Abstract the secondary evac into a separate function

This is generic enough (no longer depending on the replace type) that
it will be used for other replace modes too. So all we need to do is
to abstract it to a separate function (that can be called from
multiple branches) and to make the imove op_fn computation more
generic, instead of hard-coding it.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>

htools/Ganeti/HTools/Cluster.hs

@@ -911,16 +911,7 @@ nodeEvacInstance nl il ChangePrimary
 nodeEvacInstance nl il ChangeSecondary
                  inst@(Instance.Instance {Instance.diskTemplate = DTDrbd8})
                  gdx avail_nodes =
-  do
-    let op_fn = ReplaceSecondary
-    (nl', inst', _, ndx) <- annotateResult "Can't find any good node" $
-                            eitherToResult $
-                            foldl' (evacDrbdSecondaryInner nl inst gdx op_fn)
-                            (Left "no nodes available") avail_nodes
-    let idx = Instance.idx inst
-        il' = Container.add idx inst' il
-        ops = iMoveToJob nl' il' idx (op_fn ndx)
-    return (nl', il', ops)
+  evacOneNodeOnly nl il inst gdx avail_nodes
 
 -- The algorithm for ChangeAll is as follows:
 --
@@ -961,6 +952,31 @@ nodeEvacInstance nl il ChangeAll
 
     return (nl', il', ops)
 
+-- | Generic function for changing one node of an instance.
+--
+-- This is similar to 'nodeEvacInstance' but will be used in a few of
+-- its sub-patterns.
+evacOneNodeOnly :: Node.List         -- ^ The node list (cluster-wide)
+                -> Instance.List     -- ^ Instance list (cluster-wide)
+                -> Instance.Instance -- ^ The instance to be evacuated
+                -> Gdx               -- ^ The group we're targetting
+                -> [Ndx]             -- ^ The list of available nodes
+                                      -- for allocation
+                -> Result (Node.List, Instance.List, [OpCodes.OpCode])
+evacOneNodeOnly nl il inst gdx avail_nodes = do
+  op_fn <- case templateMirrorType (Instance.diskTemplate inst) of
+             MirrorNone -> Bad "Can't relocate/evacuate non-mirrored instances"
+             MirrorInternal -> Ok ReplaceSecondary
+             MirrorExternal -> Ok FailoverToAny
+  (nl', inst', _, ndx) <- annotateResult "Can't find any good node" $
+                          eitherToResult $
+                          foldl' (evacDrbdSecondaryInner nl inst gdx op_fn)
+                          (Left "no nodes available") avail_nodes
+  let idx = Instance.idx inst
+      il' = Container.add idx inst' il
+      ops = iMoveToJob nl' il' idx (op_fn ndx)
+  return (nl', il', ops)
+
 -- | Inner fold function for changing secondary of a DRBD instance.
 --
 -- The running solution is either a @Left String@, which means we