Instance policy command line support

[ganeti-local] / lib / locking.py

diff --git a/lib/locking.py b/lib/locking.py
index 3575085..e5861c5 100644 (file)
--- a/lib/locking.py
+++ b/lib/locking.py
@@ -1,7 +1,7 @@
 #
 #
 
 #
 #
 

-# Copyright (C) 2006, 2007, 2008, 2009, 2010 Google Inc.
+# Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011 Google Inc.

 #
 # This program is free software; you can redistribute it and/or modify
 # it under the terms of the GNU General Public License as published by
 #
 # This program is free software; you can redistribute it and/or modify
 # it under the terms of the GNU General Public License as published by

@@ -20,7 +20,7 @@
 
 """Module implementing the Ganeti locking code."""
 
 
 """Module implementing the Ganeti locking code."""
 

-# pylint: disable-msg=W0212
+# pylint: disable=W0212

 
 # W0212 since e.g. LockSet methods use (a lot) the internals of
 # SharedLock
 
 # W0212 since e.g. LockSet methods use (a lot) the internals of
 # SharedLock

@@ -32,7 +32,7 @@ import errno
 import weakref
 import logging
 import heapq
 import weakref
 import logging
 import heapq

-import operator
+import itertools

 
 from ganeti import errors
 from ganeti import utils
 
 from ganeti import errors
 from ganeti import utils

@@ -161,7 +161,7 @@ class _BaseCondition(object):
     except AttributeError:
       self._acquire_restore = self._base_acquire_restore
     try:
     except AttributeError:
       self._acquire_restore = self._base_acquire_restore
     try:

-      self._is_owned = lock._is_owned
+      self._is_owned = lock.is_owned

     except AttributeError:
       self._is_owned = self._base_is_owned
 
     except AttributeError:
       self._is_owned = self._base_is_owned
 

@@ -246,7 +246,7 @@ class SingleNotifyPipeCondition(_BaseCondition):
       self._write_fd = None
     self._poller = None
 
       self._write_fd = None
     self._poller = None
 

-  def wait(self, timeout=None):
+  def wait(self, timeout):

     """Wait for a notification.
 
     @type timeout: float or None
     """Wait for a notification.
 
     @type timeout: float or None

@@ -276,7 +276,7 @@ class SingleNotifyPipeCondition(_BaseCondition):
       if self._nwaiters == 0:
         self._Cleanup()
 
       if self._nwaiters == 0:
         self._Cleanup()
 

-  def notifyAll(self): # pylint: disable-msg=C0103
+  def notifyAll(self): # pylint: disable=C0103

     """Close the writing side of the pipe to notify all waiters.
 
     """
     """Close the writing side of the pipe to notify all waiters.
 
     """

@@ -313,7 +313,7 @@ class PipeCondition(_BaseCondition):
     self._waiters = set()
     self._single_condition = self._single_condition_class(self._lock)
 
     self._waiters = set()
     self._single_condition = self._single_condition_class(self._lock)
 

-  def wait(self, timeout=None):
+  def wait(self, timeout):

     """Wait for a notification.
 
     @type timeout: float or None
     """Wait for a notification.
 
     @type timeout: float or None

@@ -333,7 +333,7 @@ class PipeCondition(_BaseCondition):
       self._check_owned()
       self._waiters.remove(threading.currentThread())
 
       self._check_owned()
       self._waiters.remove(threading.currentThread())
 

-  def notifyAll(self): # pylint: disable-msg=C0103
+  def notifyAll(self): # pylint: disable=C0103

     """Notify all currently waiting threads.
 
     """
     """Notify all currently waiting threads.
 
     """

@@ -433,9 +433,10 @@ class SharedLock(object):
 
     # Register with lock monitor
     if monitor:
 
     # Register with lock monitor
     if monitor:

+      logging.debug("Adding lock %s to monitor", name)

       monitor.RegisterLock(self)
 
       monitor.RegisterLock(self)
 

-  def GetInfo(self, requested):
+  def GetLockInfo(self, requested):

     """Retrieves information for querying locks.
 
     @type requested: set
     """Retrieves information for querying locks.
 
     @type requested: set

@@ -488,7 +489,7 @@ class SharedLock(object):
       else:
         pending = None
 
       else:
         pending = None
 

-      return (self.name, mode, owner_names, pending)
+      return [(self.name, mode, owner_names, pending)]

     finally:
       self.__lock.release()
 
     finally:
       self.__lock.release()
 

@@ -525,7 +526,7 @@ class SharedLock(object):
     else:
       return self.__is_exclusive()
 
     else:
       return self.__is_exclusive()
 

-  def _is_owned(self, shared=-1):
+  def is_owned(self, shared=-1):

     """Is the current thread somehow owning the lock at this time?
 
