pmbootstrap init: Ask for hostname, default: device name (#1327)

* Save "" (empty string) in the user's config as hostname if the user
  let it default to the name of the device. That way, when the device
  gets changed, the user won't get the old device's name as hostname
  by accident.
* Add a test case

pmb/helpers/other.py

@@ -16,8 +16,9 @@ GNU General Public License for more details.
 You should have received a copy of the GNU General Public License
 along with pmbootstrap.  If not, see <http://www.gnu.org/licenses/>.
 """
-import os
 import logging
+import os
+import re
 import pmb.chroot
 import pmb.config
 import pmb.helpers.run
@@ -127,3 +128,26 @@ def migrate_work_folder(args):
                        " folder manually ('sudo rm -rf " + args.work +
                        "') and start over with 'pmbootstrap init'. All your"
                        " binary packages will be lost.")
+
+
+def validate_hostname(hostname):
+    """
+    Check whether the string is a valid hostname, according to
+    <http://en.wikipedia.org/wiki/Hostname#Restrictions_on_valid_host_names>
+    """
+    # Check length
+    if len(hostname) > 63:
+        logging.fatal("ERROR: Hostname '" + hostname + "' is too long.")
+        return False
+
+    # Check that it only contains valid chars
+    if not re.match("^[0-9a-z-]*$", hostname):
+        logging.fatal("ERROR: Hostname must only contain letters (a-z),"
+                      " digits (0-9) or minus signs (-)")
+        return False
+
+    # Check that doesn't begin or end with a minus sign
+    if hostname[:1] == "-" or hostname[-1:] == "-":
+        logging.fatal("ERROR: Hostname must not begin or end with a minus sign")
+        return False
+    return True