Add a WConfd call for flushing the configuration

After the call successfully returns, the configuration is saved and
fully distributed to the master candidates.
Signed-off-by: Petr Pudlak <pudlak@google.com>
Reviewed-by: Klaus Aehlig <aehlig@google.com>

src/Ganeti/WConfd/Core.hs

@@ -103,6 +103,11 @@ unlockConfig
   :: ClientId -> WConfdMonad ()
 unlockConfig cid = freeLocksLevel cid LevelConfig
 
+-- | Force the distribution of configuration without actually modifying it.
+-- It is not necessary to hold a lock for this operation.
+flushConfig :: WConfdMonad ()
+flushConfig = forceConfigStateDistribution
+
 -- ** Temporary reservations related functions
 
 computeDRBDMap :: WConfdMonad T.DRBDMap
@@ -189,6 +194,7 @@ exportedFunctions = [ 'echo
                     , 'writeConfig
                     , 'lockConfig
                     , 'unlockConfig
+                    , 'flushConfig
                     -- temporary reservations
                     , 'computeDRBDMap
                     , 'allocateDRBDMinor

src/Ganeti/WConfd/Monad.hs

@@ -43,6 +43,7 @@ module Ganeti.WConfd.Monad
   , WConfdMonad
   , daemonHandle
   , modifyConfigState
+  , forceConfigStateDistribution
   , readConfigState
   , modifyLockWaiting
   , modifyLockWaiting_
@@ -221,6 +222,17 @@ modifyConfigState f = do
     return ()
   return r
 
+-- | Force the distribution of configuration without actually modifying it.
+--
+-- We need a separate call for this operation, because 'modifyConfigState' only
+-- triggers the distribution when the configuration changes.
+forceConfigStateDistribution :: WConfdMonad ()
+forceConfigStateDistribution  = do
+  logDebug "Forcing synchronous config write together with full distribution"
+  dh <- daemonHandle
+  liftBase . triggerAndWait (Any True) . dhSaveConfigWorker $ dh
+  logDebug "Forced config write and distribution finished"
+
 -- | Atomically modifies the state of temporary reservations in
 -- WConfdMonad in the presence of possible errors.
 modifyTempResStateErr