     @param shared:
     """Is the current thread somehow owning the lock at this time?
 
     @param shared:

@@ -540,6 +541,10 @@ class SharedLock(object):
     finally:
       self.__lock.release()
 
     finally:
       self.__lock.release()
 

+  #: Necessary to remain compatible with threading.Condition, which tries to
+  #: retrieve a locks' "_is_owned" attribute
+  _is_owned = is_owned
+

   def _count_pending(self):
     """Returns the number of pending acquires.
 
   def _count_pending(self):
     """Returns the number of pending acquires.
 

@@ -561,7 +566,9 @@ class SharedLock(object):
     self.__lock.acquire()
     try:
       # Order is important: __find_first_pending_queue modifies __pending
     self.__lock.acquire()
     try:
       # Order is important: __find_first_pending_queue modifies __pending

-      return not (self.__find_first_pending_queue() or
+      (_, prioqueue) = self.__find_first_pending_queue()
+
+      return not (prioqueue or

                   self.__pending or
                   self.__pending_by_prio or
                   self.__pending_shared)
                   self.__pending or
                   self.__pending_by_prio or
                   self.__pending_shared)

@@ -595,16 +602,15 @@ class SharedLock(object):
     while self.__pending:
       (priority, prioqueue) = self.__pending[0]
 
     while self.__pending:
       (priority, prioqueue) = self.__pending[0]
 

-      if not prioqueue:
-        heapq.heappop(self.__pending)
-        del self.__pending_by_prio[priority]
-        assert priority not in self.__pending_shared
-        continue
-

       if prioqueue:
       if prioqueue:

-        return prioqueue
+        return (priority, prioqueue)

 
 

-    return None
+      # Remove empty queue
+      heapq.heappop(self.__pending)
+      del self.__pending_by_prio[priority]
+      assert priority not in self.__pending_shared
+
+    return (None, None)

 
   def __is_on_top(self, cond):
     """Checks whether the passed condition is on top of the queue.
 
   def __is_on_top(self, cond):
     """Checks whether the passed condition is on top of the queue.

@@ -612,7 +618,9 @@ class SharedLock(object):
     The caller must make sure the queue isn't empty.
 
     """
     The caller must make sure the queue isn't empty.
 
     """

-    return cond == self.__find_first_pending_queue()[0]
+    (_, prioqueue) = self.__find_first_pending_queue()
+
+    return cond == prioqueue[0]

 
   def __acquire_unlocked(self, shared, timeout, priority):
     """Acquire a shared lock.
 
   def __acquire_unlocked(self, shared, timeout, priority):
     """Acquire a shared lock.

@@ -689,7 +697,9 @@ class SharedLock(object):
       if not wait_condition.has_waiting():
         prioqueue.remove(wait_condition)
         if wait_condition.shared:
       if not wait_condition.has_waiting():
         prioqueue.remove(wait_condition)
         if wait_condition.shared:

-          del self.__pending_shared[priority]
+          # Remove from list of shared acquires if it wasn't while releasing
+          # (e.g. on lock deletion)
+          self.__pending_shared.pop(priority, None)

 
     return False
 
 
     return False
 

@@ -721,6 +731,48 @@ class SharedLock(object):
     finally:
       self.__lock.release()
 
     finally:
       self.__lock.release()
 

+  def downgrade(self):
+    """Changes the lock mode from exclusive to shared.
+
+    Pending acquires in shared mode on the same priority will go ahead.
+
+    """
+    self.__lock.acquire()
+    try:
+      assert self.__is_owned(), "Lock must be owned"
+
+      if self.__is_exclusive():
+        # Do nothing if the lock is already acquired in shared mode
+        self.__exc = None
+        self.__do_acquire(1)
+
+        # Important: pending shared acquires should only jump ahead if there
+        # was a transition from exclusive to shared, otherwise an owner of a
+        # shared lock can keep calling this function to push incoming shared
+        # acquires
+        (priority, prioqueue) = self.__find_first_pending_queue()
+        if prioqueue:
+          # Is there a pending shared acquire on this priority?
+          cond = self.__pending_shared.pop(priority, None)
+          if cond:
+            assert cond.shared
+            assert cond in prioqueue
+
+            # Ensure shared acquire is on top of queue
+            if len(prioqueue) > 1:
+              prioqueue.remove(cond)
+              prioqueue.insert(0, cond)
+
+            # Notify
+            cond.notifyAll()
+
+      assert not self.__is_exclusive()
+      assert self.__is_sharer()
+
+      return True
+    finally:
+      self.__lock.release()
+

   def release(self):
     """Release a Shared Lock.
 
   def release(self):
     """Release a Shared Lock.
 

