Alpha/Vote: do not allow multiple votes

src/proto_alpha/lib_protocol/src/alpha_context.mli

@@ -759,8 +759,10 @@ module Vote : sig
 
   val ballots_encoding : ballots Data_encoding.t
 
+  val has_recorded_ballot :
+    context -> public_key_hash -> bool Lwt.t
   val record_ballot:
-    context -> public_key_hash -> ballot -> context Lwt.t
+    context -> public_key_hash -> ballot -> context tzresult Lwt.t
   val get_ballots: context -> ballots tzresult Lwt.t
   val get_ballot_list: context -> (Signature.Public_key_hash.t * ballot) list Lwt.t
   val clear_ballots: context -> context Lwt.t

src/proto_alpha/lib_protocol/src/amendment.ml

@@ -227,9 +227,11 @@ let record_ballot ctxt delegate proposal ballot =
       Vote.get_current_proposal ctxt >>=? fun current_proposal ->
       fail_unless (Protocol_hash.equal proposal current_proposal)
         Invalid_proposal >>=? fun () ->
+      Vote.has_recorded_ballot ctxt delegate >>= fun has_ballot ->
+      fail_when has_ballot Unauthorized_ballot >>=? fun () ->
       Vote.in_listings ctxt delegate >>= fun in_listings ->
       if in_listings then
-        Vote.record_ballot ctxt delegate ballot >>= return
+        Vote.record_ballot ctxt delegate ballot
       else
         fail Unauthorized_ballot
   | Testing | Proposal ->

src/proto_alpha/lib_protocol/src/vote_storage.ml

@@ -74,7 +74,8 @@ let ballots_encoding =
     (req "nay"  int32)
     (req "pass" int32)
 
-let record_ballot = Storage.Vote.Ballots.init_set
+let has_recorded_ballot = Storage.Vote.Ballots.mem
+let record_ballot = Storage.Vote.Ballots.init
 
 let get_ballots ctxt =
   Storage.Vote.Ballots.fold ctxt

src/proto_alpha/lib_protocol/src/vote_storage.mli

@@ -45,9 +45,10 @@ type ballots = {
 
 val ballots_encoding : ballots Data_encoding.t
 
+val has_recorded_ballot : Raw_context.t -> Signature.Public_key_hash.t -> bool Lwt.t
 val record_ballot:
   Raw_context.t -> Signature.Public_key_hash.t -> Vote_repr.ballot ->
-  Raw_context.t Lwt.t
+  Raw_context.t tzresult Lwt.t
 val get_ballots: Raw_context.t -> ballots tzresult Lwt.t
 val get_ballot_list :
   Raw_context.t -> (Signature.Public_key_hash.t * Vote_repr.ballot) list Lwt.t

src/proto_alpha/lib_protocol/test/voting.ml

@@ -201,6 +201,13 @@ let test_voting () =
     delegates >>=? fun operations ->
   Block.bake ~operations b >>=? fun b ->
 
+  Op.ballot (B b) del1 Protocol_hash.zero Vote.Nay >>=? fun op ->
+  Block.bake ~operations:[op] b >>= fun res ->
+  Assert.proto_error ~loc:__LOC__ res begin function
+    | Amendment.Unauthorized_ballot -> true
+    | _ -> false
+  end >>=? fun () ->
+
   fold_left_s (fun v acc -> return Int32.(add v acc))
     0l rolls >>=? fun rolls_sum ->