xapi-consts: use a variant to reify valid values of cluster stack

Until now, the valid string values were not written anywhere, change the
situation.

Ideally this would be done using an enum in the idl, but unfortunately
this changes types of existing parameters in API calls, so it's quite
risky. Instead have a conservative approach to only enumerate the valid
values and make Constants the only source of truth for these values,
including default ones, which were scatterred around previously.

Signed-off-by: Pau Ruiz Safont <pau.safont@vates.tech>

Author: psafont

ocaml/xapi-consts/constants.ml

@@ -212,12 +212,29 @@ let ballooning_enabled = "ballooning.enabled"
 let redo_log_enabled = "redo_log.enabled"
 
 (* Valid cluster stack values *)
-let ha_cluster_stack = "ha_cluster_stack"
+module Ha_cluster_stack = struct
+  type t = Xhad | Corosync
 
-let default_smapiv3_cluster_stack = "corosync"
+  let key = "ha_cluster_stack"
 
-(* Note: default without clustering is in !Xapi_globs.default_cluster_stack *)
-let supported_smapiv3_cluster_stacks = ["corosync"]
+  let to_string = function Xhad -> "xhad" | Corosync -> "corosync"
+
+  let of_string = function
+    | "xhad" ->
+        Some Xhad
+    | "corosync" ->
+        Some Corosync
+    | _ ->
+        None
+end
+
+let ha_cluster_stack = Ha_cluster_stack.key
+
+let default_cluster_stack = Ha_cluster_stack.(to_string Xhad)
+
+let default_smapiv3_cluster_stack = Ha_cluster_stack.(to_string Corosync)
+
+let supported_smapiv3_cluster_stacks = [default_smapiv3_cluster_stack]
 
 (* Set in the local db to cause us to emit an alert when we come up as a master after
    a transition or HA failover *)

ocaml/xapi/xapi_globs.ml

@@ -858,7 +858,7 @@ let migration_https_only = ref true
 
 let cluster_stack_root = ref "/usr/libexec/xapi/cluster-stack"
 
-let cluster_stack_default = ref "xhad"
+let cluster_stack_default = ref Constants.default_cluster_stack
 
 let xen_cmdline_path = ref "/opt/xensource/libexec/xen-cmdline"