@@ -740,9 +792,14 @@ class SharedLock(object):
         self.__shr.remove(threading.currentThread())
 
       # Notify topmost condition in queue
         self.__shr.remove(threading.currentThread())
 
       # Notify topmost condition in queue

-      prioqueue = self.__find_first_pending_queue()
+      (priority, prioqueue) = self.__find_first_pending_queue()

       if prioqueue:
       if prioqueue:

-        prioqueue[0].notifyAll()
+        cond = prioqueue[0]
+        cond.notifyAll()
+        if cond.shared:
+          # Prevent further shared acquires from sneaking in while waiters are
+          # notified
+          self.__pending_shared.pop(priority, None)

 
     finally:
       self.__lock.release()
 
     finally:
       self.__lock.release()

@@ -844,8 +901,8 @@ class LockSet:
     # Lock monitor
     self.__monitor = monitor
 
     # Lock monitor
     self.__monitor = monitor
 

-    # Used internally to guarantee coherency.
-    self.__lock = SharedLock(name)
+    # Used internally to guarantee coherency
+    self.__lock = SharedLock(self._GetLockName("[lockset]"), monitor=monitor)

 
     # The lockdict indexes the relationship name -> lock
     # The order-of-locking is implied by the alphabetical order of names
 
     # The lockdict indexes the relationship name -> lock
     # The order-of-locking is implied by the alphabetical order of names

@@ -870,17 +927,68 @@ class LockSet:
     """
     return "%s/%s" % (self.name, mname)
 
     """
     return "%s/%s" % (self.name, mname)
 

-  def _is_owned(self):
-    """Is the current thread a current level owner?"""
+  def _get_lock(self):
+    """Returns the lockset-internal lock.
+
+    """
+    return self.__lock
+
+  def _get_lockdict(self):
+    """Returns the lockset-internal lock dictionary.
+
+    Accessing this structure is only safe in single-thread usage or when the
+    lockset-internal lock is held.
+
+    """
+    return self.__lockdict
+
+  def is_owned(self):
+    """Is the current thread a current level owner?
+
+    @note: Use L{check_owned} to check if a specific lock is held
+
+    """

     return threading.currentThread() in self.__owners
 
     return threading.currentThread() in self.__owners
 

+  def check_owned(self, names, shared=-1):
+    """Check if locks are owned in a specific mode.
+
+    @type names: sequence or string
+    @param names: Lock names (or a single lock name)
+    @param shared: See L{SharedLock.is_owned}
+    @rtype: bool
+    @note: Use L{is_owned} to check if the current thread holds I{any} lock and
+      L{list_owned} to get the names of all owned locks
+
+    """
+    if isinstance(names, basestring):
+      names = [names]
+
+    # Avoid check if no locks are owned anyway
+    if names and self.is_owned():
+      candidates = []
+
+      # Gather references to all locks (in case they're deleted in the meantime)
+      for lname in names:
+        try:
+          lock = self.__lockdict[lname]
+        except KeyError:
+          raise errors.LockError("Non-existing lock '%s' in set '%s' (it may"
+                                 " have been removed)" % (lname, self.name))
+        else:
+          candidates.append(lock)
+
+      return compat.all(lock.is_owned(shared=shared) for lock in candidates)
+    else:
+      return False
+

   def _add_owned(self, name=None):
     """Note the current thread owns the given lock"""
     if name is None:
   def _add_owned(self, name=None):
     """Note the current thread owns the given lock"""
     if name is None:

-      if not self._is_owned():
+      if not self.is_owned():

         self.__owners[threading.currentThread()] = set()
     else:
         self.__owners[threading.currentThread()] = set()
     else:

-      if self._is_owned():
+      if self.is_owned():

         self.__owners[threading.currentThread()].add(name)
       else:
         self.__owners[threading.currentThread()] = set([name])
         self.__owners[threading.currentThread()].add(name)
       else:
         self.__owners[threading.currentThread()] = set([name])

