Fix for qemu-nbd hang

NBD device once used seem to run into intermittent trouble when
used with mount repeatedly. Adding code to explicitly flush the
device buffers using 'blockdev --flushbufs'.

NOTE(dmllr): Also merged regression fix:

    Consolidate the blockdev related filters

    Seeing a "/usr/local/bin/nova-rootwrap: Unauthorized command" error
    in the logs when  "blockdev --flushbufs" is executed because of the
    existing blockdev in compute.filters. We need to merge both into a
    single RegExpFilter

    Change-Id: Ic323ff00e5c23786a6e376e67a4ad08f92708aef

(cherry-picked from dd3f96e915)
(cherry-picked from 31b2791e6b)

Closes-Bug: #973413
Partial-Bug: #1254890
Change-Id: I2b7053b9a069d6e82f6f6baf9ad480efa4388d91

etc/nova/rootwrap.d/compute.filters

@@ -31,6 +31,10 @@ qemu-nbd: CommandFilter, qemu-nbd, root
 # nova/virt/disk/mount/loop.py: 'losetup', '--detach', device
 losetup: CommandFilter, losetup, root
 
+# nova/virt/libvirt/utils.py: 'blockdev', '--getsize64', path
+# nova/virt/disk/mount/nbd.py: 'blockdev', '--flushbufs', device
+blockdev: RegExpFilter, blockdev, root, blockdev, (--getsize64|--flushbufs), /dev/.*
+
 # nova/virt/disk/vfs/localfs.py: 'tee', canonpath
 tee: CommandFilter, tee, root
 

nova/tests/virt/test_virt.py

@@ -125,12 +125,14 @@ class TestVirtDisk(test.NoDBTestCase):
 
         disk_api.teardown_container('/mnt/nbd/nopart')
         expected_commands += [
+                              ('blockdev', '--flushbufs', '/dev/nbd15'),
                               ('umount', '/dev/nbd15'),
                               ('qemu-nbd', '-d', '/dev/nbd15'),
                              ]
 
         disk_api.teardown_container('/mnt/nbd/part')
         expected_commands += [
+                              ('blockdev', '--flushbufs', '/dev/nbd15'),
                               ('umount', '/dev/mapper/nbd15p1'),
                               ('kpartx', '-d', '/dev/nbd15'),
                               ('qemu-nbd', '-d', '/dev/nbd15'),

nova/virt/disk/mount/api.py

@@ -205,10 +205,14 @@ class Mount(object):
         """Unmount the device from the file system."""
         if not self.mounted:
             return
+        self.flush_dev()
         LOG.debug(_("Umount %s") % self.mapped_device)
         utils.execute('umount', self.mapped_device, run_as_root=True)
         self.mounted = False
 
+    def flush_dev(self):
+        pass
+
     def do_mount(self):
         """Call the get, map and mnt operations."""
         status = False

nova/virt/disk/mount/nbd.py

@@ -129,3 +129,12 @@ class NbdMount(api.Mount):
         utils.execute('qemu-nbd', '-d', self.device, run_as_root=True)
         self.linked = False
         self.device = None
+
+    def flush_dev(self):
+        """flush NBD block device buffer."""
+        # Perform an explicit BLKFLSBUF to support older qemu-nbd(s).
+        # Without this flush, when a nbd device gets re-used the
+        # qemu-nbd intermittently hangs.
+        if self.device:
+            utils.execute('blockdev', '--flushbufs',
+                          self.device, run_as_root=True)