CA-46955: make code robust to parallel deletions of VBDs

In general we should be very careful in code like this not to expect the configuration of "other VMs" to remain static while we run. In this case a parallel thread deleted a VBD which it "owned" and this cross-talk killed this thread.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>

diff --git a/ocaml/xapi/xapi_vbd_helpers.ml b/ocaml/xapi/xapi_vbd_helpers.ml
index 0488d032aff64395f1c7f62b07f2f0a0a784765d..e778f3a14b85210342a2f08435d7b9d9edcaa45d 100644 (file)
--- a/ocaml/xapi/xapi_vbd_helpers.ml
+++ b/ocaml/xapi/xapi_vbd_helpers.ml
@@ -148,9 +148,10 @@ let valid_operations ~expensive_sharing_checks ~__context record _ref' : table =
     if not record.Db_actions.vBD_currently_attached && expensive_sharing_checks 
     then begin
       (* Perform the sharing checks *)
-      (* Careful to not count this VBD *)
-       let vbds = List.filter (fun vbd -> vbd <> _ref') vdi_record.Db_actions.vDI_VBDs in
-       let vbd_records = List.map (fun self -> Db.VBD.get_record_internal ~__context ~self) vbds in    
+      (* Careful to not count this VBD and be careful to be robust to parallel deletions of unrelated VBDs *)
+               let vbd_records = 
+                       let vbds = List.filter (fun vbd -> vbd <> _ref') vdi_record.Db_actions.vDI_VBDs in
+                       List.concat (List.map (fun self -> try [ Db.VBD.get_record_internal ~__context ~self ] with _ -> []) vbds) in   
        (* Any VBD with a current_operation is said to conflict
           EXCEPT if I'm a control_domain (running pygrub) and the VBD is being 'attach'ed for booting *)
        let any p xs = try ignore(List.find p xs); true with Not_found -> false in