@@ -888,29 +996,29 @@ class LockSet:
   def _del_owned(self, name=None):
     """Note the current thread owns the given lock"""
 
   def _del_owned(self, name=None):
     """Note the current thread owns the given lock"""
 

-    assert not (name is None and self.__lock._is_owned()), \
+    assert not (name is None and self.__lock.is_owned()), \

            "Cannot hold internal lock when deleting owner status"
 
     if name is not None:
       self.__owners[threading.currentThread()].remove(name)
 
     # Only remove the key if we don't hold the set-lock as well
            "Cannot hold internal lock when deleting owner status"
 
     if name is not None:
       self.__owners[threading.currentThread()].remove(name)
 
     # Only remove the key if we don't hold the set-lock as well

-    if (not self.__lock._is_owned() and
+    if (not self.__lock.is_owned() and

         not self.__owners[threading.currentThread()]):
       del self.__owners[threading.currentThread()]
 
         not self.__owners[threading.currentThread()]):
       del self.__owners[threading.currentThread()]
 

-  def _list_owned(self):
+  def list_owned(self):

     """Get the set of resource names owned by the current thread"""
     """Get the set of resource names owned by the current thread"""

-    if self._is_owned():
+    if self.is_owned():

       return self.__owners[threading.currentThread()].copy()
     else:
       return set()
 
   def _release_and_delete_owned(self):
     """Release and delete all resources owned by the current thread"""
       return self.__owners[threading.currentThread()].copy()
     else:
       return set()
 
   def _release_and_delete_owned(self):
     """Release and delete all resources owned by the current thread"""

-    for lname in self._list_owned():
+    for lname in self.list_owned():

       lock = self.__lockdict[lname]
       lock = self.__lockdict[lname]

-      if lock._is_owned():
+      if lock.is_owned():

         lock.release()
       self._del_owned(name=lname)
 
         lock.release()
       self._del_owned(name=lname)
 

@@ -932,7 +1040,7 @@ class LockSet:
     # If we don't already own the set-level lock acquired
     # we'll get it and note we need to release it later.
     release_lock = False
     # If we don't already own the set-level lock acquired
     # we'll get it and note we need to release it later.
     release_lock = False

-    if not self.__lock._is_owned():
+    if not self.__lock.is_owned():

       release_lock = True
       self.__lock.acquire(shared=1)
     try:
       release_lock = True
       self.__lock.acquire(shared=1)
     try:

@@ -969,8 +1077,8 @@ class LockSet:
     assert timeout is None or timeout >= 0.0
 
     # Check we don't already own locks at this level
     assert timeout is None or timeout >= 0.0
 
     # Check we don't already own locks at this level

-    assert not self._is_owned(), ("Cannot acquire locks in the same set twice"
-                                  " (lockset %s)" % self.name)
+    assert not self.is_owned(), ("Cannot acquire locks in the same set twice"
+                                 " (lockset %s)" % self.name)

 
     if priority is None:
       priority = _DEFAULT_PRIORITY
 
     if priority is None:
       priority = _DEFAULT_PRIORITY

@@ -1100,7 +1208,7 @@ class LockSet:
           # We shouldn't have problems adding the lock to the owners list, but
           # if we did we'll try to release this lock and re-raise exception.
           # Of course something is going to be really wrong after this.
           # We shouldn't have problems adding the lock to the owners list, but
           # if we did we'll try to release this lock and re-raise exception.
           # Of course something is going to be really wrong after this.

-          if lock._is_owned():
+          if lock.is_owned():

             lock.release()
           raise
 
             lock.release()
           raise
 

@@ -1111,6 +1219,42 @@ class LockSet:
 
     return acquired
 
 
     return acquired
 

+  def downgrade(self, names=None):
+    """Downgrade a set of resource locks from exclusive to shared mode.
+
+    The locks must have been acquired in exclusive mode.
+
+    """
+    assert self.is_owned(), ("downgrade on lockset %s while not owning any"
+                             " lock" % self.name)
+
+    # Support passing in a single resource to downgrade rather than many
+    if isinstance(names, basestring):
+      names = [names]
+
+    owned = self.list_owned()
+
+    if names is None:
+      names = owned
+    else:
+      names = set(names)
+      assert owned.issuperset(names), \
+        ("downgrade() on unheld resources %s (set %s)" %
+         (names.difference(owned), self.name))
+
+    for lockname in names:
+      self.__lockdict[lockname].downgrade()
+
+    # Do we own the lockset in exclusive mode?
+    if self.__lock.is_owned(shared=0):
+      # Have all locks been downgraded?
+      if not compat.any(lock.is_owned(shared=0)
+                        for lock in self.__lockdict.values()):
+        self.__lock.downgrade()
+        assert self.__lock.is_owned(shared=1)
+
+    return True
+

   def release(self, names=None):
     """Release a set of resource locks, at the same level.
 
   def release(self, names=None):
     """Release a set of resource locks, at the same level.
 

