xapi_vm_snapshot: Fail VM.revert when a VDI.revert actually fails

In case where there's an error while reverting on the Storage Backend,
the state of the SR might become unstable, and a recovery using the
clone method for reverting might introduce bigger issues. Instead stop
the revert immediately.

The cases where ignoring the errors is acceptable is when either the SR
does not advertise support for VDI_REVERT, and when the SR does not
implement revert for a particular VDI. for example, if the SR implements
the revert for only some of the formats it implements.

Signed-off-by: Pau Ruiz Safont <pau.safont@vates.tech>
(cherry picked from commit f887de1)

Author: psafont

ocaml/xapi/storage_smapiv1.ml

@@ -1136,10 +1136,13 @@ module SMAPIv1 : Server_impl = struct
     let revert _context ~dbg ~sr ~snapshot_info =
       with_dbg ~name:"VDI.revert" ~dbg @@ fun di ->
       let dbg = Debug_info.to_string di in
-      Server_helpers.exec_with_new_task "VDI.revert"
-        ~subtask_of:(Ref.of_string dbg) (fun __context ->
-          call_revert ~__context ~dbg ~sr ~snapshot_info
-      )
+      try
+        Server_helpers.exec_with_new_task "VDI.revert"
+          ~subtask_of:(Ref.of_string dbg) (fun __context ->
+            call_revert ~__context ~dbg ~sr ~snapshot_info
+        )
+      with Smint.Not_implemented_in_backend ->
+        raise (Storage_error (Unimplemented "VDI.revert"))
   end
 
   let get_by_name _context ~dbg:_ ~name:_ = assert false

ocaml/xapi/xapi_vdi.ml

@@ -1138,10 +1138,13 @@ let revert' ~__context ~snapshot =
   C.VDI.revert (Ref.string_of task) sr' snapshot_info
 
 let revert ~__context ~snapshot =
+  let __FUN = __FUNCTION__ in
   Storage_utils.transform_storage_exn @@ fun () ->
   try revert' ~__context ~snapshot
   with Storage_interface.Storage_error (Unimplemented _) ->
     let msg = [Ref.string_of (Db.VDI.get_SR ~__context ~self:snapshot)] in
+    debug "%s: Backend reported not implemented despite it offering the feature"
+      __FUN ;
     raise Api_errors.(Server_error (unimplemented_in_sm_backend, msg))
 
 let copy ~__context ~vdi ~sr ~base_vdi ~into_vdi =

ocaml/xapi/xapi_vm_snapshot.ml

@@ -274,7 +274,12 @@ let revert_vbds ~__context ~rpc ~session_id ~snapshot ~vm =
         try
           Client.VDI.revert ~rpc ~session_id ~snapshot ;
           true
-        with _ -> false
+        with
+        | Api_errors.(Server_error (e, _))
+        when e = Api_errors.sr_operation_not_supported
+             || e = Api_errors.unimplemented_in_sm_backend
+        ->
+          false
       )
       snap_disks_all
   in