add "is_parent_of" function to blockdevice module

improve pre-defined exceptions
improve few failure behaviours

src/cryptobox/core/blockdevice.py

@@ -33,6 +33,7 @@ import subprocess
 import time
 import logging
 import cryptobox.core.settings
+import cryptobox.core.exceptions
 
 OPTIONAL_PROGS = { "lvm": True }
 """remember which programs don't exist to avoid calling them in vain
@@ -99,6 +100,10 @@ class Blockdevices:
 
 
 class Blockdevice:
+    """don't instantiate this class directly!
+
+    use "_get_blockdevice" instead
+    """
 
     def __init__(self, dev,
             sysblock_dir=DEFAULT_SYSBLOCK_DIR,
@@ -115,6 +120,11 @@ class Blockdevice:
         self.devdir = dev
         self.devnode_dir = devnode_dir
         self.sysblock_dir = sysblock_dir
+        ## check if the device is valid
+        if not os.path.isdir(os.path.join(self.devnode_dir, dev)):
+            raise cryptobox.core.exceptions.CBInternalError(
+                    "invalid blockdevice given: %s (%s)" % \
+                    (self.devdir, self.sysblock_dir))
         self.name = os.path.basename(self.devdir)
         ## "reset" below will fill these values
         self.devnum = None
@@ -358,6 +368,38 @@ class Blockdevice:
         return result
 
 
+    def is_parent_of(self, ask_child):
+        """check if the blockdevice or any of its children contains the given child
+
+        the result of "foo.is_parent_of(foo)" returns False
+
+        invalid (None) "ask_child" devices return False
+
+        @param ask_child: the blockdevice that is considered to be a possible child
+        @type ask_child: BlockDevice
+        @return: True if the child is (even recursively) part of the blockdevice, otherwise False
+        @rtype: bool
+        """
+        ## return False for non existing device
+        if ask_child is None:
+            return False
+        ## throw exception for invalid call
+        if not isinstance(ask_child, Blockdevice):
+            raise cryptobox.core.exceptions.CBInternalError(\
+                    "invalid arguments for 'is_parent_of'")
+        ## recursively go through all the children
+        for child_devname in self.children:
+            child_dev = get_blockdevice(child_devname)
+            ## direct child?
+            if child_dev == ask_child:
+                return True
+            ## indirect child?
+            if child_dev.is_parent_of(ask_child):
+                return True
+        ## no matches found
+        return False
+
+
     def __get_dev_related(self, subdir):
         """return the content of sub directories (e.g. 'holders' or 'slaves')
         """