Fix loop_wait (#1462150)

The previous code used losetup --list -O to return the backing store
associated with the loop device. This can fail due to losetup truncating
the output filename if sysfs isn't setup. Instead of printing the full
path it will truncate it to 64 characters with a * at the end.

See util-linux lib/loopdev.c for the code that does this.

This commit changes it to use the existing get_loop_name function, which
uses losetup -j to lookup the loop device associated with the backing
store which avoids the truncation problem.

Resolves: rhbz#1462150

src/pylorax/imgutils.py

@@ -24,6 +24,7 @@ import os, tempfile
 from os.path import join, dirname
 from subprocess import Popen, PIPE, CalledProcessError
 import sys
+import time
 import traceback
 import multiprocessing
 from time import sleep
@@ -136,12 +137,16 @@ def loop_waitfor(loop_dev, outfile):
     """
     for x in xrange(0,5):
         runcmd(["udevadm", "settle", "--timeout", "300"])
-        buf = runcmd_output(["losetup", "--list", "-O", "BACK-FILE", loop_dev])
-        lines = buf.splitlines()
-        if len(lines) < 2:
-            continue
-        if lines[1].strip() == outfile:
+        ## XXX Note that losetup --list output can be truncated to 64 bytes in some
+        ##     situations. Don't use it to lookup backing file, go the other way
+        ##     and lookup the loop for the backing file. See util-linux lib/loopdev.c
+        ##     loopcxt_get_backing_file()
+        if get_loop_name(outfile) == os.path.basename(loop_dev):
             return
+
+        # If this really is a race, give it some time to settle down
+        time.sleep(1)
+
     raise RuntimeError("Unable to setup %s on %s" % (loop_dev, outfile))
 
 def loop_attach(outfile):