Reinitialising a composite list zeros out other values

[talex5]

Test case:

  let open Examples.Api.Builder in
  let p = Payload.init_root () in
  let content = Payload.content_get p in
  Payload.cap_table_init p 1 |> ignore;
  TestArgs.params_init content 1 |> ignore;
  Alcotest.(check int) "Args len aa" 1 (Capnp.Array.length (TestArgs.params_get content));
  Payload.cap_table_init p 1 |> ignore;
  Alcotest.(check int) "Args len bb" 1 (Capnp.Array.length (TestArgs.params_get content))

With schema (simplfied from an RPC system):

struct CapDescriptor {
  v @0 :Float64;
}

struct Payload {
  content @0 :TestArgs;
  capTable @1 :List(CapDescriptor);
}

struct TestArgs {
  params @0 :List(Float64);
}

The second assertion fails: the second call to cap_table_init zeroes out the last element of params.

Stepping through with ocamldebug, it seems the problem is at https://github.com/pelzlpj/capnp-ocaml/blob/f711ece828a5a662569c68d048671390d7fb1862/src/runtime/builderOps.ml#L834:

        (* Composite lists prefix the data with a tag word, so clean up
           the tag word along with everything else *)
        let content_slice = {
          list_storage.storage with
          RWM.Slice.start = list_storage.storage.RWM.Slice.start - sizeof_uint64;
          RWM.Slice.len   = list_storage.storage.RWM.Slice.len   + sizeof_uint64;
        } in
RWM.Slice.zero_out content_slice ~pos:0 ~len:content_slice.RWM.Slice.len

By contrast, alloc_list_storage appears to return a slice that includes the tag. This means that whatever happens to be stored just before the list gets zeroed out. Possibly it also fails to zero the list properly.