.egg/labgrid/driver/shelldriver.py", line 200, in _put_ssh_key
    self._run_check('mount --bind /tmp/keys ~/.ssh/authorized_keys')
  File "/ptx/work/dude/WORK_B/rhi/realtimeci-DistroKit/labgrid-tests/env/lib/python3.5/site-packages/labgrid-0.1.1.dev1+g0897126-py3.5.egg/labgrid/step.py", line 192, in wrapper
    _result = func(*_args, **_kwargs)
  File "/ptx/work/dude/WORK_B/rhi/realtimeci-DistroKit/labgrid-tests/env/lib/python3.5/site-packages/labgrid-0.1.1.dev1+g0897126-py3.5.egg/labgrid/driver/shelldriver.py", line 114, in _run_check
    raise ExecutionError(cmd)
labgrid.driver.exception.ExecutionError: mount --bind /tmp/keys ~/.ssh/authorized_keys

… which, in hindsight, is obvious, because on the target console:

root@DistroKit:~ mount --bind /tmp/keys ~/.ssh/authorized_keys
mount: mount point /root/.ssh/authorized_keys does not exist
root@DistroKit:~ mkdir ~/.ssh
mkdir: can't create directory '/root/.ssh': Read-only file system
root@DistroKit:~ mount -o rw,remount /
root@DistroKit:~ mkdir ~/.ssh
root@DistroKit:~ ls -la ~/.ssh
drwxr-xr-x    2 root     root             6 May 22  2017 .
drwxr-xr-x    3 1059     3000            26 May 22  2017 ..

On most freshly installed targets, /root/.ssh will most probably not exist, so on a read-only system, mkdir ~/.ssh in ShellDriver.py, line 179 will fail. I'm guessing the read-write mount is only done a few lines later with mount --bind, and if this works like I imagine, I think this issue could be solved by doing a mount --bind on the whole /root/ directory instead of only the authorized_keys file.