Make the nbd mounter respect CONF.max_nbd_devices.

We need to wait for the config parser to have been run before we
query the value of the max_nbd_devices flag. We will always get
the default value with the previous implementation.

Based on a conversation with Robert Collins. Resolves bug 1088073.

Change-Id: If7b5d6a5c7d8153530e97532aad67c42e744ffec

nova/tests/virt/disk/__init__.py

@@ -0,0 +1,16 @@
+# vim: tabstop=4 shiftwidth=4 softtabstop=4
+
+# Copyright 2012 Michael Still
+# All Rights Reserved.
+#
+#    Licensed under the Apache License, Version 2.0 (the "License"); you may
+#    not use this file except in compliance with the License. You may obtain
+#    a copy of the License at
+#
+#         http://www.apache.org/licenses/LICENSE-2.0
+#
+#    Unless required by applicable law or agreed to in writing, software
+#    distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+#    WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+#    License for the specific language governing permissions and limitations
+#    under the License.

nova/tests/virt/disk/test_nbd.py

@@ -0,0 +1,87 @@
+# vim: tabstop=4 shiftwidth=4 softtabstop=4
+
+# Copyright 2012 Michael Still
+# All Rights Reserved.
+#
+#    Licensed under the Apache License, Version 2.0 (the "License"); you may
+#    not use this file except in compliance with the License. You may obtain
+#    a copy of the License at
+#
+#         http://www.apache.org/licenses/LICENSE-2.0
+#
+#    Unless required by applicable law or agreed to in writing, software
+#    distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+#    WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+#    License for the specific language governing permissions and limitations
+#    under the License.
+
+
+import fixtures
+import os
+
+from nova import test
+
+from nova.openstack.common import cfg
+from nova.virt.disk.mount import nbd
+
+CONF = cfg.CONF
+CONF.import_opt('max_nbd_devices', 'nova.virt.disk.mount.nbd')
+
+
+class NbdTestCase(test.TestCase):
+    def setUp(self):
+        super(NbdTestCase, self).setUp()
+        self.flags(max_nbd_devices=2)
+        nbd.NbdMount._DEVICES_INITIALIZED = False
+        nbd.NbdMount._DEVICES = []
+
+    def test_nbd_initialize(self):
+        tempdir = self.useFixture(fixtures.TempDir()).path
+        n = nbd.NbdMount(None, tempdir)
+        self.assertTrue(n._DEVICES_INITIALIZED)
+        self.assertEquals(['/dev/nbd0', '/dev/nbd1'], nbd.NbdMount._DEVICES)
+
+    def test_nbd_not_loaded(self):
+        orig_exists = os.path.exists
+        tempdir = self.useFixture(fixtures.TempDir()).path
+        n = nbd.NbdMount(None, tempdir)
+
+        # Fake out os.path.exists
+        def fake_exists(path):
+            if path.startswith('/sys/block/nbd'):
+                return False
+            return orig_exists(path)
+        self.useFixture(fixtures.MonkeyPatch('os.path.exists', fake_exists))
+
+        # This should fail, as we don't have the module "loaded"
+        # TODO(mikal): work out how to force english as the gettext language
+        # so that the error check always passes
+        self.assertEquals(None, n._allocate_nbd())
+        self.assertEquals('nbd unavailable: module not loaded', n.error)
+
+        # And no device should be consumed
+        self.assertEquals(['/dev/nbd0', '/dev/nbd1'], nbd.NbdMount._DEVICES)
+
+    def test_nbd_allocation(self):
+        orig_exists = os.path.exists
+        tempdir = self.useFixture(fixtures.TempDir()).path
+        n = nbd.NbdMount(None, tempdir)
+
+        # Fake out os.path.exists
+        def fake_exists(path):
+            if path.startswith('/sys/block/nbd'):
+                if path.endswith('pid'):
+                    return False
+                return True
+            return orig_exists(path)
+        self.useFixture(fixtures.MonkeyPatch('os.path.exists', fake_exists))
+
+        # Allocate a nbd device
+        d = n._allocate_nbd()
+        self.assertEquals('/dev/nbd1', d)
+        self.assertEquals(1, len(nbd.NbdMount._DEVICES))
+
+        # Allocate another
+        d = n._allocate_nbd()
+        self.assertEquals('/dev/nbd0', d)
+        self.assertEquals([], nbd.NbdMount._DEVICES)

nova/virt/disk/mount/nbd.py

@@ -52,17 +52,33 @@ class NbdMount(api.Mount):
     # are no free devices. Note that patch currently hardcodes 16 devices.
     # We might be able to alleviate problem 2. by scanning /proc/partitions
     # like the aformentioned patch does.
-    _DEVICES = ['/dev/nbd%s' % i for i in range(CONF.max_nbd_devices)]
+
+    _DEVICES_INITIALIZED = False
+    _DEVICES = None
+
+    def __init__(self, image, mount_dir, partition=None, device=None):
+        super(NbdMount, self).__init__(image, mount_dir, partition=partition,
+                                       device=device)
+
+        # NOTE(mikal): this must be done here, because we need configuration
+        # to have been parsed before we initialize. Note the scoping to ensure
+        # we're updating the class scoped variables.
+        if not self._DEVICES_INITIALIZED:
+            NbdMount._DEVICES = ['/dev/nbd%s' % i for
+                                 i in range(CONF.max_nbd_devices)]
+            NbdMount._DEVICES_INITIALIZED = True
 
     def _allocate_nbd(self):
         if not os.path.exists("/sys/block/nbd0"):
             self.error = _('nbd unavailable: module not loaded')
             return None
+
         while True:
             if not self._DEVICES:
                 # really want to log this info, not raise
                 self.error = _('No free nbd devices')
                 return None
+
             device = self._DEVICES.pop()
             if not os.path.exists("/sys/block/%s/pid" %
                                   os.path.basename(device)):
@@ -80,6 +96,7 @@ class NbdMount(api.Mount):
         device = self._allocate_nbd()
         if not device:
             return False
+
         LOG.debug(_("Get nbd device %(dev)s for %(imgfile)s") %
                   {'dev': device, 'imgfile': self.image})
         _out, err = utils.trycmd('qemu-nbd', '-c', device, self.image,