Re: [MirageOS-devel] mirage-logs and functoria

[Drup <drupyog+caml@xxxxxxxx>]

Le 12/01/2016 15:34, Thomas Leonard a Ãcrit :
> On 12 January 2016 at 14:00, Drup <drupyog+caml@xxxxxxxx> wrote:
> > > Ideally, I'd like to integrate this with functoria. What would be the
> > > best way to do this?
> > > Is it possible to do something like this?
> > >
> > >     register "console" [mirage_logs (main $ default_console)]
> > Not directly like that. I see several possibilities.
> >
> > - Make "Main" take an extra module argument
> >        module L = struct let log = create () |> run end
> >    then config.ml is now
> >        let log = default_log $ default_console in
> >        register "foo" [main $ .. $ log]
> >
> >    This way, the functor application of Mirage_logs.Make is outside (in the
> > "default_log" combinator).
> >    Maybe it could be combined with Logs.Src.create if that's desirable.
> >
> > - Change a bit the API, so that you have a `start` function that returns a
> > thread that starts the logger
> >    (which means no automated function wrapping).
> >
> >    Then, we declare main differently:
> >         let main = foreign ~deps:[hidden default_log] "Unikernel.Main" ( ...
> > @-> job)
> >    `Main.start` will receive an additional value (at least unit, but you can
> > put whatever you want).
> >    and that ensures that logs are initialized before the main job is
> > executed.
> >    This is what is done for nocrypto.
> Do you mean "before the main job's start function is called" or
> "before any of the inputs are evaluated"? The second would be OK. The
> docs say:
>
>      {- If [deps] is set, the given list of {{!abstract_impl}abstract}
>         implementations is added as data-dependencies: they will be
>         initialized before calling [name.connect]. }
>
> However, there is no [name.connect] function. If it means [name.start]
> then I don't see what the difference is between this and declaring it
> as a regular argument.
It's the first, there are no orders between dependencies of a device.

The difference is whether you also have a functor argument or not.

"start" is called by connect for module declared with "foreign". This is 
because devices and foreigns were different, which is not the case any more.
I would gladly change it for consistency, but that would break 
everything. :)
> >    Function wrapping could probably be arranged by using the value passed
> > around.
> >    In the end, this could be like 1), but without the functor argument.
> >
> > - The problem with those two methods is similar to your tracing issue. It
> > doesn't log things started before the main job.
> >    The third possibility is, we solve
> > https://github.com/mirage/functoria/issues/45 and we add a way to put the
> > logger (and the tracer) there.
> >    Again, no automated wrapping
> Yes. I think we have a slight advantage here: libraries using Logs
> will be written to assume that there will be no log output until their
> connect function is called (whereas libraries aren't generally aware
> that they might be being traced). However, being able to trace the
> connect calls is really useful, so waiting until Main.start is too
> late.
This would mean that, for example, network initialization is not logged. 
That limits its utility as a debugging tool.

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