@@ -1122,24 +1266,24 @@ class LockSet:
         (defaults to all the locks acquired at that level).
 
     """
         (defaults to all the locks acquired at that level).
 
     """

-    assert self._is_owned(), ("release() on lock set %s while not owner" %
-                              self.name)
+    assert self.is_owned(), ("release() on lock set %s while not owner" %
+                             self.name)

 
     # Support passing in a single resource to release rather than many
     if isinstance(names, basestring):
       names = [names]
 
     if names is None:
 
     # Support passing in a single resource to release rather than many
     if isinstance(names, basestring):
       names = [names]
 
     if names is None:

-      names = self._list_owned()
+      names = self.list_owned()

     else:
       names = set(names)
     else:
       names = set(names)

-      assert self._list_owned().issuperset(names), (
+      assert self.list_owned().issuperset(names), (

                "release() on unheld resources %s (set %s)" %
                "release() on unheld resources %s (set %s)" %

-               (names.difference(self._list_owned()), self.name))
+               (names.difference(self.list_owned()), self.name))

 
     # First of all let's release the "all elements" lock, if set.
     # After this 'add' can work again
 
     # First of all let's release the "all elements" lock, if set.
     # After this 'add' can work again

-    if self.__lock._is_owned():
+    if self.__lock.is_owned():

       self.__lock.release()
       self._del_owned()
 
       self.__lock.release()
       self._del_owned()
 

@@ -1161,7 +1305,7 @@ class LockSet:
 
     """
     # Check we don't already own locks at this level
 
     """
     # Check we don't already own locks at this level

-    assert not self._is_owned() or self.__lock._is_owned(shared=0), \
+    assert not self.is_owned() or self.__lock.is_owned(shared=0), \

       ("Cannot add locks if the set %s is only partially owned, or shared" %
        self.name)
 
       ("Cannot add locks if the set %s is only partially owned, or shared" %
        self.name)
 

@@ -1172,7 +1316,7 @@ class LockSet:
     # If we don't already own the set-level lock acquired in an exclusive way
     # we'll get it and note we need to release it later.
     release_lock = False
     # If we don't already own the set-level lock acquired in an exclusive way
     # we'll get it and note we need to release it later.
     release_lock = False

-    if not self.__lock._is_owned():
+    if not self.__lock.is_owned():

       release_lock = True
       self.__lock.acquire()
 
       release_lock = True
       self.__lock.acquire()
 

@@ -1235,7 +1379,7 @@ class LockSet:
     # If we own any subset of this lock it must be a superset of what we want
     # to delete. The ownership must also be exclusive, but that will be checked
     # by the lock itself.
     # If we own any subset of this lock it must be a superset of what we want
     # to delete. The ownership must also be exclusive, but that will be checked
     # by the lock itself.

-    assert not self._is_owned() or self._list_owned().issuperset(names), (
+    assert not self.is_owned() or self.list_owned().issuperset(names), (

       "remove() on acquired lockset %s while not owning all elements" %
       self.name)
 
       "remove() on acquired lockset %s while not owning all elements" %
       self.name)
 

@@ -1252,8 +1396,8 @@ class LockSet:
         removed.append(lname)
       except (KeyError, errors.LockError):
         # This cannot happen if we were already holding it, verify:
         removed.append(lname)
       except (KeyError, errors.LockError):
         # This cannot happen if we were already holding it, verify:

-        assert not self._is_owned(), ("remove failed while holding lockset %s"
-                                      % self.name)
+        assert not self.is_owned(), ("remove failed while holding lockset %s" %
+                                     self.name)

       else:
         # If no LockError was raised we are the ones who deleted the lock.
         # This means we can safely remove it from lockdict, as any further or
       else:
         # If no LockError was raised we are the ones who deleted the lock.
         # This means we can safely remove it from lockdict, as any further or

@@ -1264,7 +1408,7 @@ class LockSet:
         # it's the job of the one who actually deleted it.
         del self.__lockdict[lname]
         # And let's remove it from our private list if we owned it.
         # it's the job of the one who actually deleted it.
         del self.__lockdict[lname]
         # And let's remove it from our private list if we owned it.

