Re: [Xen-devel] [PATCH v2 3/3] libxl: ocaml: use 'for_app_registration' in osevent callbacks

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Fri, 2013-12-13 at 18:04 +0000, Rob Hoes wrote:
> On 13 Dec 2013, at 17:24, Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx> wrote:
> > Rob Hoes writes ("[PATCH v2 3/3] libxl: ocaml: use 'for_app_registration' 
> > in osevent callbacks"):
> >> This allows the application to pass a token to libxl in the fd/timeout
> >> registration callbacks, which it receives back in modification or
> >> deregistration callbacks.
> > ...
> >> +  *for_app = caml_callbackN_exn(*func, 4, args);
> >> +  if (Is_exception_result(*for_app)) {
> >> +          ret = ERROR_OSEVENT_REG_FAIL;
> >> +          goto err;
> >> +  }
> >> +
> >> +  caml_register_global_root(for_app);
> >> +  *for_app_registration_out = for_app;
> >> +
> >> +err:
> >>    CAMLdone;
> >>    caml_enter_blocking_section();
> >> -  return 0;
> >> +  return ret;
> > 
> > Doesn't this have the effect of throwing away the exception
> > information ?  Perhaps it should be logged somewhere.
> 
> Thatâs true.

*for_app eventually ends up in a caller which is in the C bindings,
doesn't it? Can it not be picked up there and reused with caml_raise
after the appropriate C level cleanup?

> What would be the best way to do that? Can we get the logger from the
> context in these stubs? If not, itâs going to be a little more
> complicated... If we can, then still we need to add the ctx to the
> âuserâ value that is passed around.
> 
> Rob



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