Provide means to allocate new job ids

Add utility functions to allocate new job ids by increasing
the value stored in the serial file. As this function is
used in a multi-threaded program, synchronize access over
an internal lock.
Signed-off-by: Klaus Aehlig <aehlig@google.com>
Reviewed-by: Michele Tartara <mtartara@google.com>

src/Ganeti/JQueue.hs

@@ -46,8 +46,11 @@ module Ganeti.JQueue
     , loadJobFromDisk
     , noSuchJob
     , readSerialFromDisk
+    , allocateJobIds
+    , allocateJobId
     ) where
 
+import Control.Concurrent.MVar
 import Control.Exception
 import Control.Monad
 import Data.List
@@ -66,8 +69,10 @@ import Ganeti.BasicTypes
 import qualified Ganeti.Constants as C
 import Ganeti.JSON
 import Ganeti.Logging
+import Ganeti.Objects (Node)
 import Ganeti.OpCodes
 import Ganeti.Path
+import Ganeti.Rpc (executeRpcCall, RpcCallJobqueueUpdate(..))
 import Ganeti.THH
 import Ganeti.Types
 import Ganeti.Utils
@@ -338,3 +343,40 @@ readSerialFromDisk = do
   filename <- jobQueueSerialFile
   tryAndLogIOError (readFile filename) "Failed to read serial file"
                    (makeJobIdS . rStripSpace)
+
+-- | Allocate new job ids.
+-- To avoid races while accessing the serial file, the threads synchronize
+-- over a lock, as usual provided by an MVar.
+allocateJobIds :: [Node] -> MVar () -> Int -> IO (Result [JobId])
+allocateJobIds mastercandidates lock n =
+  if n <= 0
+    then return . Bad $ "Can only allocate positive number of job ids"
+    else do
+      takeMVar lock
+      rjobid <- readSerialFromDisk
+      case rjobid of
+        Bad s -> do
+          putMVar lock ()
+          return . Bad $ s
+        Ok jid -> do
+          let current = fromJobId jid
+              serial_content = show (current + n) ++  "\n"
+          serial <- jobQueueSerialFile
+          write_result <- try $ atomicWriteFile serial serial_content
+                          :: IO (Either IOError ())
+          putMVar lock ()
+          case write_result of
+            Left e -> do
+              let msg = "Failed to write serial file: " ++ show e
+              logError msg
+              return . Bad $ msg 
+            Right () -> do
+              _ <- executeRpcCall mastercandidates
+                     $ RpcCallJobqueueUpdate serial serial_content
+              return $ mapM makeJobId [(current+1)..(current+n)]
+
+-- | Allocate one new job id.
+allocateJobId :: [Node] -> MVar () -> IO (Result JobId)
+allocateJobId mastercandidates lock = do
+  jids <- allocateJobIds mastercandidates lock 1
+  return (jids >>= monadicThe "Failed to allocate precisely one Job ID")