-        if self._is_owned():
+        if self.is_owned():

           self._del_owned(name=lname)
 
     return removed
           self._del_owned(name=lname)
 
     return removed

@@ -1283,20 +1427,31 @@ LEVEL_CLUSTER = 0
 LEVEL_INSTANCE = 1
 LEVEL_NODEGROUP = 2
 LEVEL_NODE = 3
 LEVEL_INSTANCE = 1
 LEVEL_NODEGROUP = 2
 LEVEL_NODE = 3

+LEVEL_NODE_RES = 4

 
 

-LEVELS = [LEVEL_CLUSTER,
-          LEVEL_INSTANCE,
-          LEVEL_NODEGROUP,
-          LEVEL_NODE]
+LEVELS = [
+  LEVEL_CLUSTER,
+  LEVEL_INSTANCE,
+  LEVEL_NODEGROUP,
+  LEVEL_NODE,
+  LEVEL_NODE_RES,
+  ]

 
 # Lock levels which are modifiable
 
 # Lock levels which are modifiable

-LEVELS_MOD = [LEVEL_NODE, LEVEL_NODEGROUP, LEVEL_INSTANCE]
-
+LEVELS_MOD = frozenset([
+  LEVEL_NODE_RES,
+  LEVEL_NODE,
+  LEVEL_NODEGROUP,
+  LEVEL_INSTANCE,
+  ])
+
+#: Lock level names (make sure to use singular form)

 LEVEL_NAMES = {
   LEVEL_CLUSTER: "cluster",
   LEVEL_INSTANCE: "instance",
   LEVEL_NODEGROUP: "nodegroup",
   LEVEL_NODE: "node",
 LEVEL_NAMES = {
   LEVEL_CLUSTER: "cluster",
   LEVEL_INSTANCE: "instance",
   LEVEL_NODEGROUP: "nodegroup",
   LEVEL_NODE: "node",

+  LEVEL_NODE_RES: "node-res",

   }
 
 # Constant for the big ganeti lock
   }
 
 # Constant for the big ganeti lock

@@ -1335,13 +1490,25 @@ class GanetiLockManager:
     # The keyring contains all the locks, at their level and in the correct
     # locking order.
     self.__keyring = {
     # The keyring contains all the locks, at their level and in the correct
     # locking order.
     self.__keyring = {

-      LEVEL_CLUSTER: LockSet([BGL], "BGL", monitor=self._monitor),
-      LEVEL_NODE: LockSet(nodes, "nodes", monitor=self._monitor),
-      LEVEL_NODEGROUP: LockSet(nodegroups, "nodegroups", monitor=self._monitor),
-      LEVEL_INSTANCE: LockSet(instances, "instances",
+      LEVEL_CLUSTER: LockSet([BGL], "cluster", monitor=self._monitor),
+      LEVEL_NODE: LockSet(nodes, "node", monitor=self._monitor),
+      LEVEL_NODE_RES: LockSet(nodes, "node-res", monitor=self._monitor),
+      LEVEL_NODEGROUP: LockSet(nodegroups, "nodegroup", monitor=self._monitor),
+      LEVEL_INSTANCE: LockSet(instances, "instance",

                               monitor=self._monitor),
       }
 
                               monitor=self._monitor),
       }
 

+    assert compat.all(ls.name == LEVEL_NAMES[level]
+                      for (level, ls) in self.__keyring.items())
+
+  def AddToLockMonitor(self, provider):
+    """Registers a new lock with the monitor.
+
+    See L{LockMonitor.RegisterLock}.
+
+    """
+    return self._monitor.RegisterLock(provider)
+

   def QueryLocks(self, fields):
     """Queries information from all locks.
 
   def QueryLocks(self, fields):
     """Queries information from all locks.
 

