[Libguestfs] [PATCH] virt-sysprep:add logging feature

[Richard W.M. Jones]

On Fri, Apr 06, 2012 at 01:37:33PM +0800, Wanlong Gao wrote:
> Hi Rich,
> 
> I tried to implement the logging feature, but I can't though
> compiling with this patch now, could you please give me
> some comments?
> 
> The error message is below,
> ---
> ocamlfind ocamlopt -g -warn-error CDEFLMPSUVYZX -package unix -I ../src/.libs -I ../ocaml -c sysprep_operation.ml -o sysprep_operation.cmx
> File "sysprep_operation.ml", line 1, characters 0-1:
> Error: The implementation sysprep_operation.ml
>        does not match the interface sysprep_operation.cmi:
>        Type declarations do not match:
>          type operation = {
>            name : string;
>            pod_description : string;
>            extra_args : ((Arg.key * Arg.spec * Arg.doc) * string) list;
>            perform : Guestfs.guestfs -> string -> bool -> flag list;
>          }
>        is not included in
>          type operation = {
>            name : string;
>            pod_description : string;
>            extra_args : ((Arg.key * Arg.spec * Arg.doc) * string) list;
>            perform : Guestfs.guestfs -> string -> flag list;
>          }
>        The types for field perform are not equal.

Indeed they are not!  The implementation (*.ml) has changed:

> -  perform : Guestfs.guestfs -> string -> flag list;
> +  perform : Guestfs.guestfs -> string -> bool -> flag list;

but you must make an equivalent change to the interface (in
sysprep_operation.mli) -- adding the extra debug flag.

May I suggest another way to do this which is a bit more natural.  Use
a function.  Declare it as:

  type debug_fn = ('a, unit, string, unit) format4 -> 'a
  type operation = { ...
    perform : Guestfs.guestfs -> debug:debug_fn -> string -> flag list
  }

Implementations would just do this:

  let foo_perform g ~debug root =
    debug "I'm calling foo %s" root;
    ...

The main function does this:

  let debug fs =
    ksprintf (fun str -> if show_log then prerr_endline str) fs

  ...

  Sysprep_operations.perform_operations ?operations ~debug g root

Note that the debug function prints conditionally, based on whether
show_log is set or not.

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
libguestfs lets you edit virtual machines.  Supports shell scripting,
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