check if already unlocked and mounted

[sssoleileraaa]

Description

Fixes #12, going with option 2 in the expected results: #12 (comment)

We now check if the device is already unlocked and mounted. If the device is mounted, we don't attempt to mount again, instead we use the current mountpoint to copy files to.

We will need to create a followup issue with a new status code to inform the client that the device is already unlocked and no user passphrase is necessary.

**Important: ** If the device is unlocked by an external program we will still lock it after a file transfer. This is to err on the side of caution.

Test Plan

1. checkout master branch and follow STR in cannot export file if drive is already unlocked #12
2. checkout this pr branch and follow STR in cannot export file if drive is already unlocked #12 and see that the problem is fixed

[redshiftzero]

in testing this I keep getting an ERROR_GENERIC due to umount /media/user/<drivename>: not mounted in the finally block of copy_submission, not sure why I'm getting in this state... 🤔 (this is in the case when I've unlocked the drive using Files)

[sssoleileraaa]

umount /media/user/: not mounted

could you copy your logs around the log line Unmounting drive from {}'.format(self.mountpoint))? it's strange because i added a line of code that checks if the drive is mounted before unmounting:

if os.path.exists(self.mountpoint):
     logger.info('Unmounting drive from {}'.format(self.mountpoint))
     subprocess.check_call(["sudo", "umount", self.mountpoint])

it's possible that if you restarted the export vm that you forgot to copy the export deb package with my changes.

[sssoleileraaa]

or actually looks like i accidentally used rstrip instead of strip here:

    def mount_volume(self):
        # If the drive is already mounted then we don't need to mount it again
        output = subprocess.check_output(
            ["lsblk", "-o", "MOUNTPOINT", "--noheadings", self.device])
        mountpoint = output.decode('utf-8').strip()
        if mountpoint:
            logger.debug('The device is already mounted')
            self.mountpoint = mountpoint
            return

fixed, ready for a retest

[redshiftzero]

OK this looks good now.

For the record, in reproducing #12 I could reproduce that an ERROR_GENERIC occurs but that was after the files are successfully exported (and thus the success message popped up). Regardless, I can confirm on this diff that I no longer see this behavior for both 1. the drive is already mounted/unlocked and 2. the drive is not mounted/unlocked cases - the export is successful in both cases.