Add data structure representing current lock status

To allow for jobs as processes, a central daemon (wconfd) will
handle allocation and release of locks. Add an appropriate data
structure to describe the current status of the locks.
Signed-off-by: Klaus Aehlig <aehlig@google.com>
Reviewed-by: Petr Pudlak <pudlak@google.com>

Makefile.am

@@ -126,6 +126,7 @@ HS_DIRS = \
 	src/Ganeti/HTools/Program \
 	src/Ganeti/Hypervisor \
 	src/Ganeti/Hypervisor/Xen \
+        src/Ganeti/Locking \
 	src/Ganeti/Monitoring \
 	src/Ganeti/Query \
 	src/Ganeti/Storage \
@@ -730,6 +731,7 @@ HS_LIB_SRCS = \
 	src/Ganeti/JSON.hs \
 	src/Ganeti/Jobs.hs \
 	src/Ganeti/Kvmd.hs \
+        src/Ganeti/Locking/Allocation.hs \
 	src/Ganeti/Logging.hs \
 	src/Ganeti/Luxi.hs \
 	src/Ganeti/Monitoring/Server.hs \

src/Ganeti/Locking/Allocation.hs

+{-| Implementation of lock allocation.
+
+-}
+
+{-
+
+Copyright (C) 2014 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
+the Free Software Foundation; either version 2 of the License, or
+(at your option) any later version.
+
+This program is distributed in the hope that it will be useful, but
+WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+General Public License for more details.
+
+You should have received a copy of the GNU General Public License
+along with this program; if not, write to the Free Software
+Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
+02110-1301, USA.
+
+-}
+
+module Ganeti.Locking.Allocation
+  ( LockAllocation
+  , emptyAllocation
+  , OwnerState(..)
+  , listLocks
+  ) where
+
+import qualified Data.Map as M
+import Data.Maybe (fromMaybe)
+import qualified Data.Set as S
+
+{-
+
+This module is parametric in the type of locks and lock owners.
+While we only state minimal requirements for the types, we will
+consistently use the type variable 'a' for the type of locks and
+the variable 'b' for the type of the lock owners throughout this
+module.
+
+-}
+
+-- | The state of a lock that is taken.
+data AllocationState a = Exclusive a | Shared (S.Set a) deriving (Eq, Show)
+
+-- | Data type describing the way a lock can be owned.
+data OwnerState = OwnShared | OwnExclusive deriving (Ord, Eq, Show)
+
+{-| Representation of a Lock allocation
+
+To keep queries for locks efficient, we keep two
+associations, with the invariant that they fit
+together: the association from locks to their
+allocation state, and the association from an
+owner to the set of locks owned. As we do not
+export the constructor, the problem of keeping
+this invariant reduces to only exporting functions
+that keep the invariant.
+
+-}
+
+data LockAllocation a b =
+  LockAllocation { laLocks :: M.Map a (AllocationState b)
+                 , laOwned :: M.Map b (M.Map a OwnerState)
+                 }
+  deriving (Eq, Show)
+
+-- | A state with all locks being free.
+emptyAllocation :: (Ord a, Ord b) => LockAllocation a b
+emptyAllocation =
+  LockAllocation { laLocks = M.empty
+                 , laOwned = M.empty
+                 }
+
+-- | Obtain the set of locks held by a given owner.
+listLocks :: Ord b => b -> LockAllocation a b -> M.Map a OwnerState
+listLocks owner = fromMaybe M.empty . M.lookup owner . laOwned