Re: [Qemu-devel] [PATCH 35/35] docs: writing-qmp-commands.txt: update error section

[Pavel Hrdina <phrdina@redhat.com>]

On 08/07/2012 05:53 PM, Luiz Capitulino wrote:
> Add information about the new error format and improve the text a bit.
>
> Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
> ---
>   docs/writing-qmp-commands.txt | 47 +++++++++++++++++++++++++------------------
>   1 file changed, 27 insertions(+), 20 deletions(-)
>
> diff --git a/docs/writing-qmp-commands.txt b/docs/writing-qmp-commands.txt
> index 0ad51aa..10ecd97 100644
> --- a/docs/writing-qmp-commands.txt
> +++ b/docs/writing-qmp-commands.txt
> @@ -210,19 +210,17 @@ if you don't see these strings, then something went wrong.
>   === Errors ===
>   
>   QMP commands should use the error interface exported by the error.h header
> -file. The basic function used to set an error is the error_set() one.
> +file. Basically, errors are set by calling the error_set() function.
>   
>   Let's say we don't accept the string "message" to contain the word "love". If
> -it does contain it, we want the "hello-world" command to the return the
> -InvalidParameter error.
> -
> -Only one change is required, and it's in the C implementation:
> +it does contain it, we want the "hello-world" command to return an error:
>   
>   void qmp_hello_world(bool has_message, const char *message, Error **errp)
>   {
>       if (has_message) {
>           if (strstr(message, "love")) {
> -            error_set(errp, QERR_INVALID_PARAMETER, "message");
> +            error_set(errp, ERROR_CLASS_GENERIC_ERROR,
> +                      "the word 'love' is not allowed");
>               return;
>           }
>           printf("%s\n", message);
> @@ -231,30 +229,40 @@ void qmp_hello_world(bool has_message, const char *message, Error **errp)
>       }
>   }
>   
> -Let's test it. Build qemu, run it as defined in the "Testing" section, and
> -then issue the following command:
> +The first argument to the error_set() function is the Error pointer to pointer,
> +which is passed to all QMP functions. The second argument is a ErrorClass
> +value, which should be ERROR_CLASS_GENERIC_ERROR most of the time (more
> +details about error classes are given below). The third argument is a human
> +description of the error, this is a free-form printf-like string.
> +
> +Let's test the example above. Build qemu, run it as defined in the "Testing"
> +section, and then issue the following command:
>   
> -{ "execute": "hello-world", "arguments": { "message": "we love qemu" } }
> +{ "execute": "hello-world", "arguments": { "message": "all you need is love" } }
>   
>   The QMP server's response should be:
>   
>   {
>       "error": {
> -        "class": "InvalidParameter",
> -        "desc": "Invalid parameter 'message'",
> -        "data": {
> -            "name": "message"
> -        }
> +        "class": "GenericError",

you have here trailing white-space  ^

> +        "desc": "the word 'love' is not allowed"
>       }
>   }
>   
> -Which is the InvalidParameter error.
> +As a general rule, all QMP errors should use ERROR_CLASS_GENERIC_ERROR. There
> +are two exceptions to this rule:
> +
> + 1. A non-generic ErrorClass value exists* for the failure you want to report
> +    (eg. DeviceNotFound)
> +
> + 2. Management applications have to take special action on the failure you
> +    want to report, hence you have to add a new ErrorClass value so that they
> +    can check for it
>   
> -When you have to return an error but you're unsure what error to return or
> -which arguments an error takes, you should look at the qerror.h file. Note
> -that you might be required to add new errors if needed.
> +If the failure you want to report doesn't fall in one of the two cases above,
> +just report ERROR_CLASS_GENERIC_ERROR.
>   
> -FIXME: describe better the error API and how to add new errors.
> + * All existing ErrorClass values are defined in the qapi-schema.json file
>   
>   === Command Documentation ===
>   
> @@ -275,7 +283,6 @@ here goes "hello-world"'s new entry for the qapi-schema.json file:
>   # @message: #optional string to be printed
>   #
>   # Returns: Nothing on success.
> -#          If @message contains "love", InvalidParameter
>   #
>   # Notes: if @message is not provided, the "Hello, world" string will
>   #        be printed instead