Re: [MirageOS-devel] console interface

[Hannes Mehnert <hannes@xxxxxxxxxxx>]

please see https://github.com/mirage/mirage/pull/667 (and
https://github.com/mirage/mirage-dev/pull/207 for a happy travis)


hannes

On 12/11/2016 15:45, Hannes Mehnert wrote:
> I tried to find the reasoning behind the log functions in the module
> type CONSOLE (using the git history).
> 
> Some facts:
>  - Nowadays (since 15th October 2014, or Mirage-2.0.0) a CONSOLE is a
> FLOW (and thus has read/write/writev/close)
>  - there is a log_s : t -> string -> unit io
>  - and a log : t -> string -> unit
> 
> From the documentation of log (which mirage-console-xen follows AFAIU,
> -unix and -solo5 implementations write the entire string): "writes as
> many characters of string that can be written in one write operation to
> the console."
> 
> Both log and log_s append line breaks to the given string - that is
> convenient for hello world like applications and printf debugging.
> 
> But there are several problems with log and log_s: They cannot fail (in
> contrast to write/writev which can return `Eof).  Furthermore, if you
> emit ANSII escape codes to colour your output and use log, since it only
> attempts to write as many characters as can be written, the output
> console can be in a strange state.
> 
> I don't know what the intended semantics of the following program should be:
> 
> module Main (C: V1_LWT.CONSOLE) = struct
>   let start c =
>     C.log_s c "hello" >>= fun () ->
>     C.close c >>= fun () ->
>     C.log_s c "world"
> end
> 
> (From the documentation of FLOW.close: flushes all pending writes and
> signals the remote endpoint that there will be no future writes.)
> 
> 
> I propose the following:
> - remove CONSOLE.log
> - rename CONSOLE.log_s to CONSOLE.log
> - document that CONSOLE.log_s may fail silently if the console device
> has been closed
> 
> Searching through mirage-skeleton using grep(1) results in few
> unikernels where log is used -- all of which are inside the Lwt monad,
> and thus log_s can be used instead.
> 
> 
> Please let me know if I missed something.  Expect a PR soon,
> 
> hannes
> 
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> 


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