Fix host choice for vGPU allocation

The ordering for the group_by function when grouping hosts based on "best" PGPU
was not specified. Now the hosts should be ordered and grouped correctly
according to GPU_group.allocation_algorithm.

Signed-off-by: Si Beaumont <simon.beaumont@citrix.com>

ocaml/xapi/helpers.ml

@@ -505,10 +505,10 @@ let group_by f list =
 
 (** Groups list elements by equality of result of function application sorted
  *  in order of that result *)
-let group_by ?(descending=false) f list =
-	match descending with
-	| true -> group_by f list
-	| false -> List.rev (group_by f list)
+let group_by ~ordering f list =
+	match ordering with
+	| `descending -> group_by f list
+	| `ascending -> List.rev (group_by f list)
 
 (** Schwarzian transform sort *)
 let sort_by_schwarzian ?(descending=false) f list =

ocaml/xapi/xapi_vm_helpers.ml

@@ -574,12 +574,13 @@ let group_hosts_by_best_pgpu_in_group ~__context gpu_group vgpu_type =
 		(List.map (fun pgpu -> Db.PGPU.get_host ~__context ~self:pgpu)
 			viable_pgpus)
 	in
-	Helpers.group_by
+	let ordering =
+		match Db.GPU_group.get_allocation_algorithm ~__context ~self:gpu_group with
+		| `depth_first -> `ascending | `breadth_first -> `descending
+	in
+	Helpers.group_by ~ordering
 		(fun host ->
-			let group_by_capacity pgpus =
-				(match Db.GPU_group.get_allocation_algorithm ~__context ~self:gpu_group with
-				| `depth_first -> Helpers.group_by ~descending:false
-				| `breadth_first -> Helpers.group_by ~descending:true)
+			let group_by_capacity pgpus = Helpers.group_by ~ordering
 				(fun pgpu -> Xapi_pgpu_helpers.get_remaining_capacity ~__context ~self:pgpu ~vgpu_type)
 				pgpus
 			in