@@ -1369,19 +1536,25 @@ class GanetiLockManager:
     assert level in LEVELS, "Invalid locking level %s" % level
     return self.__keyring[level]._names()
 
     assert level in LEVELS, "Invalid locking level %s" % level
     return self.__keyring[level]._names()
 

-  def _is_owned(self, level):
+  def is_owned(self, level):

     """Check whether we are owning locks at the given level
 
     """
     """Check whether we are owning locks at the given level
 
     """

-    return self.__keyring[level]._is_owned()
-
-  is_owned = _is_owned
+    return self.__keyring[level].is_owned()

 
 

-  def _list_owned(self, level):
+  def list_owned(self, level):

     """Get the set of owned locks at the given level
 
     """
     """Get the set of owned locks at the given level
 
     """

-    return self.__keyring[level]._list_owned()
+    return self.__keyring[level].list_owned()
+
+  def check_owned(self, level, names, shared=-1):
+    """Check if locks at a certain level are owned in a specific mode.
+
+    @see: L{LockSet.check_owned}
+
+    """
+    return self.__keyring[level].check_owned(names, shared=shared)

 
   def _upper_owned(self, level):
     """Check that we don't own any lock at a level greater than the given one.
 
   def _upper_owned(self, level):
     """Check that we don't own any lock at a level greater than the given one.

@@ -1389,18 +1562,18 @@ class GanetiLockManager:
     """
     # This way of checking only works if LEVELS[i] = i, which we check for in
     # the test cases.
     """
     # This way of checking only works if LEVELS[i] = i, which we check for in
     # the test cases.

-    return compat.any((self._is_owned(l) for l in LEVELS[level + 1:]))
+    return compat.any((self.is_owned(l) for l in LEVELS[level + 1:]))

 
 

-  def _BGL_owned(self): # pylint: disable-msg=C0103
+  def _BGL_owned(self): # pylint: disable=C0103

     """Check if the current thread owns the BGL.
 
     Both an exclusive or a shared acquisition work.
 
     """
     """Check if the current thread owns the BGL.
 
     Both an exclusive or a shared acquisition work.
 
     """

-    return BGL in self.__keyring[LEVEL_CLUSTER]._list_owned()
+    return BGL in self.__keyring[LEVEL_CLUSTER].list_owned()

 
   @staticmethod
 
   @staticmethod

-  def _contains_BGL(level, names): # pylint: disable-msg=C0103
+  def _contains_BGL(level, names): # pylint: disable=C0103

     """Check if the level contains the BGL.
 
     Check if acting on the given level and set of names will change
     """Check if the level contains the BGL.
 
     Check if acting on the given level and set of names will change

@@ -1445,6 +1618,22 @@ class GanetiLockManager:
     return self.__keyring[level].acquire(names, shared=shared, timeout=timeout,
                                          priority=priority)
 
     return self.__keyring[level].acquire(names, shared=shared, timeout=timeout,
                                          priority=priority)
 

+  def downgrade(self, level, names=None):
+    """Downgrade a set of resource locks from exclusive to shared mode.
+
+    You must have acquired the locks in exclusive mode.
+
+    @type level: member of locking.LEVELS
+    @param level: the level at which the locks shall be downgraded
+    @type names: list of strings, or None
+    @param names: the names of the locks which shall be downgraded
+        (defaults to all the locks acquired at the level)
+
+    """
+    assert level in LEVELS, "Invalid locking level %s" % level
+
+    return self.__keyring[level].downgrade(names=names)
+

   def release(self, level, names=None):
     """Release a set of resource locks, at the same level.
 
   def release(self, level, names=None):
     """Release a set of resource locks, at the same level.
 

@@ -1463,7 +1652,7 @@ class GanetiLockManager:
             not self._upper_owned(LEVEL_CLUSTER)), (
             "Cannot release the Big Ganeti Lock while holding something"
             " at upper levels (%r)" %
             not self._upper_owned(LEVEL_CLUSTER)), (
             "Cannot release the Big Ganeti Lock while holding something"
             " at upper levels (%r)" %

-            (utils.CommaJoin(["%s=%r" % (LEVEL_NAMES[i], self._list_owned(i))
+            (utils.CommaJoin(["%s=%r" % (LEVEL_NAMES[i], self.list_owned(i))

                               for i in self.__keyring.keys()]), ))
 
     # Release will complain if we don't own the locks already
                               for i in self.__keyring.keys()]), ))
 
     # Release will complain if we don't own the locks already

@@ -1508,12 +1697,25 @@ class GanetiLockManager:
     # Check we either own the level or don't own anything from here
     # up. LockSet.remove() will check the case in which we don't own
     # all the needed resources, or we have a shared ownership.
     # Check we either own the level or don't own anything from here
     # up. LockSet.remove() will check the case in which we don't own
     # all the needed resources, or we have a shared ownership.

