Behavior of jobs without dependencies

[reynir]

In the following code we have a unikernel with a job that does not depend on any devices, but logs a message.

config.ml:

open Mirage

let hello = main "Unikernel.Hello" job
let () = register "hello-nodev" [ hello ]

unikernel.ml:

module Hello = struct
  (* this is imediately evaluated *)
  let start =
    Logs.info (fun f -> f "hello");
    Lwt.return_unit
end

This successfully compiles and runs, but on the unix target this does not print anything to the console. My suspicion is the log reporter is not set up before Unikernel.Hello.start is evaluated and therefore nothing is printed.

Then my idea was to suspend the computation with a dummy/null/unit device. It was not very clear, and I had to consult the source code, but I found that I could use Mirage.noop in config.ml:

open Mirage

let hello = main "Unikernel.Hello" (job @-> job)
let () = register "hello-nodev" [ hello $ noop ]

And in unikernel.ml:

module Hello (_ : sig end) = struct
  let start () =
    Logs.info (fun f -> f "hello");
    Lwt.return_unit
end 

I guess for the first code we can't do much about the evaluation order(??) to ensure the log reporter is registered first.

My question then is what is a job? The documentation says a "main task" (but we can have a list of them!?). Should we have a unit typ and unit impl? Can we chain jobs like this?

This was discovered with @PizieDust.

[palainp]

Hi @reynir and @PizieDust for finding this :)

I'm not sure if this is a plausible path to track this issue but looking in caml_program, calling camlDune__exe__Unikernel__entry, it seems that the start function is inline in the first version.

I then tried to un-inline it but, as it's no longer a function, the compiler doesn't let me to do that :/.

I'm now at the point where I've added a single unit to the start function when no args are provided (see main...palainp:mirage:fix-nodev). This is a breaking change as now an unikernel without functorized device will have to take a unit as argument (but I'm not sure this was used before).
@mirage/core, any thoughts on this possible fix?

[hannesm]

There's #873 which looks very similar to this issue. I agree with @palainp solution to add a unit argument to start functions without any dependencies.

[reynir]

Thank you for looking into this! When I was going through mirage-skeleton and playing with mirage with @PizieDust my intuition was that zero device jobs would be a special case with a unit argument. I'm not sure if others share that intuition. On the other hand it can seem inconsistent that it's special cased.

[hannesm]

Another path is to error out (at mirage configure time) if there aren't any dependencies -- then you have to manually add the noop job as dependency (which imho would be fine, and not special casing anything... the error message could even point to that thing).

[hannesm]

fixed by #1428