Release the crypt device if encrypt_image is interrupted

Nothing unwound the loop device and dm-crypt mapping when the copy loop
raised, so interrupting a pack stranded both, still holding the profile's
rootimg.sfs.

Tear them down from a finally.  The retry loop moves with them, so also
honour its tries counter, as unpack_image already does; spinning forever
inside a finally would hang the interrupt it is meant to clean up after.

A bounded retry loop can also give up, and the detach that follows would
then fail with EBUSY and, raising from a finally, replace the exception
that brought us here.  Warn and leave both in place instead.

imgutil/imgutil

@@ -385,28 +385,37 @@ def encrypt_image(plainfile, cryptfile, keyfile):
         neededblocks += 1
     loopdev = subprocess.check_output(['losetup', '-f']).decode('utf8').strip()
     subprocess.check_call(['losetup', loopdev, cryptfile])
-    subprocess.check_call(['dmsetup', 'create', dmname, '--table', '0 {} crypt aes-xts-plain64 {} 0 {} 8'.format(neededblocks, key, loopdev)])
-    subprocess.check_call(['dmsetup', 'mknodes', dmname])
-    with open('/dev/mapper/{}'.format(dmname), 'wb') as cryptout:
-        with open(plainfile, 'rb+') as plainin:
-            lastoffset = 0
-            chunk = plainin.read(2097152)
-            while chunk:
-                fallocate(plainin.fileno(), FALLOC_FL_KEEP_SIZE|FALLOC_FL_PUNCH_HOLE, lastoffset, len(chunk))
-                lastoffset = plainin.tell()
-                cryptout.write(chunk)
+    try:
+        subprocess.check_call(['dmsetup', 'create', dmname, '--table', '0 {} crypt aes-xts-plain64 {} 0 {} 8'.format(neededblocks, key, loopdev)])
+        subprocess.check_call(['dmsetup', 'mknodes', dmname])
+        with open('/dev/mapper/{}'.format(dmname), 'wb') as cryptout:
+            with open(plainfile, 'rb+') as plainin:
+                lastoffset = 0
                 chunk = plainin.read(2097152)
-    mounted = True
-    tries = 30
-    time.sleep(0.1)
-    while mounted:
-        tries -= 1
-        try:
-            subprocess.check_call(['dmsetup', 'remove', dmname])
-            mounted = False
-        except subprocess.CalledProcessError:
-            time.sleep(0.1)
-    subprocess.check_call(['losetup', '-d', loopdev])
+                while chunk:
+                    fallocate(plainin.fileno(), FALLOC_FL_KEEP_SIZE|FALLOC_FL_PUNCH_HOLE, lastoffset, len(chunk))
+                    lastoffset = plainin.tell()
+                    cryptout.write(chunk)
+                    chunk = plainin.read(2097152)
+    finally:
+        mounted = True
+        tries = 30
+        time.sleep(0.1)
+        while mounted and tries:
+            tries -= 1
+            try:
+                subprocess.check_call(['dmsetup', 'remove', dmname])
+                mounted = False
+            except subprocess.CalledProcessError:
+                time.sleep(0.1)
+        if mounted:
+            sys.stderr.write(
+                'Warning: unable to remove {0}, it and {1} are left behind\n'.format(dmname, loopdev))
+        else:
+            try:
+                subprocess.check_call(['losetup', '-d', loopdev])
+            except subprocess.CalledProcessError:
+                sys.stderr.write('Warning: unable to detach {0}\n'.format(loopdev))
     oum = os.umask(0o077)
     with open(keyfile, 'w') as keyout:
         keyout.write('aes-xts-plain64\n{}\n'.format(key))