-    assert self._is_owned(level) or not self._upper_owned(level), (
+    assert self.is_owned(level) or not self._upper_owned(level), (

            "Cannot remove locks at a level while not owning it or"
            " owning some at a greater one")
     return self.__keyring[level].remove(names)
 
 
            "Cannot remove locks at a level while not owning it or"
            " owning some at a greater one")
     return self.__keyring[level].remove(names)
 
 

+def _MonitorSortKey((item, idx, num)):
+  """Sorting key function.
+
+  Sort by name, registration order and then order of information. This provides
+  a stable sort order over different providers, even if they return the same
+  name.
+
+  """
+  (name, _, _, _) = item
+
+  return (utils.NiceSortKey(name), num, idx)
+
+

 class LockMonitor(object):
   _LOCK_ATTR = "_lock"
 
 class LockMonitor(object):
   _LOCK_ATTR = "_lock"
 

@@ -1523,27 +1725,50 @@ class LockMonitor(object):
     """
     self._lock = SharedLock("LockMonitor")
 
     """
     self._lock = SharedLock("LockMonitor")
 

+    # Counter for stable sorting
+    self._counter = itertools.count(0)
+

     # Tracked locks. Weak references are used to avoid issues with circular
     # references and deletion.
     self._locks = weakref.WeakKeyDictionary()
 
   @ssynchronized(_LOCK_ATTR)
     # Tracked locks. Weak references are used to avoid issues with circular
     # references and deletion.
     self._locks = weakref.WeakKeyDictionary()
 
   @ssynchronized(_LOCK_ATTR)

-  def RegisterLock(self, lock):
+  def RegisterLock(self, provider):

     """Registers a new lock.
 
     """Registers a new lock.
 

+    @param provider: Object with a callable method named C{GetLockInfo}, taking
+      a single C{set} containing the requested information items
+    @note: It would be nicer to only receive the function generating the
+      requested information but, as it turns out, weak references to bound
+      methods (e.g. C{self.GetLockInfo}) are tricky; there are several
+      workarounds, but none of the ones I found works properly in combination
+      with a standard C{WeakKeyDictionary}
+

     """
     """

-    logging.debug("Registering lock %s", lock.name)
-    assert lock not in self._locks, "Duplicate lock registration"
-    assert not compat.any(lock.name == i.name for i in self._locks.keys()), \
-           "Found duplicate lock name"
-    self._locks[lock] = None
+    assert provider not in self._locks, "Duplicate registration"
+
+    # There used to be a check for duplicate names here. As it turned out, when
+    # a lock is re-created with the same name in a very short timeframe, the
+    # previous instance might not yet be removed from the weakref dictionary.
+    # By keeping track of the order of incoming registrations, a stable sort
+    # ordering can still be guaranteed.
+
+    self._locks[provider] = self._counter.next()

 
 

-  @ssynchronized(_LOCK_ATTR)

   def _GetLockInfo(self, requested):
   def _GetLockInfo(self, requested):

-    """Get information from all locks while the monitor lock is held.
+    """Get information from all locks.

 
     """
 
     """

-    return [lock.GetInfo(requested) for lock in self._locks.keys()]
+    # Must hold lock while getting consistent list of tracked items
+    self._lock.acquire(shared=1)
+    try:
+      items = self._locks.items()
+    finally:
+      self._lock.release()
+
+    return [(info, idx, num)
+            for (provider, num) in items
+            for (idx, info) in enumerate(provider.GetLockInfo(requested))]

 
   def _Query(self, fields):
     """Queries information from all locks.
 
   def _Query(self, fields):
     """Queries information from all locks.

@@ -1554,11 +1779,13 @@ class LockMonitor(object):
     """
     qobj = query.Query(query.LOCK_FIELDS, fields)
 
     """
     qobj = query.Query(query.LOCK_FIELDS, fields)
 

-    # Get all data and sort by name
-    lockinfo = utils.NiceSort(self._GetLockInfo(qobj.RequestedData()),
-                              key=operator.itemgetter(0))
+    # Get all data with internal lock held and then sort by name and incoming
+    # order
+    lockinfo = sorted(self._GetLockInfo(qobj.RequestedData()),
+                      key=_MonitorSortKey)

 
 

-    return (qobj, query.LockQueryData(lockinfo))
+    # Extract lock information and build query data
+    return (qobj, query.LockQueryData(map(compat.fst, lockinfo)))

 
   def QueryLocks(self, fields):
     """Queries information from all locks.
 
   def QueryLocks(self, fields):
     """Queries information from all locks.