Re: [PATCH v4 02/45] error: Improve error.h's big comment

[Greg Kurz <groug@kaod.org>]

On Tue,  7 Jul 2020 18:05:30 +0200
Markus Armbruster <armbru@redhat.com> wrote:

> Add headlines to the big comment.
> 
> Explain examples for NULL, &error_abort and &error_fatal argument
> better.
> 
> Tweak rationale for error_propagate_prepend().
> 
> Signed-off-by: Markus Armbruster <armbru@redhat.com>
> ---

Reviewed-by: Greg Kurz <groug@kaod.org>

>  include/qapi/error.h | 51 +++++++++++++++++++++++++++++++-------------
>  1 file changed, 36 insertions(+), 15 deletions(-)
> 
> diff --git a/include/qapi/error.h b/include/qapi/error.h
> index e8960eaad5..6d079c58b7 100644
> --- a/include/qapi/error.h
> +++ b/include/qapi/error.h
> @@ -15,6 +15,8 @@
>  /*
>   * Error reporting system loosely patterned after Glib's GError.
>   *
> + * = Creating errors =
> + *
>   * Create an error:
>   *     error_setg(&err, "situation normal, all fouled up");
>   *
> @@ -27,6 +29,8 @@
>   *     error_setg(&err, "invalid quark\n" // WRONG!
>   *                "Valid quarks are up, down, strange, charm, top, bottom.");
>   *
> + * = Reporting and destroying errors =
> + *
>   * Report an error to the current monitor if we have one, else stderr:
>   *     error_report_err(err);
>   * This frees the error object.
> @@ -40,6 +44,30 @@
>   *     error_free(err);
>   * Note that this loses hints added with error_append_hint().
>   *
> + * Call a function ignoring errors:
> + *     foo(arg, NULL);
> + * This is more concise than
> + *     Error *err = NULL;
> + *     foo(arg, &err);
> + *     error_free(err); // don't do this
> + *
> + * Call a function aborting on errors:
> + *     foo(arg, &error_abort);
> + * This is more concise and fails more nicely than
> + *     Error *err = NULL;
> + *     foo(arg, &err);
> + *     assert(!err); // don't do this
> + *
> + * Call a function treating errors as fatal:
> + *     foo(arg, &error_fatal);
> + * This is more concise than
> + *     Error *err = NULL;
> + *     foo(arg, &err);
> + *     if (err) { // don't do this
> + *         error_report_err(err);
> + *         exit(1);
> + *     }
> + *
>   * Handle an error without reporting it (just for completeness):
>   *     error_free(err);
>   *
> @@ -47,6 +75,11 @@
>   * reporting it (primarily useful in testsuites):
>   *     error_free_or_abort(&err);
>   *
> + * = Passing errors around =
> + *
> + * Errors get passed to the caller through the conventional @errp
> + * parameter.
> + *
>   * Pass an existing error to the caller:
>   *     error_propagate(errp, err);
>   * where Error **errp is a parameter, by convention the last one.
> @@ -54,11 +87,10 @@
>   * Pass an existing error to the caller with the message modified:
>   *     error_propagate_prepend(errp, err,
>   *                             "Could not frobnicate '%s': ", name);
> - *
> - * Avoid
> - *     error_propagate(errp, err);
> + * This is more concise than
> + *     error_propagate(errp, err); // don't do this
>   *     error_prepend(errp, "Could not frobnicate '%s': ", name);
> - * because this fails to prepend when @errp is &error_fatal.
> + * and works even when @errp is &error_fatal.
>   *
>   * Create a new error and pass it to the caller:
>   *     error_setg(errp, "situation normal, all fouled up");
> @@ -70,15 +102,6 @@
>   *         handle the error...
>   *     }
>   *
> - * Call a function ignoring errors:
> - *     foo(arg, NULL);
> - *
> - * Call a function aborting on errors:
> - *     foo(arg, &error_abort);
> - *
> - * Call a function treating errors as fatal:
> - *     foo(arg, &error_fatal);
> - *
>   * Receive an error and pass it on to the caller:
>   *     Error *err = NULL;
>   *     foo(arg, &err);
> @@ -86,8 +109,6 @@
>   *         handle the error...
>   *         error_propagate(errp, err);
>   *     }
> - * where Error **errp is a parameter, by convention the last one.
> - *
>   * Do *not* "optimize" this to
>   *     foo(arg, errp);
>   *     if (*errp) { // WRONG!